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PALEOTOPOGRAPHY ON THE INTRA-SWAN HILLS FORMATION UNCONFORMITY IN AN ISOLATED PLATFORM, CARSON CREEK NORTH FIELD (UPPER DEVONIAN, FRASNIAN), AND IMPLICATIONS FOR REGIONAL STRATIGRAPHIC CORRELATION IN THE BEAVERHILL LAKE GROUP, SOUTHERN ALBERTA, CANADA: THE CASE OF THE MISSING REGRESSION

By
Joel F. Collins
Joel F. Collins
ExxonMobil Development Co., 22777 Springwoods Village Parkway, Spring, Texas 77389, USA e-mail: joel.f.collins@exxonmobil.com
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Published:
January 01, 2017
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Abstract:

The Carson Creek North Field is an Upper Devonian isolated reef-rimmed buildup in the Swan Hills Formation of the subsurface Western Canada Sedimentary Basin. The Swan Hills Formation belongs to the Beaverhill Lake Group, which contains three regionally defined sequences. The sequence boundaries at the base of the middle sequence (BHL2.1 SB) and the upper sequence (BHL3.1 SB) are present within the Carson Creek North buildup, dividing it into three evolutionary stages (lower atoll stage, upper atoll stage, and shoal stage). The BHL2.1 SB is a locally exposed surface at the top of the lower atoll stage, during which the Carson Creek North buildup evolved from a low-angle ramp to a steep-sided reef margin enclosing a restricted lagoon. The BHL3.1 SB is a regional subaerial exposure surface at the top of the upper atoll stage, also known as the intra-Swan Hills unconformity (ISHU). The upper atoll stage consists of backstepping reef margin cycles, and the shoal stage overlying the ISHU contains backstepping ramps culminating in drowning of the Carson Creek North buildup; therefore, the ISHU occurs within a continuously backstepping margin succession. The ISHU demonstrates ~13 m of paleotopographic relief within the Carson Creek North buildup, yet it is not associated with a regressive margin.

The Beaverhill Lake Group also contains shallow-water carbonates in the southeastern part of the Western Canada basin (Eastern Shelf) that are partly equivalent to the Swan Hills Formation. The Eastern Shelf carbonates are separated from Carson Creek North by more than 100 km across the Waterways shale basin. A regressive facies succession is present just below the BHL3.1 SB in the Eastern Shelf area, followed by an intrabasin lowstand corresponding to a hiatus during which paleotopography developed on the ISHU at Carson Creek North. The Eastern Shelf succession thus contains the missing regression across the ISHU at Carson Creek North. The lowstand was followed by gradual regional flooding that included deposition of the Carson Creek North shoal stage ramps. Stratigraphic comparisons among Carson Creek North, the Swan Hills Formation, and the Eastern Shelf areas indicate that regional differential subsidence patterns and initial basin floor topography were the most likely reasons for the divergent stratigraphic architecture. The origin of the ISHU itself remains unresolved, but it could be linked to a global eustatic event near the base of the Frasnian Stage, enhanced by tectonic activity in the Western Canada Basin.

REGIONAL SETTING, PREVIOUS WORK, AND OBJECTIVES

The Upper Devonian (upper Givetian–lower Frasnian) Swan Hills Formation forms an extensive carbonate complex along the southwestern margin of the Western Canada Basin, consisting of a regional shelf that breaks up into a series of oil-bearing isolated buildups toward the northern end. The Swan Hills Formation was historically divided into informal platform and reef members, with the platform member serving as a foundation upon which the isolated buildups developed. Within the reef member, intervals characterized as “green shale and breccia,” or “erosional breccia,” or “subaerial exposure surfaces associated with cryptalgal laminite” were noted separating reef-rimmed lagoonal successions from overlying ramp or shoal successions in several locations along the shelf margin. Older case studies used this green shale marker for internal zonation of the reef member (Murray 1966, Fischbuch 1968, Viau 1983, Kaufman and Meyers 1988, Kaufman et al. 1991). These markers were first proposed to represent a regional exposure surface within the Beaverhill Lake Group by Potma et al. (1992). The exposure surface was called the intra-Swan Hills unconformity (ISHU) in Wendte and Muir (1995), and instances in the Judy Creek and Snipe Lake buildups were subsequently intensely studied from stratigraphic (Wendte and Muir 1995) and petrographic/geochemical (Chow and Wendte 2011) perspectives. Swan Hills Formation studies conducted after 1995 have generally accepted similar markers or thick green shale intervals in the Swan Hills Formation as equivalent to the ISHU (e.g., Saller et al. 2001, Atchley et al. 2008).

The Swan Hills Formation is part of the regional Beaverhill Lake Group in the Western Canada Basin, which contains shallow-water carbonate units in the Eastern Shelf area that are partly equivalent to the Swan Hills Formation. The Eastern Shelf carbonates and the Swan Hills complex are separated by the Waterways Basin (Fig. 1), and comparison between the Swan Hills shelf complex and the Eastern shelf carbonates inevitably entails well-log correlations across the Waterways Basin. The Beaverhill Lake and Swan Hills strata have been studied for several decades, particularly during an active period of oil and gas exploration from the late 1960s to the mid-1990s, yet correlations have not been unequivocally established. Regional studies by Potma et al. (2001) and Wendte and Uyeno (2005) contain different interpretations for the number of stratigraphic sequences making up the Beaverhill Lake Group, methods of correlation across the basin, and position of the ISHU correlative equivalent in the Eastern Shelf area. Wendte and Uyeno (2005) correlated two regional sequences within the Beaverhill Lake Group separated by the ISHU and its correlative equivalent. Potma et al. (2001, 2002) identified three Beaverhill Lake Group sequences as part of a more comprehensive framework for the entire Frasnian in the Western Canada Basin, with the ISHU forming the BHL3.1 sequence boundary at the base of the BHL3 sequence (Wong et al. 2016).

Fig. 1.

—Index map showing the distribution of carbonates in the Swan Hills shelf complex and the Eastern Shelf area, including wells and cores used or referenced in the study. Regional well-log cross sections that appear in Figures 21 and 22 are indicated. The Waterways Formation downlap trends are from Wendte and Uyeno (2005). The westward extent of the Firebag Member is a depositional marker indicating positive basin-floor relief under the Swan Hills complex (see Figs. 21 and 22). Well names represent their locations within the Alberta grid system. The grid system consists of square townships (6 by 6 miles, 9.7 by 9.7 km), each containing 36 sections, with each section divided into 16 legal subdivisions (LSD). Township numbers increase in a northward direction, with their longitudinal positions indicated by westward-increasing range numbers referenced to north-south meridians (w4 and w5). The well name “1-1-50-20w4,” for example, specifies the LSD, section, township number, range number, and reference meridian containing the well.

Fig. 1.

—Index map showing the distribution of carbonates in the Swan Hills shelf complex and the Eastern Shelf area, including wells and cores used or referenced in the study. Regional well-log cross sections that appear in Figures 21 and 22 are indicated. The Waterways Formation downlap trends are from Wendte and Uyeno (2005). The westward extent of the Firebag Member is a depositional marker indicating positive basin-floor relief under the Swan Hills complex (see Figs. 21 and 22). Well names represent their locations within the Alberta grid system. The grid system consists of square townships (6 by 6 miles, 9.7 by 9.7 km), each containing 36 sections, with each section divided into 16 legal subdivisions (LSD). Township numbers increase in a northward direction, with their longitudinal positions indicated by westward-increasing range numbers referenced to north-south meridians (w4 and w5). The well name “1-1-50-20w4,” for example, specifies the LSD, section, township number, range number, and reference meridian containing the well.

The Carson Creek North Field is southernmost among the northern isolated Swan Hills buildups in Figure 1. As it does elsewhere, the ISHU divides the reef member at Carson Creek North into distinct stages, one represented by low-energy lagoonal successions below the ISHU (atoll stage), and another by higher-energy shoal/ramp deposits above the ISHU (shoal stage). The Carson Creek North reef member was first divided into growth stages by Leavitt (1968), based on detailed lithofacies successions in the buildup interior combined with the stratigraphic architecture of the reef margin. The different lithofacies in the atoll and shoal stages cause them to have some residual interpretive value because the green shale marker associated with the ISHU is often readily visible on gamma-ray (GR) logs, which can then be used to identify the same stages in areas with less well and core control along the margin of the Swan Hills complex. The atoll and shoal stages of the reef member, along with the Swan Hills platform member, conform to a modification of the regional sequence stratigraphic framework in Potma et al. (2001, 2002) as indicated in Figure 2.

Fig. 2.

—Beaverhill Lake Group sequences, Waterways Formation correlation units, and Carson Creek North reef stages, including the interpreted intrabasin lowstand interval corresponding to the Swan Hills ISHU.

Fig. 2.

—Beaverhill Lake Group sequences, Waterways Formation correlation units, and Carson Creek North reef stages, including the interpreted intrabasin lowstand interval corresponding to the Swan Hills ISHU.

The study of the cores presented in this paper indicates that the ISHU is associated with paleotopographic relief at the top of the upper atoll stage in Carson Creek North, implying a significant associated hiatus. The durations of any depositional gaps in the Swan Hills Formation are poorly constrained however. Direct age dating of the ISHU within the Swan Hills Formation is not generally feasible because of the difficulty of obtaining conodonts from the shallow carbonate facies (Wendte and Uyeno 2005). This study examines the ISHU hiatus by comparing the transgressive–regressive architecture in the Carson Creek North reef member with the transgressive–regressive architecture of the equivalent Eastern Shelf carbonates. The ISHU occurs within a continuously backstepping margin formed by the Carson Creek North upper atoll stage (BHL2 sequence) and the shoal stage (part of the BHL3 sequence) and does not appear to be associated with a significant highstand or regressive interval, while the Eastern Shelf carbonates exhibit pronounced regressive architecture in the upper part of the BHL2 sequence and the base of the BHL3 sequence. Potma et al. (2001, 2002) and Wendte and Uyeno (2005) agreed that the ISHU corresponds to a regional sequence boundary in the Eastern Shelf area, but they placed the boundary at different stratigraphic levels (Wendte and Embry 2002). In this study, the appearance of evaporitic facies in the Eastern Shelf area associated with the regressive interval is interpreted to indicate development of lowstand conditions in the Eastern Shelf area during the ISHU hiatus in the Swan Hills complex, coeval with development of paleotopography on the ISHU at Carson Creek North. The regional biostratigraphic framework established for the Beaverhill Lake Group in Wendte and Uyeno (2005) allows the lowstand interval and the ISHU hiatus to be constrained in time within the Frasnian Stage (Fig. 3), showing that the Eastern Shelf area contains the “missing regression” associated with the ISHU at Carson Creek North and other Swan Hills buildups.

FIG. 3.

—Stratigraphic chart showing relationships among the Swan Hills units in this study, members and correlation units of the Waterways Formation, and the architecture of the Eastern Shelf carbonates. The Frasnian Montaigne Noire (MN) standard conodont zonation referenced in the text is shown, as applied to the Beaverhill Lake Group by Wendte and Uyeno (2005). Absolute ages and sea-level data scaled to the conodont zones are from Becker et al. (2012). Beaverhill Lake Group sequence terminology is derived from Potma et al. (2001, 2002) and Wong et al. (2016). Note the eustatic sea-level falls approximately coincident with the BHL2.1 and 3.1 sequence boundaries.

FIG. 3.

—Stratigraphic chart showing relationships among the Swan Hills units in this study, members and correlation units of the Waterways Formation, and the architecture of the Eastern Shelf carbonates. The Frasnian Montaigne Noire (MN) standard conodont zonation referenced in the text is shown, as applied to the Beaverhill Lake Group by Wendte and Uyeno (2005). Absolute ages and sea-level data scaled to the conodont zones are from Becker et al. (2012). Beaverhill Lake Group sequence terminology is derived from Potma et al. (2001, 2002) and Wong et al. (2016). Note the eustatic sea-level falls approximately coincident with the BHL2.1 and 3.1 sequence boundaries.

DATA AND METHODS

Carson Creek North and other nearby Swan Hills buildups are in an advanced stage of oil development with excellent data sets of well logs and cores. At the time of the study, the Carson Creek North Field encompassed more than 90 cored wells, spaced at ~0.8 km on average in the most densely drilled part of the field. Carson Creek North was interpreted by combining core descriptions from these wells with GR logs and neutron, density, and/or sonic logs, depending on availability, to represent porosity. Core descriptions were calibrated to log data by shifting relatively continuous laboratory core porosity data (usually measured for ~30 cm core pieces prior to slabbing) to the porosity logs. The GR logs respond strongly to shale and clay beds thicker than 5–10 cm, and weakly to variations in dispersed argillaceous and/or organic material in some Carson Creek North lithofacies. GR variability associated with the latter is difficult to observe at normal log scales (0–150 API [American Petroleum Institute] units); therefore, for many figures and displays in this report, an expanded scale (typically 0–20 or 0–50 API units) was used to enhance the subtle radioactivity patterns necessary for correlation.

The Beaverhill Lake Group and Eastern Shelf carbonates are represented in this study by the wells and cores in Figure 1. The extent of the Eastern Shelf area shown on this map is based on the maximum basinward extent of carbonate members in the Beaverhill Lake Group. The adjacent Waterways Basin contains cycles of alternating shale and argillaceous limestone that comprise the Waterways Formation. For regional correlations and comparison between Carson Creek North and the Eastern Shelf, the Waterways Formation was divided into informal log-based correlation units (Waterways A, B, C, D, E, and upper Waterways). Nonstandard sequential nomenclature is commonly applied to studies involving the Waterways Formation, but the number of units typically varies from study to study (e.g., Sheasby 1971, Campbell 1992b, Potma et al. 2001, Wendte and Uyeno 2005, Schneider et al. 2013). Lateral facies changes between the Eastern Shelf carbonates and the Waterways Basin units are generally gradual relative to the average well spacing and are related to the Beaverhill Lake sequences as shown in Figures 2 and 3. The transgressive–regressive architecture of the Eastern Shelf area was examined in five cored wells (16-36-41-23w4, 1-7-36-18w4, 6-4-37-16w4, 10-30-36-18w4, 10-34-35-17w4) representing a shelf-to-basin profile. Finally, selected wells along the margin of the Swan Hills complex in which the ISHU was recognized, and selected wells from the Waterways Basin are used to discuss regional thickness patterns within the Beaverhill Lake sequences. Core descriptions for the additional Swan Hills wells are not included in this study; however, the pertinent stratigraphic data for the regional wells are provided in Table 1.

Table 1.

—Stratigraphic tops data and examined core intervals for regional wells penetrating the Elk Point/Watt Mountain Formations. Well locations are plotted in Figure 1. Interval tops are listed as measured depths, in feet (ft) or meters (m) according to the well’s original measurement units in column 3. Thickness values for the Beaverhill Lake (BHL) Group, and the BHL1, BHL2, and BHL3 regional sequences were calculated from the data in the columns on the right.

Well location Area Units Beaverhill Lake Gp. Swan Hills Fm. ISHU (BHL3.1) Lower atoll stage (BHL2.1) Platform member Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lower atoll stage 
Swan Hills Complex 
6-1-62-12w5 CCN A-Pool ft 8667 8754 8809 8905 8993 9125 8804–9000 140 43 29 67 27 
6-3-62-12w5 CCN A-Pool ft 8506 8547 8607 8720 8797 8935 8550–8823 131 31 34 66 23 
16-11-62-12w5 CCN A-Pool ft 8660 8734 8756 8883 8953 9070 8710–8973 125 29 39 57 21 
6-17-62-12w5 CCN B-Pool ft 8572 8610 8675 8816 8890 9030 8634–8834 140 31 43 65 23 
13-14-52-17w5 Edson 3648 3650 3701 3729 3742 3773 3742–3769 125 53 28 44 13 
9-3-52-17w5 Edson 3728 3738 3785 3816 3830 3859 3760–3835 131 57 31 43 14 
11-33-51-16w5 Edson ft 11,965 12,060 12,092 12,172 12,220 12,340 12,025–12,198 114 39 24 51 15 
6-30-46-17w5 Hanlan N ft 15,265 15,360 15,430 15,515 15,555 15,635 15,438–15,498 113 50 26 37 12 
11-8-47-17w5 Hanlan N ft 15,040 15,100 15,145 15,234 15,270 15,340 15,115–15,252 91 32 27 32 11 
11-19-47-17w5 Hanlan N ft 14,940 15,024 15,071 15,145 15,167 15,235 15,054–15,246 90 40 23 27 
11-22-44-16w5 Hanlan S 4673 4714 4714 4741 4752 4777 4726–4778 104 41 27 36 11 
6-20-44-16w5 Hanlan S 4764 4776 4801 4832 NP 4846 4794–4835 82 37 31 14  
2-15-36-9w5 Strachan 4573 4596 4603 4640 4655 4687 4590–4608 114 30 37 47 15 
10-33-36-10w5 Strachan ft 15,607 15,670 15,725 15,825 15,880 15,950 NC/NE 105 36 30 38 17 
6-12-38-10w5 Strachan 4351 4388 4409 4431 4445 4482 4399–4418 131 58 22 51 14 
6-20-33-4w5 Caroline 3580 3637 3648 3669 3680 3712 3625–3676 132 68 21 43 11 
6-13-33-5w5 Caroline 3698 3736 3757 3782 3791 3825 3730–3797 127 59 25 43 
7-18-34-4w5 Caroline 3586 3645 3657 3680 3691 3723 3655–3708 137 71 23 43 11 
6-29-34-5w5 Caroline 3737 3770 3798 3823 3834 3864 3775–3873 127 61 25 41 11 
10-33-34-5w5 Caroline 3684 3731 3747 3770 3782 3815 3725–3824 131 63 23 45 12 
3-10-35-5w5 Caroline 3652 3705 3716 3743 3758 3792 3710–3766 140 64 27 49 15 
10-15-35-5w5 Caroline 3653 3721 3721 3745 3762 3794 3723–3758 141 68 24 49 17 
5-32-35-5w5 Caroline 3650 3717 3718 3746 3762 3791 3723–3762 141 68 28 45 16 
10-24-31-4w5 Twining 3605 3655 3660 3691 3707 3738 3670–3699 133 55 31 47 16 
15-16-31-5w5 Twining 3959 3998 4015 4046 4058 4088 3995–4023 129 56 31 42 12 
7-32-32-28w4 Stewart 2864 2915 2944 2968 2978 3022 2940–2976 158 80 24 54 10 
Well location Area Units Beaverhill
Lake Gp. 
WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl2.1) Elk Point/Watt Mtn. Core interval BHL Gp. BHL3 BHL2 BHL1 Lowstand 
Waterways Basin 
8-13-27-3w5 WWYS Basin 3635 3666 3675 3696 3743 3761 3696–3753 126 61 47 18 21 
5-15-28-3w5 WWYS Basin 3659 3690 3698 3731 3764 3786 3710–3746 127 72 33 22 33 
6-6-29-2w5 WWYS Basin 3462 3507 3518 3560 3593 3611 NC/NE 149 98 33 18 42 
6-14-33-2w5 WWYS Basin 3115 3153 3162 3207 3228 3266 3138–3157 151 92 21 38 45 
9-17-34-25w4 WWYS Basin ft 7980 8149 8190 8310 8403 8575 8071–8088 181 101 28 52 37 
16-16-37-25w4 WWYS Basin ft 8016 8181 8215 8348 8450 8630 NC/NE 187 101 31 55 41 
6-16-38-3w5 WWYS Basin 3048 3086 3109 3142 3161 3215 NC/NE 167 94 19 54 33 
13-36-39-25w4 WWYS Basin ft 7390 7555 7590 7695 7838 8046 NC/NE 200 93 44 63 32 
Well location Area Units Beaverhill Lake Gp. WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl3.1) Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lowstand 
6-11-40-21w4 WWYS Basin 1940 1991 2004 2031 2080 2146 NC/NE 206 91 49 66 27 
12-14-40-26w4 WWYS Basin 2378 2424 2435 2474 2508 2571 NC/NE 193 96 34 63 39 
9-23-41-9w5 WWYS Basin ft 12,435 12,520 12,550 12,695 12,800 12,944 NC/NE 155 79 32 44 44 
6-20-41-24w4 WWYS Basin ft 7260 7420 7460 7556 7707 7922 NC/NE 202 90 46 66 29 
6-4-41-1w5 WWYS Basin ft 8682 8790 8830 8975 9060 9258 NC/NE 176 89 26 60 44 
7-1-41-3w5 WWYS Basin 2888 2923 2944 2979 3006 3063 NC/NE 175 91 27 57 35 
16-16-42-2w5 WWYS Basin 2695 2730 2753 2789 2819 2877 NC/NE 182 94 30 58 36 
15-35-44-2w5 WWYS Basin 2559 2591 2606 2648 2682 2746 NC/NE 187 89 34 64 42 
10-25-46-2w5 WWYS Basin ft 8162 8270 8340 8450 8575 8789 NC/NE 191 88 38 65 34 
6-15-48-3w5 WWYS Basin ft 8078 8178 8232 8358 8480 8697 NC/NE 189 85 37 66 38 
9-18-49-9w5 WWYS Basin 3065 3103 3125 3153 3174 3227 NC/NE 162 88 21 53 28 
8-30-52-5w5 WWYS Basin 2408 2445 2454 2483 2523 2585 NC/NE 177 75 40 62 29 
6-3-51-7w5 WWYS Basin ft 8645 8765 8824 8923 9005 9208 NC/NE 172 85 25 62 30 
3-18-52-8w5 WWYS Basin ft 9000 9130 9205 9292 9360 9545 NC/NE 166 89 21 56 27 
16-5-55-9w5 WWYS Basin ft 8628 8733 8798 8890 8950 9140 NC/NE 156 80 18 58 28 
10-26-59-9w5 WWYS Basin ft 7550 7651 7718 7795 7897 8042 NC/NE 150 75 31 44 23 
6-29-61-9w5 WWYS Basin ft 7464 7570 7642 7722 7844 7947 NC/NE 147 79 37 31 24 
Eastern Shelf area 
6-5-30-21w4 Stewart 2069 2108 2121 2136 2195 2250 2201–2257 181 67 59 55 15 
8-33-31-22w4 Stewart 2149 2184 2198 2233 2279 2326 2245–2277 177 84 46 47 35 
10-30-32-23w4 Stewart 2295 2338 2354 2370 2430 2482 2372–2426 187 75 60 52 16 
16-27-32-24w4 Stewart 2346 2386 2410 2435 2480 2531 2414–2443 185 89 45 51 25 
6-11-33-24w4 Stewart 2343 2380 2402 2420 2470 2516 2422–2425 173 77 50 46 18 
16-36-41-23w4 Eastern Shelf 2040 2092 2103 2134 2191 2250 2132–2185 210 94 57 59 31 
1-7-36-18w4 Eastern Shelf 1881 1923 1940 1971 2020 2080 1944–1962 199 90 49 60 31 
6-4-37-16w4 Eastern Shelf 1752 1792 1809 1844 1890 1951 1850–1868 199 92 46 61 35 
10-30-36-16w4 Eastern Shelf ft 5806 5946 5985 6095 6304 6465 6060–6180 201 88 64 49 34 
10-34-35-17w4 Eastern Shelf ft 5890 6030 6070 6172 6348 NP 6065–6365  86 54  31 
14-35-34-17w4 Eastern Shelf 1818 1860 1869 1903 1950 2009 NC/NE 191 85 47 59 34 
14-11-39-17w4 Eastern Shelf 1705 1740 1757 1793 1846 1905 NC/NE 200 88 53 59 36 
6-11-40-21W4 Eastern Shelf 1940 1990 2005 2030 2080 2146 NC/NE 206 90 50 66 25 
6-10-41-19W4 Eastern Shelf ft 5820 5985 6012 6130 6280 6500 NC/NE 207 94 46 67 36 
7-9-41-15W4 Eastern Shelf ft 4915 5040 5090 5212 5380 5590 NC/NE 206 91 51 64 37 
2-2-42-14w4 Eastern Shelf 1390 1428 1449 1484 1545 1585 NC/NE 195 94 61 40 35 
8-23-43-16w4 Eastern Shelf 1487 1520 1544 1580 1638 1698 NC/NE 211 93 58 60 36 
6-35-46-15w4 Eastern Shelf 1322 1374 1390 1420 1477 1540 NC/NE 218 98 57 63 30 
Well location Area Units Beaverhill Lake Gp. Swan Hills Fm. ISHU (BHL3.1) Lower atoll stage (BHL2.1) Platform member Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lower atoll stage 
Swan Hills Complex 
6-1-62-12w5 CCN A-Pool ft 8667 8754 8809 8905 8993 9125 8804–9000 140 43 29 67 27 
6-3-62-12w5 CCN A-Pool ft 8506 8547 8607 8720 8797 8935 8550–8823 131 31 34 66 23 
16-11-62-12w5 CCN A-Pool ft 8660 8734 8756 8883 8953 9070 8710–8973 125 29 39 57 21 
6-17-62-12w5 CCN B-Pool ft 8572 8610 8675 8816 8890 9030 8634–8834 140 31 43 65 23 
13-14-52-17w5 Edson 3648 3650 3701 3729 3742 3773 3742–3769 125 53 28 44 13 
9-3-52-17w5 Edson 3728 3738 3785 3816 3830 3859 3760–3835 131 57 31 43 14 
11-33-51-16w5 Edson ft 11,965 12,060 12,092 12,172 12,220 12,340 12,025–12,198 114 39 24 51 15 
6-30-46-17w5 Hanlan N ft 15,265 15,360 15,430 15,515 15,555 15,635 15,438–15,498 113 50 26 37 12 
11-8-47-17w5 Hanlan N ft 15,040 15,100 15,145 15,234 15,270 15,340 15,115–15,252 91 32 27 32 11 
11-19-47-17w5 Hanlan N ft 14,940 15,024 15,071 15,145 15,167 15,235 15,054–15,246 90 40 23 27 
11-22-44-16w5 Hanlan S 4673 4714 4714 4741 4752 4777 4726–4778 104 41 27 36 11 
6-20-44-16w5 Hanlan S 4764 4776 4801 4832 NP 4846 4794–4835 82 37 31 14  
2-15-36-9w5 Strachan 4573 4596 4603 4640 4655 4687 4590–4608 114 30 37 47 15 
10-33-36-10w5 Strachan ft 15,607 15,670 15,725 15,825 15,880 15,950 NC/NE 105 36 30 38 17 
6-12-38-10w5 Strachan 4351 4388 4409 4431 4445 4482 4399–4418 131 58 22 51 14 
6-20-33-4w5 Caroline 3580 3637 3648 3669 3680 3712 3625–3676 132 68 21 43 11 
6-13-33-5w5 Caroline 3698 3736 3757 3782 3791 3825 3730–3797 127 59 25 43 
7-18-34-4w5 Caroline 3586 3645 3657 3680 3691 3723 3655–3708 137 71 23 43 11 
6-29-34-5w5 Caroline 3737 3770 3798 3823 3834 3864 3775–3873 127 61 25 41 11 
10-33-34-5w5 Caroline 3684 3731 3747 3770 3782 3815 3725–3824 131 63 23 45 12 
3-10-35-5w5 Caroline 3652 3705 3716 3743 3758 3792 3710–3766 140 64 27 49 15 
10-15-35-5w5 Caroline 3653 3721 3721 3745 3762 3794 3723–3758 141 68 24 49 17 
5-32-35-5w5 Caroline 3650 3717 3718 3746 3762 3791 3723–3762 141 68 28 45 16 
10-24-31-4w5 Twining 3605 3655 3660 3691 3707 3738 3670–3699 133 55 31 47 16 
15-16-31-5w5 Twining 3959 3998 4015 4046 4058 4088 3995–4023 129 56 31 42 12 
7-32-32-28w4 Stewart 2864 2915 2944 2968 2978 3022 2940–2976 158 80 24 54 10 
Well location Area Units Beaverhill
Lake Gp. 
WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl2.1) Elk Point/Watt Mtn. Core interval BHL Gp. BHL3 BHL2 BHL1 Lowstand 
Waterways Basin 
8-13-27-3w5 WWYS Basin 3635 3666 3675 3696 3743 3761 3696–3753 126 61 47 18 21 
5-15-28-3w5 WWYS Basin 3659 3690 3698 3731 3764 3786 3710–3746 127 72 33 22 33 
6-6-29-2w5 WWYS Basin 3462 3507 3518 3560 3593 3611 NC/NE 149 98 33 18 42 
6-14-33-2w5 WWYS Basin 3115 3153 3162 3207 3228 3266 3138–3157 151 92 21 38 45 
9-17-34-25w4 WWYS Basin ft 7980 8149 8190 8310 8403 8575 8071–8088 181 101 28 52 37 
16-16-37-25w4 WWYS Basin ft 8016 8181 8215 8348 8450 8630 NC/NE 187 101 31 55 41 
6-16-38-3w5 WWYS Basin 3048 3086 3109 3142 3161 3215 NC/NE 167 94 19 54 33 
13-36-39-25w4 WWYS Basin ft 7390 7555 7590 7695 7838 8046 NC/NE 200 93 44 63 32 
Well location Area Units Beaverhill Lake Gp. WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl3.1) Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lowstand 
6-11-40-21w4 WWYS Basin 1940 1991 2004 2031 2080 2146 NC/NE 206 91 49 66 27 
12-14-40-26w4 WWYS Basin 2378 2424 2435 2474 2508 2571 NC/NE 193 96 34 63 39 
9-23-41-9w5 WWYS Basin ft 12,435 12,520 12,550 12,695 12,800 12,944 NC/NE 155 79 32 44 44 
6-20-41-24w4 WWYS Basin ft 7260 7420 7460 7556 7707 7922 NC/NE 202 90 46 66 29 
6-4-41-1w5 WWYS Basin ft 8682 8790 8830 8975 9060 9258 NC/NE 176 89 26 60 44 
7-1-41-3w5 WWYS Basin 2888 2923 2944 2979 3006 3063 NC/NE 175 91 27 57 35 
16-16-42-2w5 WWYS Basin 2695 2730 2753 2789 2819 2877 NC/NE 182 94 30 58 36 
15-35-44-2w5 WWYS Basin 2559 2591 2606 2648 2682 2746 NC/NE 187 89 34 64 42 
10-25-46-2w5 WWYS Basin ft 8162 8270 8340 8450 8575 8789 NC/NE 191 88 38 65 34 
6-15-48-3w5 WWYS Basin ft 8078 8178 8232 8358 8480 8697 NC/NE 189 85 37 66 38 
9-18-49-9w5 WWYS Basin 3065 3103 3125 3153 3174 3227 NC/NE 162 88 21 53 28 
8-30-52-5w5 WWYS Basin 2408 2445 2454 2483 2523 2585 NC/NE 177 75 40 62 29 
6-3-51-7w5 WWYS Basin ft 8645 8765 8824 8923 9005 9208 NC/NE 172 85 25 62 30 
3-18-52-8w5 WWYS Basin ft 9000 9130 9205 9292 9360 9545 NC/NE 166 89 21 56 27 
16-5-55-9w5 WWYS Basin ft 8628 8733 8798 8890 8950 9140 NC/NE 156 80 18 58 28 
10-26-59-9w5 WWYS Basin ft 7550 7651 7718 7795 7897 8042 NC/NE 150 75 31 44 23 
6-29-61-9w5 WWYS Basin ft 7464 7570 7642 7722 7844 7947 NC/NE 147 79 37 31 24 
Eastern Shelf area 
6-5-30-21w4 Stewart 2069 2108 2121 2136 2195 2250 2201–2257 181 67 59 55 15 
8-33-31-22w4 Stewart 2149 2184 2198 2233 2279 2326 2245–2277 177 84 46 47 35 
10-30-32-23w4 Stewart 2295 2338 2354 2370 2430 2482 2372–2426 187 75 60 52 16 
16-27-32-24w4 Stewart 2346 2386 2410 2435 2480 2531 2414–2443 185 89 45 51 25 
6-11-33-24w4 Stewart 2343 2380 2402 2420 2470 2516 2422–2425 173 77 50 46 18 
16-36-41-23w4 Eastern Shelf 2040 2092 2103 2134 2191 2250 2132–2185 210 94 57 59 31 
1-7-36-18w4 Eastern Shelf 1881 1923 1940 1971 2020 2080 1944–1962 199 90 49 60 31 
6-4-37-16w4 Eastern Shelf 1752 1792 1809 1844 1890 1951 1850–1868 199 92 46 61 35 
10-30-36-16w4 Eastern Shelf ft 5806 5946 5985 6095 6304 6465 6060–6180 201 88 64 49 34 
10-34-35-17w4 Eastern Shelf ft 5890 6030 6070 6172 6348 NP 6065–6365  86 54  31 
14-35-34-17w4 Eastern Shelf 1818 1860 1869 1903 1950 2009 NC/NE 191 85 47 59 34 
14-11-39-17w4 Eastern Shelf 1705 1740 1757 1793 1846 1905 NC/NE 200 88 53 59 36 
6-11-40-21W4 Eastern Shelf 1940 1990 2005 2030 2080 2146 NC/NE 206 90 50 66 25 
6-10-41-19W4 Eastern Shelf ft 5820 5985 6012 6130 6280 6500 NC/NE 207 94 46 67 36 
7-9-41-15W4 Eastern Shelf ft 4915 5040 5090 5212 5380 5590 NC/NE 206 91 51 64 37 
2-2-42-14w4 Eastern Shelf 1390 1428 1449 1484 1545 1585 NC/NE 195 94 61 40 35 
8-23-43-16w4 Eastern Shelf 1487 1520 1544 1580 1638 1698 NC/NE 211 93 58 60 36 
6-35-46-15w4 Eastern Shelf 1322 1374 1390 1420 1477 1540 NC/NE 218 98 57 63 30 

NC = no core available; NE = not examined; NP = not picked; WWYS = Waterways Formation.

Beaverhill Lake Environments and Lithofacies

Core descriptions for this study incorporate the lithofacies in Figure 4, which are based on Swan Hills and Beaverhill Lake shelf-to-basin profiles previously established in the literature (e.g., Klovan 1964, Leavitt 1968, Harvard and Oldershaw 1976, Heckel and Witzke 1979, Wendte 1992a, Wendte and Muir 1995, Wendte and Uyeno 2005). Environmental interpretation is largely based on the skeletal morphology of in situ stromatoporoids, which were major reef builders during the Middle to Late Devonian (Wilson 1974, James 1983, Witzke and Heckel 1988, Tucker 1990, James and Wood 2010). The following paragraphs briefly describe the depositional environments and associated lithofacies observed in the Carson Creek North reef and in the Eastern Shelf area carbonates presented in this study.

FIG. 4.

—Symbols and color legend for Swan Hills and Beaverhill Lake lithofacies and environments used throughout this study.

FIG. 4.

—Symbols and color legend for Swan Hills and Beaverhill Lake lithofacies and environments used throughout this study.

Lagoon–Tidal Flat: Lagoon and tidal flat environments are abundant in the interior of the upper atoll stage at Carson Creek North. They are also found at the top of the lower atoll stage, and at the base of the shoal stage. The Lagoon–tidal flat environment is represented by shallowing-upward cycles, with subtidal limestones at the base, and intertidal to supratidal limestones toward the top of the cycle. Subtidal lagoons are represented by Amphipora floatstone to packstone, including nodular dark brown (lithofacies LTF1, Fig. 5A) and massive light brown (lithofacies LTF2, Fig. 5B) varieties. Skeletal-peloidal grainstone with abundant Amphipora (lithofacies LTF3a) is also present in lagoonal successions. Intertidal to supratidal limestones are represented by Amphipora floatstones with fine laminations and fenestral fabric (lithofacies LTF4a). They are also associated with cream-colored fenestral mudstone containing scattered Amphipora and occasional larger stromatoporoids (lithofacies LTF4b), or with localized beaches or shoals composed of Amphipora rudstone (lithofacies LTF3b). The Carson Creek North Lagoon–tidal flat cycles are similar to meter-scale cycles described from the interior of the Swan Hills Kaybob reef (Wong and Oldershaw 1980) and from the interior of the Judy Creek reef (Wendte 1992b).

Fig. 5.

—Lithofacies and environments in the Swan Hills complex and Eastern Shelf area. A) Dark-colored Amphipora floatstone (lithofacies LTF1) representing subtidal lagoons; 9-10-62-12w5, 2668.2 m. B) Light-colored lagoonal grainstone with Amphipora, lithofacies LTF2, 6-9-62-12w5, 8582 ft. (2615.8 m) C) Bedded reef flat grainstone with stromatoporoid fragments (lithofacies RM1), 6-31-61-11w5, 8656 ft. (2638.3 m) D) Massive stromatoporoid boundstone, reef margin (lithofacies RM2), 16-6-62-11w5, 8777 ft. (2675.2 m) E) Lithofacies RM2 boundstone with branching and tabular stromatoporoids, 6-31-61-11w5, 8710 ft. (2654.8 m) F) Upper foreslope boundstone (lithofacies FS1) with tabular stromatoporoids (t) and brachiopods (b), 16-6-62-11w5, 8811 ft. (2685.6 m) G) Floatstone with bulbous and cylindrical stromatoporoids (lithofacies FS2) and micrite matrix, lower to middle foreslope, 6-36-61-12w5, 8653 ft. (2637.4 m) H) Nodular skeletal wackestone–packstone (lithofacies SB3) with gastropods (g), Thamnopora (th), and brachiopod shell fragments (b), 16-36-41-23w4, 2142 m. I) Basinal nodular mudstone–wackestone (lithofacies SB2) with brachiopods (b), 9-15-62-12w5, 8719 ft. (2657.6 m) J) Vuggy ramp margin rudstone with stromatoporoid fragments (s) and grainstone matrix (lithofacies RA2), 16-11-62-12w5, 8747 ft. (2666.1 m) Some stromatoporoids (arrows) have light-colored micrite or oncolite coatings. K) Lithofacies (RA3) in low-energy ramp environments containing rounded stromatoporoid fragments (s), Amphipora (a), and cylindrical stromatoporoids (c), 8-2-62-12w5, 2637.9 m. L) Stromatolitic mudstone (lithofacies SI3) with authigenic anhydrite (an) from a restricted environment in the Eastern Shelf area, 10-30-36-16w4, 6095 ft. (1857.8 m) M) Green shale breccia (GSB) from the ISHU, 9-10-62-12w5, 2638.0 m. N) Submarine scour (lithofacies SS1) with rounded gray intraclasts matching the lithology below the scour surface (dashed line), 10-34-35-17w4, 6080 ft. (1853.2 m) All scale bars = 2 cm.

Fig. 5.

—Lithofacies and environments in the Swan Hills complex and Eastern Shelf area. A) Dark-colored Amphipora floatstone (lithofacies LTF1) representing subtidal lagoons; 9-10-62-12w5, 2668.2 m. B) Light-colored lagoonal grainstone with Amphipora, lithofacies LTF2, 6-9-62-12w5, 8582 ft. (2615.8 m) C) Bedded reef flat grainstone with stromatoporoid fragments (lithofacies RM1), 6-31-61-11w5, 8656 ft. (2638.3 m) D) Massive stromatoporoid boundstone, reef margin (lithofacies RM2), 16-6-62-11w5, 8777 ft. (2675.2 m) E) Lithofacies RM2 boundstone with branching and tabular stromatoporoids, 6-31-61-11w5, 8710 ft. (2654.8 m) F) Upper foreslope boundstone (lithofacies FS1) with tabular stromatoporoids (t) and brachiopods (b), 16-6-62-11w5, 8811 ft. (2685.6 m) G) Floatstone with bulbous and cylindrical stromatoporoids (lithofacies FS2) and micrite matrix, lower to middle foreslope, 6-36-61-12w5, 8653 ft. (2637.4 m) H) Nodular skeletal wackestone–packstone (lithofacies SB3) with gastropods (g), Thamnopora (th), and brachiopod shell fragments (b), 16-36-41-23w4, 2142 m. I) Basinal nodular mudstone–wackestone (lithofacies SB2) with brachiopods (b), 9-15-62-12w5, 8719 ft. (2657.6 m) J) Vuggy ramp margin rudstone with stromatoporoid fragments (s) and grainstone matrix (lithofacies RA2), 16-11-62-12w5, 8747 ft. (2666.1 m) Some stromatoporoids (arrows) have light-colored micrite or oncolite coatings. K) Lithofacies (RA3) in low-energy ramp environments containing rounded stromatoporoid fragments (s), Amphipora (a), and cylindrical stromatoporoids (c), 8-2-62-12w5, 2637.9 m. L) Stromatolitic mudstone (lithofacies SI3) with authigenic anhydrite (an) from a restricted environment in the Eastern Shelf area, 10-30-36-16w4, 6095 ft. (1857.8 m) M) Green shale breccia (GSB) from the ISHU, 9-10-62-12w5, 2638.0 m. N) Submarine scour (lithofacies SS1) with rounded gray intraclasts matching the lithology below the scour surface (dashed line), 10-34-35-17w4, 6080 ft. (1853.2 m) All scale bars = 2 cm.

Reef and Reef Flat (Reef Margin): Carson Creek North reef margins are characterized by massive to bedded reef flat grainstones and stromatoporoid rudstones (lithofacies RM1), and in situ reefs consisting of stromatoporoid rudstone or boundstone (lithofacies RM2). Reef flat grainstones commonly contain scattered stromatoporoid fragments or thin (several centimeters) stromatoporoid rudstone beds (Fig. 5C). Reef rudstones and boundstones contain massive, domal, and thick tabular stromatoporoids (Fig. 5D), or nearly in situ bulbous and branching forms (Fig. 5E). The matrix in reef bound-stone/rudstone is micritic to grainy. Well-developed reef margins occur in the lower and upper atoll stages. Lithofacies RM1 may also be associated with Carson Creek North ramp margins, and with shelf margins in the Eastern Shelf area.

Slope and Basin: Foreslope environments are predominantly associated with the lower and upper atoll stages, with slope paleobathymetry related to the morphology of in situ stromatoporoids. Thick tabular and branching stromatoporoids (lithofacies FS1) are prevalent in the upper foreslope (Fig. 5F), while thin platy stromatoporoids, and cylindrical and smaller branching forms (lithofacies FS2) are more abundant in the middle to lower foreslope (Fig. 5G). Brachiopods, Thamnopora, crinoids, and tabulate corals are also commonly observed in foreslope settings. Lower slope to basinal limestones consist of nodular skeletal wackestone to packstone with abundant crinoids (lithofacies SB3, Fig. 5H), nodular mudstone to skeletal wackestone (SB2, Fig. 5I), and laminated argillaceous mudstone or shale (SB1).

Ramps and Mounds: Ramps and mounds occur in the lower atoll stage and the shoal stage at Carson Creek North. They are characterized by relatively high-energy margins (Fig. 5J) containing massive grainstone (lithofacies RA1), or grainstone with a mix of large and small stromatoporoid fragments (lithofacies RA2). Ramps also contain lower-energy facies (Fig. 5K) represented by grainstone–packstone with cylindrical, bulbous, or branching stromatoporoids (lithofacies RA3), or grainstone–packstone with mainly small cylindrical stromatoporoids (lithofacies RA4).

Low-Energy Ramps and Shelves: Low-energy shelves occur in the Eastern Shelf area. Shelf margins contain fine-laminated to locally cross-laminated peloidal or fine coated-grain grainstone and peloidal mudstone (lithofacies SM1). Lithofacies RA3 and RA4, representing low-energy ramps, may also occur in Eastern Shelf margins. Eastern shelf interiors are generally more restricted than the Swan Hills lagoons and contain a wide range of lithofacies, including Amphipora floatstone with authigenic anhydrite (lithofacies SI2), laminated or stromatolitic mudstone (Fig. 5L) with occasional angular intraclasts (lithofacies SI3), bedded to nodular anhydrite with stromatolites and occasional Amphipora beds (lithofacies SI4), and bedded anhydrite with stromatolitic or laminated mudstone (lithofacies SI5).

Green Shale Breccia, Green Shale, and Submarine Scour: Green shale and green shale breccia are associated with the tops of some lagoon–tidal flat cycles at Carson Creek North, with facies discontinuities or subaerially exposed surfaces in shallow shelf or reef settings, and with the ISHU. Green shale breccias (lithofacies GSB) consist of poorly sorted, subrounded to angular limestone clasts in a matrix of green to brown shale or argillaceous mudstone (Fig. 5M). The clasts are usually identical to the lithofacies immediately below the GSB, and they do not appear to be significantly transported or reworked. Submarine scours (lithofacies SS1) are observed in settings ranging from shallow shelf or ramp to basinal. They contain argillaceous rudstone or packstone with a mix of skeletal grains and intraclasts (Fig. 5N). Intraclasts are usually micritic and rounded to subrounded, and they resemble the lithology underlying the scour. Skeletal grains are varied and include crinoids, brachiopods, gastropods, coated grains, and undifferentiated shell fragments. Laminated micritic or oncolite coatings are sometimes present around larger skeletal grains.

Allochthonous Limestones: Allochthonous limestones occur in narrow channels adjacent to the Carson Creek North buildup, and locally along the east–facing Swan Hills buildup margins, interbedded with in situ lithofacies FS1 and FS2 in foreslope settings, or with lithofacies SB1 to SB3 in basinal settings. Allochthonous limestones are divided into three main lithofacies based on allochem composition: Lithofacies AL1 (skeletal) is represented by grainstone, rudstone, or floatstone containing mainly skeletal detritus, including crinoids, small stromatoporoids, and undifferentiated shell fragments. Lithofacies AL2 (skeletal-breccia) contains skeletal debris mixed with breccia. The breccia clasts are mainly angular and micritic, up to several centimeters in size. Lithofacies AL3 (breccia) consists of mainly micritic breccia clasts with minor amounts of coarse skeletal detritus or occasional large stromatoporoid fragments.

RESULTS

Carson Creek North Sequences and Architecture

The Swan Hills Formation averages ~115 m thick at Carson Creek North, including a platform member that is ~40 m thick and a reef member up to ~75 m thick. The Carson Creek North Field contains two separate isolated buildups, a larger one (~55.1 km2) to the south (A-pool) and a smaller, narrow buildup (~20.6 km2) to the north (B-pool), separated by a narrow channel (Fig. 6). Most field development wells extend only a few meters into the top of the platform member, so this study is focused on the stratigraphy of the Carson Creek North reef member. The reef member contains three growth stages with distinct transgressive–regressive architecture (Fig. 7A): the lower atoll stage, representing the upper part of the BHL1 sequence, the upper atoll stage (BHL2 sequence), and the shoal stage at the base of the BHL3 sequence. The lower atoll stage is ~25 m thick and is associated with evolution from a backstepping ramp to a prograding reef-rimmed platform enclosing a restricted central lagoon. The upper atoll stage is ~30–35 m thick and contains backstepping reef-rimmed lagoonal megacycles capped by the ISHU. The overlying shoal stage ranges up to 20 m thick and consists of backstepping high-energy ramp cycles.

Fig. 6.

—Upper atoll and shoal stages in the Carson Creek North Swan Hills A- and B-pools, based on available well and core control at the time of the study, including the lines of cross sections shown in Figures 9 and 10.

Fig. 6.

—Upper atoll and shoal stages in the Carson Creek North Swan Hills A- and B-pools, based on available well and core control at the time of the study, including the lines of cross sections shown in Figures 9 and 10.

FIG. 7

—A) Schematic diagram showing stratigraphic units and lithofacies distribution (see Fig. 4 for legend) in the Carson Creek North buildup, including surfaces for which subaerial exposure has been observed or interpreted from cores. The ISHU is exposed across the buildup, whereas local exposure is indicated schematically for multiple levels in the lower atoll stage. B) Stratigraphic model of the Judy Creek buildup showing the R4 and R0.5 markers discussed in the text, modified from an illustration in Wendte and Uyeno (2005) to match the vertical scale of Carson Creek in A). Double vertical lines indicate well control, and black bars indicate core control for the Judy Creek model. Note similarities between the Judy Creek R0, R0.5, and R4 markers and the Carson Creek North M1c, A1c, and ISHU surfaces, respectively, in relationship to the overall stratigraphic architecture of the two buildups.

FIG. 7

—A) Schematic diagram showing stratigraphic units and lithofacies distribution (see Fig. 4 for legend) in the Carson Creek North buildup, including surfaces for which subaerial exposure has been observed or interpreted from cores. The ISHU is exposed across the buildup, whereas local exposure is indicated schematically for multiple levels in the lower atoll stage. B) Stratigraphic model of the Judy Creek buildup showing the R4 and R0.5 markers discussed in the text, modified from an illustration in Wendte and Uyeno (2005) to match the vertical scale of Carson Creek in A). Double vertical lines indicate well control, and black bars indicate core control for the Judy Creek model. Note similarities between the Judy Creek R0, R0.5, and R4 markers and the Carson Creek North M1c, A1c, and ISHU surfaces, respectively, in relationship to the overall stratigraphic architecture of the two buildups.

The Carson Creek North stratigraphic framework is similar to the one used for the Judy Creek buildup, located ~20 km to the north of Carson Creek. Judy Creek has been well documented in the literature and is useful for comparison with Carson Creek North because of its proximity. The Judy Creek stratigraphic framework has been published several times (Murray 1966, Wendte and Stoakes 1982, Wendte 1992b, Wendte and Muir 1995). A recent version (Fig. 7B) shows two platform-top exposure surfaces within the reef member. The upper exposure surface (R4 marker) also represents the BHL3.1 sequence boundary, and it separates a “rimmed reef’ stage from an overlying “ramp-bounded shoal” stage, similar to the ISHU at the top of the Carson Creek upper atoll stage. The lower exposure surface (R0.5 marker) occurs near the base of the reef member, and it is inferred to represent the BHL2.1 sequence boundary (Wong et al. 2016). The Judy Creek R0.5 marker occurs near the top of a prograding margin complex underlying a thick lagoonal succession, analogous to the Carson Creek North lower atoll stage. Only the regressive portion of the equivalent Judy Creek stage is indicated in Figure 7B—the equivalent backstepping ramps are possibly indicated by depositional topography on the R0 marker. The top of the lower atoll stage (A1c unit) is therefore adopted as the BHL2.1 sequence boundary at Carson Creek North. The detailed stratigraphic zonation for Carson Creek North used in this study is illustrated in Figure 8. The GR log pattern corresponds to a relatively consistent lithologic succession in the buildup interior of the Carson Creek North reef member. The equivalent positions of the Judy Creek R0.5 and R4 markers within this zonation are also indicated. No attempt was made, however, to replicate the rest of the Judy Creek stratigraphic nomenclature at Carson Creek North.

Fig. 8.

—Stratigraphic nomenclature and Swan Hills reef stages in a Carson Creek North “type well,” illustrating typical facies, GR log, and porosity log (sonic, neutron, or density) patterns in the buildup interior. The Core Phi track contains porosity data from core analysis (scale: 1 division = 2% porosity) used to assist with calibration of core facies to logs and stratigraphic correlation. The stratigraphic framework also shows the interpreted Carson Creek North equivalents of the Judy Creek R0.5 and R4 markers as discussed in the text. The facies legend for symbols is given in Figure 4.

Fig. 8.

—Stratigraphic nomenclature and Swan Hills reef stages in a Carson Creek North “type well,” illustrating typical facies, GR log, and porosity log (sonic, neutron, or density) patterns in the buildup interior. The Core Phi track contains porosity data from core analysis (scale: 1 division = 2% porosity) used to assist with calibration of core facies to logs and stratigraphic correlation. The stratigraphic framework also shows the interpreted Carson Creek North equivalents of the Judy Creek R0.5 and R4 markers as discussed in the text. The facies legend for symbols is given in Figure 4.

Lower Atoll Stage and BHL1 Sequence: The lower atoll stage is associated with a significant backstep above the Swan Hills platform member. Regionally, the Swan Hills margin retreated 8 to 30 km above the platform member (Oldale and Mundy 1994, Wendte and Uyeno 2005). The top of the platform member does not show evidence of subaerial exposure at Carson Creek North, and pronounced transgressive–regressive architecture is observed within the lower atoll stage itself, indicated by changes in porosity and facies from the buildup interior to the eastern margin. In the buildup interior, the lower atoll stage contains two zones in which a ramp succession is capped by thin lagoonal or tidal flat beds. The lower zone is broken out into three units (M1a, M1b, M1c). The M1a and M1b units represent porous cycles within the ramp succession, and a tight lagoon–tidal flat capping interval forms the M1c unit (Fig. 8). The upper zone is similarly divided into three units (A1a, A1b, A1c). The A1c unit is a low-porosity lagoon–tidal flat interval at the top of the zone, and the A1a and A1b units are similarly based on correlative porosity patterns observed within the underlying ramp succession.

Basinward, the cyclic porosity pattern associated with the M1a–M1c and A1a–A1c intervals in the interior (Fig. 9A) is replaced by a gradational upward trend from low to high porosity. The M1a–M1c units reduce to a thin porous layer at the base of the lower atoll stage, while the overlying A1a–A1c interval is replaced by a thick shallowing–upward facies succession consisting of FS2-FS1-RM2-RM1 lithofacies (lower slope to reef margin and reef flat). Higher porosity is associated with lithofacies FS1, RM1, and RM2 toward the top of the sequence, producing a gradational porosity pattern in the 16-6, 16-31, and 6-5 wells (Fig. 9B). The same general shallowing–upward succession is present in other wells containing this porosity pattern. The M1a–M1c units thus represent the transgressive, backstepping ramp system of the lower atoll stage, overlain by a progradational highstand with slope facies developed in the A1a and A1b units. At its most basinward extent (6-5 well), the top of the lower atoll stage contains a reef margin interval in the A1c unit.

Fig. 9.

—A) East-west stratigraphic correlation across the eastern margin of the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Fig. 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The cross section is continued in Figure 9B, with the 14-1 and 16-1 wells repeated for cross-reference. B) Continuation of the cross section in A). See Figure 6 for the line of section location.

Fig. 9.

—A) East-west stratigraphic correlation across the eastern margin of the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Fig. 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The cross section is continued in Figure 9B, with the 14-1 and 16-1 wells repeated for cross-reference. B) Continuation of the cross section in A). See Figure 6 for the line of section location.

While platform-wide subaerial exposure is indicated for the Judy Creek R0.5 marker (Wendte and Muir 1995), representing the BHL2.1 sequence boundary, the same degree of exposure is not evident at the top of the Carson Creek North A1c unit. Exposure is commonly noted within the lower atoll stage in general, but it is locally developed at various stratigraphic levels. Development of GSB and pedant cements is noted in only a few wells across the field within the A1c unit; however, the A1c unit represents the culmination of a major regressive phase, and on that basis, it is interpreted to correspond to the Judy Creek R0.5 marker and BHL2.1 sequence boundary. The A1c unit contains abundant lithofacies LTF3a, LTF4a, and LTF4b, indicating widespread shallowing in the buildup interior. These lithofacies are responsible for the characteristic elevated radioactivity and low porosity observed in the A1c interval. The A1b and A1c units both become increasingly difficult log picks toward the buildup margin because they are associated with porous reef margin grainstones, rudstones, and boundstones (lithofacies RM1–RM2) with lower radioactivity comparable to similar facies in the overlying upper atoll stage. A weak GR marker is sometimes associated with a thin zone of reduced porosity within an otherwise continuous porous reef margin–reef flat interval that extends above the lower atoll stage. Similar suggestions of exposure or meteoric diagenesis (pendant cements and vugs containing geopetal sediment and green clay) sometimes present at this level in cores (see Figs. 9B and 10) are the only indications that the BHL2.1 sequence boundary may be present.

Fig. 10.

—North-south stratigraphic correlation across the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Figure 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The 8-3 well is also included in Figure 9A. See Figure 6 for the line of section location.

Fig. 10.

—North-south stratigraphic correlation across the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Figure 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The 8-3 well is also included in Figure 9A. See Figure 6 for the line of section location.

Upper Atoll Stage (BHL2 Sequence): The upper atoll stage is dominated by meter-scale reef interior lagoon–tidal flat cycles, a typical example of which is shown in Figure 11. Cycles range from less than 1 m to ~3 m thick, and two end-member types are distinguished: mudstone-rich type A cycles (Fig. 12A), containing repetitions of lithofacies LTF1, LTF2, and LTF3a; and grainstone-rich type B cycles (Fig. 12B) containing mixed lithofacies (LTF2, LTF3a, LTF3b, RA2, RA4), with larger stromatoporoid fragments often present. Lithofacies LTF1 is often missing at the base of type B cycles. Type A and B cycles are both capped by lithofacies LTF4a or LTF4b, and some have a shale layer or GSB interval thick enough to be visible on GR logs. The facies successions in these cycles were interpreted to represent an autocyclic lagoon fill process (Wong and Oldershaw 1980).

Fig. 11.

—Cored example of a buildup interior shallowing-upward cycle in the Carson Creek North upper atoll stage (1-18-62-12w5, ~2727 m), showing thin vuggy zones and pendant cements approaching the cycle tops. Cycle tops (indicated by solid lines) are overlain by subtidal lagoons indicated by lithofacies LTF1. Other lithofacies variations (see legend in Fig. 4) are indicated by dashed lines. The core “up–direction” is shown by the large arrow and scale bar.

Fig. 11.

—Cored example of a buildup interior shallowing-upward cycle in the Carson Creek North upper atoll stage (1-18-62-12w5, ~2727 m), showing thin vuggy zones and pendant cements approaching the cycle tops. Cycle tops (indicated by solid lines) are overlain by subtidal lagoons indicated by lithofacies LTF1. Other lithofacies variations (see legend in Fig. 4) are indicated by dashed lines. The core “up–direction” is shown by the large arrow and scale bar.

Fig. 12.

—A) Type A reef interior cycle in core (16-35-61-12w5, ~8680 ft (2645.7 m)), showing subtidal lagoons indicated by nonporous, dark brown Amphipora floatstone (lithofacies LTF1). B) Cored example of a type B interior cycle with porous grainstone in lithofacies RA2 and LTF3a (6-9-62-12w5, ~8568 ft (2611.5 m)). Most of the visible macrofossils are Amphipora, mixed with a few larger stromatoporoid fragments (small arrows). Note that subtidal lithofacies (LTF1–LTF2) are absent at the base of the cycle, resulting in more continuous porosity across cycles. Cycle tops are indicated in both examples by solid white lines, with lithofacies changes indicated by dashed lines. Yellow markers were used during core description and have no particular meaning. The core “up-direction” is shown by the large arrows.

Fig. 12.

—A) Type A reef interior cycle in core (16-35-61-12w5, ~8680 ft (2645.7 m)), showing subtidal lagoons indicated by nonporous, dark brown Amphipora floatstone (lithofacies LTF1). B) Cored example of a type B interior cycle with porous grainstone in lithofacies RA2 and LTF3a (6-9-62-12w5, ~8568 ft (2611.5 m)). Most of the visible macrofossils are Amphipora, mixed with a few larger stromatoporoid fragments (small arrows). Note that subtidal lithofacies (LTF1–LTF2) are absent at the base of the cycle, resulting in more continuous porosity across cycles. Cycle tops are indicated in both examples by solid white lines, with lithofacies changes indicated by dashed lines. Yellow markers were used during core description and have no particular meaning. The core “up-direction” is shown by the large arrows.

The upper atoll stage is subdivided into four megacycle units (A2–A5), each representing packages of individual meter-scale interior cycles. The megacycle boundaries occur either below abnormally thick intervals dominated by lithofacies LTF1–LTF2 (representing several successive type A cycles, and persistent deepening of the interior environment), or above thick lithofacies LTF4a–LTF4b intervals (representing the tops of type A or B cycles, and the persistence of shallow interior environments). The A2 megacycle at the base of the upper atoll stage commonly contains abundant dark–colored subtidal lithofacies (LTF1) and has an elevated GR signature even higher than the underlying A1c unit, making the top of the A2 megacycle more easily recognizable on GR logs than the BHL2.1 sequence boundary (see Fig. 8). Lithofacies LTF1 is also abundant at the base of the A4 megacycle in the 14-1 and 16-2 wells (see Fig. 9A). Conversely, several stacked cycles with LTF4a–LTF4b are present at the top of the A3 megacycle in the 8-3 well.

Where precise megacycle correlation could not be resolved, boundaries were chosen at the closest or most appropriate meter-scale cycle within larger-scale vertical porosity and GR log patterns. Such patterns often persist for several consecutive wells, however, in general, correlations using porosity logs are less reliable than those involving GR logs because type A and B cycles have different porosity profiles (Fig. 13), and because their distribution within the upper atoll stage produces spatially variable porosity patterns. Refined megacycle correlation was achieved through arriving at reasonable-looking environmental map patterns, such as the example in Figure 14, based on the most abundant lithofacies or cycle type near the top or bottom of the megacycle. Along the Carson Creek North margin, successive backstepping of the A3 to A5 megacycles is indicated by vertical facies changes representing abrupt upward shifts to more margin-proximal environments (e.g., lagoon to reef flat, or reef flat to reef margin). Higher-energy facies (lithofacies RM1–RM2) tend to extend further into the Carson Creek North buildup interior with height in the upper atoll stage (see Figs. 9B and 10), also suggesting a predominantly backstepping architecture.

Fig. 13.

—Lithofacies (legend in Fig. 4) and log porosity profiles associated with type A and Type B shallowing-upward cycles in the Carson Creek North buildup interior, illustrating how repetitions of each cycle type manifest as distinct vertical porosity patterns. Type B cycles are dominated by interparticle pores in grainstone facies, while type A cycles often feature a thin vuggy interval toward the top of the cycle. In type B cycles, reduced porosity is associated with thin muddy tidal flats (lithofacies LTF4a and LTF4b) at the top of the cycle, or with thin subtidal lagoons (lithofacies LTF2) at the base.

Fig. 13.

—Lithofacies (legend in Fig. 4) and log porosity profiles associated with type A and Type B shallowing-upward cycles in the Carson Creek North buildup interior, illustrating how repetitions of each cycle type manifest as distinct vertical porosity patterns. Type B cycles are dominated by interparticle pores in grainstone facies, while type A cycles often feature a thin vuggy interval toward the top of the cycle. In type B cycles, reduced porosity is associated with thin muddy tidal flats (lithofacies LTF4a and LTF4b) at the top of the cycle, or with thin subtidal lagoons (lithofacies LTF2) at the base.

FIG. 14.

—Depositional environments interpreted for the A3 megacycle (upper atoll stage), based on the dominant lithofacies comprising interior cycles in the bottom portion of the megacycle.

FIG. 14.

—Depositional environments interpreted for the A3 megacycle (upper atoll stage), based on the dominant lithofacies comprising interior cycles in the bottom portion of the megacycle.

ISHU and BHL3.1 Sequence Boundary: The ISHU is indicated by a prominent GR log marker above the A5 megacycle. In cores, the GR marker corresponds to a zone ranging from less than 0.5 m to more than 1 m containing green clay or a GSB with cream-colored limestone clasts (Fig. 15A). The green shale associated with the ISHU is a strong and laterally continuous log marker in the buildup interior (see Fig. 9A) capping the transgressive phase of the BHL2 sequence. Based on occupying a position above backstepping megacycles similar to Judy Creek, the marker is accepted herein as equivalent to the Judy Creek R4 marker and BHL3.1 sequence boundary.

FIG. 15.

—Depositional and diagenetic features associated with the ISHU at Carson Creek North. A) Green shale breccia with cream-colored limestone clasts in a matrix of green shale and dark brown micrite; 16-10-62-12w5, 8689 ft. (2648.4 m) B) Cored example of the “cream unit” with elongate cemented voids (arrows) often developed in lithofacies LTF4b (see legend in Fig. 4), localized oxidation (red-brown coloration), and cement-filled shell molds (s); 6-9-62-12w5, 8493 ft. (2586.7 m) C) Matrix dissolution voids filled with coarse calcite cements from the cream unit, with Amphipora (arrows) and larger stromatoporoid fragments (s) indicated; 1-18-62-12w5, 2694.7 m. D) Thin beds of green shale breccia, abundant stylolites, and gray-green clay seams associated with the ISHU in lithofacies LTF4a (see legend Fig. 4); 6-1-62-12w5, 8806 ft. (2684.1 m) E) Reef margin stromatoporoid rudstone below the ISHU with pendant cements (arrows), oxide minerals (red-brown color), and dissolution voids layered with light green micrite (m), fine-grained dolomite (d), and coarse calcite cement (c); 6-34-61-12w5, 8488 ft. (2587.1 m) F) Close-up photo stained with Alizarin Red-S to show the distribution of dolomite. G) Reef flat to reef margin stromatoporoid rudstone with geopetal sediment and cement in irregular dissolution voids below the ISHU. Stromatoporoid fragments (s) are rounded, and some are associated with thin pendant cement (not visible in the photo); 9-9-62-12w5, 2608.9 m. All scale bars = 2 cm.

FIG. 15.

—Depositional and diagenetic features associated with the ISHU at Carson Creek North. A) Green shale breccia with cream-colored limestone clasts in a matrix of green shale and dark brown micrite; 16-10-62-12w5, 8689 ft. (2648.4 m) B) Cored example of the “cream unit” with elongate cemented voids (arrows) often developed in lithofacies LTF4b (see legend in Fig. 4), localized oxidation (red-brown coloration), and cement-filled shell molds (s); 6-9-62-12w5, 8493 ft. (2586.7 m) C) Matrix dissolution voids filled with coarse calcite cements from the cream unit, with Amphipora (arrows) and larger stromatoporoid fragments (s) indicated; 1-18-62-12w5, 2694.7 m. D) Thin beds of green shale breccia, abundant stylolites, and gray-green clay seams associated with the ISHU in lithofacies LTF4a (see legend Fig. 4); 6-1-62-12w5, 8806 ft. (2684.1 m) E) Reef margin stromatoporoid rudstone below the ISHU with pendant cements (arrows), oxide minerals (red-brown color), and dissolution voids layered with light green micrite (m), fine-grained dolomite (d), and coarse calcite cement (c); 6-34-61-12w5, 8488 ft. (2587.1 m) F) Close-up photo stained with Alizarin Red-S to show the distribution of dolomite. G) Reef flat to reef margin stromatoporoid rudstone with geopetal sediment and cement in irregular dissolution voids below the ISHU. Stromatoporoid fragments (s) are rounded, and some are associated with thin pendant cement (not visible in the photo); 9-9-62-12w5, 2608.9 m. All scale bars = 2 cm.

The absence of a regressive margin below the ISHU is contrary to the manner in which reduced accommodation is manifested in the highstands of most carbonate sequences approaching a sequence boundary. While not evident in the margin architecture, the BHL2 highstand at Carson Creek North is indicated by the disappearance of deeper lagoonal environments (lithofacies LTF1–LTF2) in the A5 megacycle leading up to the ISHU. The A5 megacycle includes a distinctive light-colored interval below the ISHU, also known as the “cream unit,” featuring abundant cemented fenestral voids (Fig. 15B and C) with pendant cements and geopetal sediment fills, along with numerous thin green clay layers or seams, and stylolites (Fig. 15D). In some places along the buildup margin, the ISHU is developed above a reef margin succession (Fig. 15E, F, and G). In these settings, the GR marker associated with the ISHU is suppressed because the green shale layer or GSB is thin or absent, replaced by a thick diagenetic interval with greenish clay dispersed throughout. Diagenesis below the ISHU in marginal settings consists of dissolution voids filled with green clay, green-colored micrite, dolomitized greenish silt, and coarse calcite cement deposited in a geopetal sequence. Pendant cements, vugs, and red-stained dissolution voids are also concentrated within the A5 megacycle. Similar red staining associated with the Judy Creek R4 surface was determined to be the product of oxidation (Chow and Wendte 2011).

Shoal Stage and BHL3 Sequence: The shoal stage represents the transgressive part of the BHL3 sequence, and it culminates with drowning of the Swan Hills Formation at Carson Creek North. It contains high-energy ramps with abundant mixed stromatoporoid types, including many large fragments, representing mainly lithofacies RA2, RA3, and RA4. Extensive Amphipora-rich lagoons are not present, although Amphipora are mixed with larger stromatoporoids in the western (leeward) portion of the shoal stage. Even with cores available, individual ramp units are not easily correlated from well to well; hence, only a single cycle (S1L) is named within the shoal stage. The S1L unit is associated with a zone of elevated radioactivity, or a second GR peak above the ISHU (see Fig. 8). The S1L unit is analogous to the lagoon–tidal flat cycles in the upper atoll Stage, ranges up to 5 m thick, and is distinct from the overlying ramp cycles. Shallow-water grainstones with Amphipora (LTF3a), and Amphipora rudstones (LTF3b) comprise much of the S1L unit. Lithofacies LTF1–LTF2 (Fig. 16A) indicate moderately deep lagoons at the base of the unit, while tidal flats (Fig. 16B) or reef flat grainstones and rudstones (Fig. 16C) are observed toward the top of the unit. The S1L unit is thickest in the center of the buildup and thins toward the buildup margin. The upper contact of the S1L unit is often a sharp or encrusted surface overlain by Higher-energy facies of the ramp cycles (Fig. 16D and E). The shoal stage is capped by a marine hardground surface (Fig. 16F) in both the Carson Creek North A- and B-pools.

Fig. 16.

—Cored examples of the S1L unit and the Carson Creek North shoal stage. A) Subtidal lagoon in the interior of the S1L unit, represented by dark brown, nodular Amphipora floatstone (lithofacies LTF1), 14-16-62-12w5, 2603.2 m. B) Dark-colored, fine-laminated mudstone and light brown stromatolitic tidal flat mudstone overlying the ISHU (marked by the green shale breccia at the bottom of the photo), 8-2-62-12w5, 2642.5 m. C) Stromatoporoid rudstone (lithofacies LTF3b-RM1) with pendant cements (arrows) below a sharp surface at the top of the S1L unit, 16-11-62-12w5, 8750 ft. (2667.0 m) D) Columnar stromatolite growth (arrows) on top of a cemented Amphipora rudstone (lithofacies LTF3b) at the top of the S1L unit, 6-9-62-12w5, 8470 ft. (2581.7 m) E) Sharp contact between the “cream unit” and the basal shoal stage ramp cycle along the ISHU (solid white line) in 8-12-62-13w5, 2744.0 m. The S1L unit is missing at this location, and the ISHU is encrusted with coarse-laminated micrite (m, dashed line). The overlying ramp cycle contains grainstone and stromatoporoid fragments (lithofacies RA2). F) Sharp contact at the top of the shoal stage encrusted with pyrite (p) and overlain by dark gray-brown basinal shale in the Waterways Formation; 9-10-62-12w5, 2621.0 m. All scale bars = 2 cm. See Figure 4 for lithofacies descriptions.

Fig. 16.

—Cored examples of the S1L unit and the Carson Creek North shoal stage. A) Subtidal lagoon in the interior of the S1L unit, represented by dark brown, nodular Amphipora floatstone (lithofacies LTF1), 14-16-62-12w5, 2603.2 m. B) Dark-colored, fine-laminated mudstone and light brown stromatolitic tidal flat mudstone overlying the ISHU (marked by the green shale breccia at the bottom of the photo), 8-2-62-12w5, 2642.5 m. C) Stromatoporoid rudstone (lithofacies LTF3b-RM1) with pendant cements (arrows) below a sharp surface at the top of the S1L unit, 16-11-62-12w5, 8750 ft. (2667.0 m) D) Columnar stromatolite growth (arrows) on top of a cemented Amphipora rudstone (lithofacies LTF3b) at the top of the S1L unit, 6-9-62-12w5, 8470 ft. (2581.7 m) E) Sharp contact between the “cream unit” and the basal shoal stage ramp cycle along the ISHU (solid white line) in 8-12-62-13w5, 2744.0 m. The S1L unit is missing at this location, and the ISHU is encrusted with coarse-laminated micrite (m, dashed line). The overlying ramp cycle contains grainstone and stromatoporoid fragments (lithofacies RA2). F) Sharp contact at the top of the shoal stage encrusted with pyrite (p) and overlain by dark gray-brown basinal shale in the Waterways Formation; 9-10-62-12w5, 2621.0 m. All scale bars = 2 cm. See Figure 4 for lithofacies descriptions.

ISHU Paleotopography: The upper atoll stage does not have uniform thickness across the Carson Creek North area, generally increasing in thickness toward the buildup margins. Measured from the top of the A2 megacycle (which forms a more reliable GR log marker than the actual base of the upper atoll stage), the upper atoll stage thickness across the A-pool (23–36 m) indicates ~13 m of paleotopographic relief on the ISHU (Fig. 17A). There is no indication of significant ISHU topography in the B-pool, and in the Judy Creek buildup, the R4 marker is approximately parallel to the underlying stratigraphy and has low relief (Wendte 1992b, Wendte and Muir 1995).

FIG. 17

.—A) Thickness of the upper atoll stage above the A2 megacycle, used to demonstrate ISHU paleotopography. The map does not precisely represent the BHL2 sequence, but for a reliable map, the top of the A2 megacycle is the best reference log marker below the ISHU. Note that the depositional limit of the shoal stage is aligned with remnant thicknesses in the upper atoll stage. B) Lithofacies in the A5 megacycle below the ISHU, demonstrating the absence of subtidal lagoons. C) Depositional environments in the S1L unit at the base of the shoal stage, showing the absence of a reef margin. Lateral continuity of the reef flat area is broken up by absences of the S1L unit along the northern and eastern margins of the south buildup.

FIG. 17

.—A) Thickness of the upper atoll stage above the A2 megacycle, used to demonstrate ISHU paleotopography. The map does not precisely represent the BHL2 sequence, but for a reliable map, the top of the A2 megacycle is the best reference log marker below the ISHU. Note that the depositional limit of the shoal stage is aligned with remnant thicknesses in the upper atoll stage. B) Lithofacies in the A5 megacycle below the ISHU, demonstrating the absence of subtidal lagoons. C) Depositional environments in the S1L unit at the base of the shoal stage, showing the absence of a reef margin. Lateral continuity of the reef flat area is broken up by absences of the S1L unit along the northern and eastern margins of the south buildup.

The most likely explanation for the thickness pattern in the A-pool is local selective denudation and/or shoreface erosion of the A5 megacycle during the ISHU hiatus. Figure 17A may include a few anomalous thickness values due to correlation errors arising from poor log quality, but the overall map pattern indicates preferential thinning in the interior of the A-pool buildup. The thickness variation is confined to the A5 megacycle, eliminating differential compaction of the entire upper atoll stage due to contrasting mechanical properties between the buildup margin and interior environments as a cause for the map pattern. Incomplete lagoon fill during deposition of the A5 megacycle is also an improbable explanation, based on the general absence of deeper subtidal lagoons (lithofacies LTF1 and LTF2) in the megacycle (Fig. 17B). The absence of deep lagoonal environments in reef cycles below the ISHU was also noted in the Snipe Lake and Judy Creek reefs (Wendte and Muir 1995). Denudation/erosion is suggested by a partial mismatch between the A5 megacycle lithofacies and the ISHU paleotopography. Paleotopographic highs (thick areas in Fig. 17A) coincide with different environments in the A5 megacycle, including shallow lagoons, tidal flats, and reef margins (Fig. 17B). The margin of the A5 megacycle is broken up into isolated regions, in contrast with the symmetrical facies pattern associated with upper atoll stage megacycles illustrated by the example in Figure 14. Paleotopography is also consistent with lithofacies variations in the S1L unit at the base of the shoal stage. The interval is locally absent and lacks a reef margin (Fig. 17C), instead containing tidal flats and shoals that appear to onlap the underlying A5 megacycle in the more elevated parts of the ISHU along the eastern buildup margin. The S1L cycle is the only shoal stage unit containing a subtidal lagoon, suggesting that the topographic high on the ISHU served as an elevated “margin” and provided energy restriction in the surrounding lows during deposition.

Collectively, these observations suggest that the ISHU is associated with a significant depositional hiatus at Carson Creek North. Complete lagoon fill was apparently the main manifestation of a brief highstand in the BHL2 sequence leading up to the ISHU (Fig. 18A). Relative sea-level fall accompanied by denudation or erosion of the A5 megacycle followed, resulting in ~13 m of topographic relief (Fig. 18B). The S1L unit was deposited during initial transgression of the ISHU surface, and the underlying topography influenced the depositional environments comprising the unit. The thickness of the S1L interval (~5 m maximum) was insufficient to completely fill the topographic lows (Fig.18C), explaining its absence along the eastern edge of the shoal stage and the lack of a reef margin. The residual topography along the eastern buildup margin then provided a locus of deposition for the high-energy shoal stage ramps (Fig. 18D), resulting in relative conformance between the eastern limit of the shoal stage and the ISHU thickness pattern.

Fig. 18.

—Schematic diagrams illustrating the development of paleotopography on the ISHU at Carson Creek North. A) Deposition and highstand lagoon fill at the end of the upper atoll stage A5 megacycle. B) sea-level fall and denudation/erosion to form the ISHU. C) Subsequent sea-level rise and deposition of the S1L unit, with facies and environments controlled by ISHU paleotopography. D) Continued transgression and deposition of shoal stage high-energy ramp cycles.

Fig. 18.

—Schematic diagrams illustrating the development of paleotopography on the ISHU at Carson Creek North. A) Deposition and highstand lagoon fill at the end of the upper atoll stage A5 megacycle. B) sea-level fall and denudation/erosion to form the ISHU. C) Subsequent sea-level rise and deposition of the S1L unit, with facies and environments controlled by ISHU paleotopography. D) Continued transgression and deposition of shoal stage high-energy ramp cycles.

Eastern Shelf Sequences and Architecture

A shelf-to-basin profile for the Eastern Shelf carbonates was reconstructed from cores across the Waterways B, C, D, and E units, representing an interval from the upper part of the BHL1 sequence to the lower portion of the BHL3 sequence. The BHL2.1 sequence boundary is placed at the base of the Waterways C unit based on Potma et al. (2001, 2002). The correlative equivalent of the ISHU (BHL3.1 sequence boundary) in the Eastern Shelf area is disputed, occupying a position as low as the base of the Waterways D unit (based on Wendte and Uyeno 2005) or as high as the approximate base of the Waterways E unit (based on Potma et al. 2002).

BHL2 Sequence: The regional BHL2 sequence is represented by the Waterways C unit in the Eastern Shelf, and it contains transgressive–regressive architecture (Fig. 19). In the shelfward direction (10-34 well), the basal sequence boundary is marked by the abrupt appearance of porous shallow-water grainstones (lithofacies RA2 and AL1) at the base of the Waterways C unit, above basinal mudstone and skeletal floatstone (lithofacies SB1–SB2) in the Waterways B unit. The basal Waterways C grainstones are 12 m thick and grade upward to a lagoonal succession with cyclic porosity variations comprising the remainder of the Waterways C unit. Basinward, the BHL2 sequence is dominated by an argillaceous succession (lithofacies SB1–SB2 and shale) in the 16-36 well down to within 3 m of the Waterways B unit, with no indication of shallow-water carbonate facies. This argillaceous succession is part of a wedge containing basinal lithofacies that rises shelfward and thins within the Waterways C unit (Fig. 19), defining the transgressive phase of the BHL2 sequence.

Fig. 19.

—Stratigraphic section illustrating the transgressive–regressive architecture associated with the Eastern Shelf. Well logs shown are sonic (SON) for porosity and gamma-ray (GR). The top of the Waterways D unit is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Lithofacies described from core (see legend in Fig. 4) are plotted next to the depth track. Horizontal red arrows indicate features in core associated with possible subaerial exposure, as described in the text. See Figure 1 for well locations.

Fig. 19.

—Stratigraphic section illustrating the transgressive–regressive architecture associated with the Eastern Shelf. Well logs shown are sonic (SON) for porosity and gamma-ray (GR). The top of the Waterways D unit is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Lithofacies described from core (see legend in Fig. 4) are plotted next to the depth track. Horizontal red arrows indicate features in core associated with possible subaerial exposure, as described in the text. See Figure 1 for well locations.

The basinal wedge is associated with an elevated GR signal, and it is ~50 m thick above the BHL2.1 sequence boundary at the 16-36 well location. Shelfward, the wedge forms an ~18-m-thick argillaceous deeper-water interval in the middle to upper Waterways C unit in the 6-4 well, containing nodular mudstones (lithofacies SB2) and nodular skeletal-intraclast wackestones (lithofacies SB3) with occasional brachiopods. It caps a deepening-upward progression consisting of porous, coarse-grained rudstone with large stromatoporoids (lithofacies RA2), porous grainstone with small bulbous stromatoporoids (lithofacies RA3), and grainstone with occasional crinoids (lithofacies RA1). The Waterways C unit in the 1-7 location between these two wells was not cored, but the GR log indicates an ~30-m-thick argillaceous interval wedged between clean limestones at the top and base of the unit. Further shelfward (10-30 well), the argillaceous interval is not present, and the BHL2 sequence contains a lagoon–tidal flat interval (lithofacies LTF1–LTF2) overlain by a marginal succession containing porous skeletal floatstone with brachiopods (lithofacies AL1), and lithofacies RA2 with Amphipora, small bulbous stromatoporoids, and tabular stromatoporoid fragments. In the most shelfward location (10-34 well), the BHL2 sequence is entirely made up of lagoonal shelf interior deposits. The 10-30 and 10-34 wells mark the shelfward termination of the deeper-water facies wedge.

The regressive phase of the BHL2 sequence occupies the upper 5–10 m of the Waterways C unit (Fig. 19). It includes a shallowing-upward succession from nonporous slope to basin carbonates (lithofacies SB2 and SB3) to porous upper foreslope and shelfmargin lithofacies containing tabular, bulbous, and domal stromatoporoids (lithofacies FS1 and RM2) in the basinward 16-36 well location, and it shallows upward from deeper-water argillaceous mudstone to bioclastic wackestone (lithofacies SB3) with crinoids and Amphipora to argillaceous, silty to peloidal, laminated, low-energy shelf interior mudstones (lithofacies SM1) in the 6-4 well location.

Basal Waterways D Facies: The base of the Waterways D unit is associated with an abrupt shift to restricted facies from deeper-water or less restricted shallow-water facies at the top of the Waterways C unit in Figure 19. In the 16-36 basinward location, the base of the Waterways D unit contains dolomitic laminated mudstone and bedded anhydrite (lithofacies SI4) abruptly overlying a scour interval (lithofacies SS1). The scour interval overlies the porous upper foreslope and shelf margin interval (lithofacies FS1 and RM2) at the top of the Waterways C unit. Abrupt lithofacies shifts are also observed at the 10-34 and 10-30 shelfward locations. At 10-34, the Waterways D unit contains restricted shelf interior environments represented by stromatolitic limestone (lithofacies SI3), and laminated evaporitic/stromatolitic mudstone with intraclasts and rare Amphipora (lithofacies SI4) that abruptly overlie less restricted lagoon to tidal flat carbonate environments (LTF2 and LTF4a) in the upper Waterways C unit. At well 10-30, a sudden upward shift occurs from relatively high-energy, porous stromatoporoid rudstone (lithofacies RA2 and RA3) at the top of the Waterways C unit to nonporous, restricted shelf interior laminated mudstone and bedded anhydrite (lithofacies SI5), lagoonal Amphipora floatstone (lithofacies LTF2), and stromatolitic shelf interior mudstone with bedded anhydrite (lithofacies SI5) in the Waterways D unit.

BHL3 Transgression: Restricted to evaporitic shelf interior facies occupy at least half of the Waterways D unit in the well 10-30 location, and the entire Waterways D unit at the shelfward well 10-34 location (Fig. 19). In the 10-34 well, flooding of the shelf interior occurred at the base of the Waterways E unit, represented by a submarine scour (lithofacies SS1) composed of oncolite–skeletal floatstone and a green clay bed, overlain by a complex succession representing conditions ranging from restricted shelf to open-marine, deeper water. Lithofacies include bedded anhydrite (lithofacies SI5), laminated mudstone, nodular intraclast–skeletal rudstone with stromatoporoids (lithofacies AL1–SB3), and intraclast–skeletal rudstone with brachiopods and multiple scour surfaces (lithofacies SS1). Flooding is also associated with the base of the Waterways E unit in the basinward 1-7 well. A restricted shelf interior interval at the top of the Waterways D unit (lithofacies SI3, laminated evaporitic mudstone, stromatolitic mudstone, and intraclast rudstone with authigenic anhydrite crystals and anhydrite beds) is overlain by several submarine scour intervals mixed with open-marine crinoid–skeletal wackestones (lithofacies SB3), and nodular mudstone (lithofacies SB2). In this well, an interval of open-marine nodular intraclast–skeletal packstone to grainstone (lithofacies SB3–AL1) with brachiopods and occasional small stromatoporoids is also present in the middle of the Waterways D unit below the restricted shelf interior deposits, indicating that episodic flooding may have started earlier in the basinward direction.

DISCUSSION

Eastern Shelf–Carson Creek North Comparison

The transgressive portion of the BHL2 sequence in the Eastern Shelf area is indicated by shelfward thickening of shallow margin and restricted interior successions above the base of the sequence. Shelf margins appear above lagoonal successions progressively higher within the Eastern Shelf successions, suggesting margin backstepping analogous to the upper atoll stage megacycles in the BHL2 sequence at Carson Creek North. The Eastern Shelf carbonates thicken shelfward at the expense of the argillaceous basinal wedge, as indicated by the cores and GR logs from more basinward wells. The top of the BHL2 sequence contains a regression, marked by the appearance of shelf slope, margin, and interior facies above the basinal wedge in the upper 10 m of the Waterways C unit. The abrupt shift to more restricted facies in the Waterways D unit represents an apparent continuation of the regression. Similar margin architecture is indicated by the GR logs from a string of wells at the eastern edge of the Stewart area (see Fig. 1 for well locations). An argillaceous wedge in the middle of the Waterways C unit is overlain by less argillaceous limestone toward the top of the unit, with clean limestone appearing in the overlying Waterways D unit. This progression is consistent with an upper Waterways C regression (increase in carbonate content) followed by shallowing at the base of the Waterways D unit (clean limestone); however, not all of the log-based intervals in this area are well represented by cores. The complete Eastern Shelf regression represented by the upper Waterways C and basal Waterways D units forms the missing regression across the ISHU at Carson Creek North, and thus includes the BHL3.1 sequence boundary.

Regional flooding of the Eastern Shelf and eventual replacement of carbonate by shale toward the top of the BHL3 sequence are analogous to deposition of the Carson Creek North shoal stage and ultimate drowning of the Swan Hills buildups. While the shoal stage backstepping ramps were deposited directly above the ISHU, flooding of the eastern regressive shelf was apparently more complex, involving multiple episodes of submarine scour and redevelopment of shallow or restricted shelves and isolated buildups in the upper Waterways D and basal Waterways E units of the BHL3 sequence. The base of the Waterways E unit is a consistent regional GR log marker in the Eastern Shelf area and Waterways Basin, but it is probably only approximately coeval with the base of the Carson Creek North shoal stage.

ISHU and the BHL3.1 Sequence Boundary

The Eastern Shelf regression and the backstepping Swan Hills buildups, illustrated by the Carson Creek North upper atoll and shoal stages, represent different local sedimentary responses across the BHL3.1 sequence boundary. The sudden appearance of evaporitic facies in the Waterways D unit above the upper Waterways C regression may indicate an episode of increased restriction and reduced circulation that developed under lowstand conditions in the Eastern Shelf area during an extended exposure event that produced the ISHU at Carson Creek North, Judy Creek, and other Swan Hills buildups. On this basis, the BHL3.1 sequence boundary is positioned at the base of the Waterways D unit, which is represented by the cored succession in the 10-34-35-17w4 well originally described by Wendte and Uyeno (2005) containing the subaerial exposure surface they correlated with the R4 marker in the Judy Creek buildup. The BHL3.1 sequence boundary in the 10-34 core is represented by a green shale bed and GSB overlying cream-colored floatstone to rudstone containing Amphipora and assorted skeletal fragments (lithofacies LTF4b). Large stromatoporoid fragments are present immediately below the green shale/GSB, and the restricted shelf interior deposits (lithofacies SI2) are present directly above it. The cream-colored interval contains numerous green-gray stylolitic seams, strongly resembling the “cream unit” in the A5 megacycle below the ISHU at Carson Creek North. The more restricted basal Waterways D lithofacies are generally more argillaceous than carbonate facies in the Waterways C unit; therefore, it may be possible to interpret the BHL3.1 sequence boundary using logs alone within the Eastern Shelf area, provided the lithofacies change could be consistently linked to a distinctive log pattern.

ISHU Hiatus and Lowstand Duration

The marker representing the BHL3.1 sequence boundary in this study was assigned by Wendte and Uyeno (2005) to the upper part of the MN2 conodont zone (MN indicates the Montaigne Noire conodont zones in the Frasnian Composite Standard of Klapper et al. [1995], Klapper [1997], and McLean and Klapper [1998], often used in studies of the Western Canada Basin). This age determination was based on its position (as described above) in the Eastern Shelf 10-34-35-17w4 well, compared with conodont ages obtained from foreslope limestones overlying partial Swan Hills reef member penetrations below the R4 marker along the flanks of the Judy Creek buildup. Due to the complex flooding history recorded in the Waterways D unit, the top of the lowstand interval cannot be firmly established. The Waterways D unit in the current study probably represents an upper limit for the lowstand interval thickness. It closely parallels two markers named in the Wendte and Uyeno (2005) study (Upper C and Top D markers) within the MN2 and MN3 conodont zones. As a result, the ISHU is shown in Figure 3 as a hiatus across the MN2 and MN3 zones corresponding to most of the Waterways D unit.

The 13 conodont zones comprising the Frasnian Composite Standard represent an estimated 10.5 Myr (Cohen et al. 2013, updated). The MN1 to lower MN4 zones containing the Swan Hills reef member and its Eastern Shelf equivalents may represent 2–2.5 Myr of this range (Becker et al. 2012), and the hiatus/lowstand itself perhaps represents 300–500 kyr. The Waterways D interval is ~30–35 m thick in the Eastern Shelf area (Table 1), requiring uncorrected accommodation rates of at least 60–100 m/Myr, assuming a shallow water depth for the carbonates and evaporites. The ISHU paleorelief at Carson Creek North (13 m) suggests uncorrected net denudation/erosion rates in the 25–40 m/Myr range or higher over the same time interval. These rates cannot be compared with carbonate denudation rates or basin accommodation rates reviewed in the literature because they are simplistically derived from present-day interval thicknesses. Even if they were to be at least corrected for compaction, however the accumulation rates are lower than those used previously to forward-model the Swan Hills buildup at Judy Creek. A model in Scaturo et al. (1989) accurately reproduced backstepping geometries in the Judy Creek reef member using maximum carbonate accumulation rates of 300–500 m/Myr, with accommodation rates based on estimated thermal subsidence (50–100 m/Myr) combined with a series of punctuated sea-level rises at variable rates (70–500 m/Myr). The input parameters for this model were constrained by an assumption that the Swan Hills reef member accumulated over a period of 600–700 kyr, and that individual reef member megacycles represented ~100,000 year Milankovitch cycles. An alternative three-dimensional (3D) forward model by Warrlich et al. (2008) also replicated the Judy Creek reef member stratigraphic architecture using successive phases of increasing linear sea level (60, 140, and 280 m/Myr). The increases were needed to duplicate the early progradational (equivalent to the Carson Creek lower atoll stage), subsequent aggradational (upper atoll stage), and ultimate drowning (shoal stage) successions. Higher-frequency Milankovitch cycles (100, 40, and 20 kyr) were superimposed on the overall regime of sea-level rise to match observed smaller-scale shallowing-upward cycles. This model required accommodation-limited margin production of 400 m/Myr over a time interval of 480 kyr representing the reef member. The age scale in Figure 3 suggests the Swan Hills reef member represents a somewhat longer total time period, perhaps ~1.5 Myr. Excluding the ISHU hiatus, the actual time for accumulation may have been about 1.0–1.2 Myr.

Origin of the ISHU

Approximate correlations between published eustatic sea-level events and the ISHU suggest a possible global eustatic origin. A eustatic fluctuation of ~50 m magnitude is noted ~2 Myr above the base of the Frasnian in Haq and Schutter (2008). The relative sea-level curve from Becker et al. (2012) suggests eustatic events could be aligned with all three regional Beaverhill Lake sequences (see Fig. 3). Specific examples from other basins indicating a significant eustatic event with similar timing are not as clearly documented. For example, the Canning Basin also features a series of backstepping Frasnian platform sequences (Playford 1980, Playford et al. 1989). Several of these sequences are associated with siliciclastic-rich slope and basin floor fans interpreted as lowstand deposits (Kennard et al. 1992, Whittam et al. 1994). Multiple such lowstands are documented throughout the upper Givetian to Famennian in the Canning Basin (Jackson et al. 1992); however, studies of the lowstands mostly lack the appropriate biostratigraphic resolution to correlate a specific deposit or event with the western Canadian ISHU. In the Canning Basin platforms themselves, sequence boundaries are associated with two major Frasnian margin backsteps (Playford et al. 2009), one occurring within MN zones 1–2 (Elliot Ridge Event) and the second near the MN3-4 boundary (Kelly’s Pass Event). The Kelly’s Pass event is noteworthy, having been described by Playford et al. (2009) as a tectonically induced angular unconformity associated with karstification; however, both the Elliot Ridge and Kelly’s Pass events have been alternatively interpreted as maximum flooding surfaces (Playton 2008, Playton and Kerans 2015).

It is also possible that the ISHU reflects local tectonic activity that affected subsidence and accommodation rates in the Western Canada Basin rather than a eustatic event. Thickness patterns in the combined BHL1 and BHL2 sequences may be taken to reflect the basin- floor configuration during early Beaverhill Lake time because shallow-water carbonates were exposed in both the Swan Hills and Eastern Shelf areas at the end of BHL2 deposition. The highest positive relief was present under the contiguous Swan Hills shelf complex, where the BHL1 and BHL2 sequences are thinnest below the ISHU (50–80 m; Table 1). The combined sequences are significantly thicker (95–100 m) at Carson Creek North, indicating a deeper basement, which may be one reason why the Swan Hills shelf breaks up into isolated buildups in the Carson Creek area. The BHL1 + BHL2 thickness at Carson Creek North represents an uncorrected net accommodation rate of ~45–50 m/Myr over the time interval indicated in Figure 3. The combined BHL1 and BHL2 sequences are slightly thicker (105–120 m) in the Eastern Shelf and include significant basinal limestone and shale in the Waterways A and B units, consistent with an even deeper basin floor. A deeper eastern basin is also suggested by the Eastern Shelf carbonates at the base of the BHL2 sequence, which may have initially developed in favorable basinal sites while the BHL2.1 sequence boundary in the Swan Hills complex (Carson Creek North lower atoll stage; Judy Creek R0.5 marker) was exposed (Fig. 20A).

Fig. 20.

—Schematic evolution of two-dimensional (2D) basin-floor topography and sea level, accounting for regional thickness trends and carbonate-shale facies patterns in the Beaverhill Lake Group using example wells from Carson Creek North (CCN), the Waterways Basin (WB), and the Eastern Shelf area (ESA). A) Elevated basin floor underlying the BHL1 sequence in the Carson Creek area, with sea-level fall and exposure of the Swan Hills complex promoting initiation of carbonate deposition above west-dipping basinal clinoforms in ESA. B) sea-level rise combined with possible basin-floor subsidence in the ESA, resulting in Swan Hills backstepping in the CCN BHL2 sequence coincident with near-drowning of the ESA carbonates. Depositional bathymetry increased in the WB as a result. C) sea-level fall combined with possible uplift along the West Alberta Ridge, resulting in exposure of the BHL2 sequence and formation of the ISHU, coincident with maximum westward development of shelf carbonates in the ESA at the base of the lowstand. D) Increased basin-floor subsidence in the ESA combined with rising sea level and initial transgression of the ISHU after deposition of the Waterways D unit lowstand. E) Regional flooding at CCN and the ESA resulting in termination of carbonate deposition during the BHL3 sequence, with depositional bathymetry due to drape over CCN and a residual low in the WB caused by low-angle, west-dipping basinal clinoforms. Accommodation in both areas may have been provided by rapid sea-level rise following the ISHU combined with onset of a more regional rapid subsidence regime.

Fig. 20.

—Schematic evolution of two-dimensional (2D) basin-floor topography and sea level, accounting for regional thickness trends and carbonate-shale facies patterns in the Beaverhill Lake Group using example wells from Carson Creek North (CCN), the Waterways Basin (WB), and the Eastern Shelf area (ESA). A) Elevated basin floor underlying the BHL1 sequence in the Carson Creek area, with sea-level fall and exposure of the Swan Hills complex promoting initiation of carbonate deposition above west-dipping basinal clinoforms in ESA. B) sea-level rise combined with possible basin-floor subsidence in the ESA, resulting in Swan Hills backstepping in the CCN BHL2 sequence coincident with near-drowning of the ESA carbonates. Depositional bathymetry increased in the WB as a result. C) sea-level fall combined with possible uplift along the West Alberta Ridge, resulting in exposure of the BHL2 sequence and formation of the ISHU, coincident with maximum westward development of shelf carbonates in the ESA at the base of the lowstand. D) Increased basin-floor subsidence in the ESA combined with rising sea level and initial transgression of the ISHU after deposition of the Waterways D unit lowstand. E) Regional flooding at CCN and the ESA resulting in termination of carbonate deposition during the BHL3 sequence, with depositional bathymetry due to drape over CCN and a residual low in the WB caused by low-angle, west-dipping basinal clinoforms. Accommodation in both areas may have been provided by rapid sea-level rise following the ISHU combined with onset of a more regional rapid subsidence regime.

Toward the end of the BHL2 sequence, the basin floor configuration described above combined with compaction of the Waterways A–B shales can perhaps be invoked to account for the additional room needed for the brief regression at the top of the Waterways C unit in the Eastern Shelf area (Fig. 20B), while maintaining the same net accommodation rate in the Carson Creek area as a regional value. In order to accommodate lowstand deposition in the Waterways D unit at the same time as denudation/erosion at the ISHU however, a subsidence increase in the Eastern Shelf area is likely required. A higher average net accommodation rate (~60 m/Myr) is needed to account for the entire Eastern Shelf Beaverhill Lake succession (~200–210 m), which incorporates both the uncorrected regional 45–50 m/Myr rate estimated for the BHL1 and BHL2 sequences and the 60–100 m/Myr rate required for the lowstand based on the duration of the ISHU hiatus discussed in the previous section. This change in accommodation could reflect a combination of uplift in the Swan Hills area and increased subsidence in the Waterways Basin and Eastern Shelf regions (Fig. 20C and D). The Swan Hills margin and the Eastern Shelf areas were both covered over by shale successions toward the top of the BHL3 sequence (Fig. 20E), perhaps suggesting greater accommodation rates in both areas following the ISHU.

Implications for Intrabasin Correlation

Stratigraphic interpretation using well-log correlations in the Beaverhill Lake Group is sensitive to the orientation of the cross section, the choice of cross-section datum, and the well spacing available. Basin-floor topography and subsidence patterns in time and space are also important considerations. Carbonate-shale facies changes in the Beaverhill Lake Group that are coordinated with regional interval thickness patterns may be portrayed to indicate clinoforms deposited on a flat surface, or as lateral facies changes reflecting deepening paleobathymetry in the opposite direction resulting from basin-floor topography or subsidence. Some of the Waterways argillaceous units are consistent with clinoform geometries. At a finer scale, the Waterways D unit contains west- to southwest-prograding clinoforms associated with accelerated fill of the Waterways Basin from the Eastern Shelf area during the lowstand interval. With the available well control, these clinoforms appear to downlap or onlap the BHL3.1 boundary approaching the Carson Creek North (Fig. 21). Similar behavior is observed for basinward-dipping Waterways D clinoforms between the Eastern Shelf and the Steward-Twining channel at the southern end of the Swan Hills complex (Fig. 22). At a more regional scale, clinoforms are indicated by radioactivity patterns on GR logs. The perception of coarse-scale vs. fine-scale clinoform geometries in GR log patterns depends on the rapidity with which the associated lateral facies changes occur, as well as the density of available well control.

Fig. 21.

—Stratigraphic correlation using GR logs between the Eastern Shelf area and Carson Creek North, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and Carson Creek North shown in B). The 6-15-48-3w5 and 15-35-44-2w5 wells are repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum, except at Carson Creek North, where the top of the Beaverhill Lake Group is used, positioned so that the Carson Creek North ISHU is approximately level with the top of the Waterways D unit in the Eastern Shelf area. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are shown with black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward shelf margin step-out and west-dipping clinoforms in the Waterways D lowstand unit.

Fig. 21.

—Stratigraphic correlation using GR logs between the Eastern Shelf area and Carson Creek North, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and Carson Creek North shown in B). The 6-15-48-3w5 and 15-35-44-2w5 wells are repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum, except at Carson Creek North, where the top of the Beaverhill Lake Group is used, positioned so that the Carson Creek North ISHU is approximately level with the top of the Waterways D unit in the Eastern Shelf area. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are shown with black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward shelf margin step-out and west-dipping clinoforms in the Waterways D lowstand unit.

Fig. 22.

—Stratigraphic correlation using GR logs between the Eastern Shelf and the Stewart-Twining channel, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and the Stewart-Twining area shown in B). The 16-36 well location is repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are indicated by black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward step-out of shelf carbonates (gray-shaded intervals), and west-dipping clinoforms in the interpreted Waterways D unit. Gray arrows next to the 8-13-27-3w5 well in the Stewart-Twining channel mark allochthonous lithofacies (AL1, AL2, AL3; see Fig. 4) noted in the core that were probably derived from Swan Hills shelf carbonates making up the BHL2 sequence adjacent to the channel (see Fig. 1).

Fig. 22.

—Stratigraphic correlation using GR logs between the Eastern Shelf and the Stewart-Twining channel, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and the Stewart-Twining area shown in B). The 16-36 well location is repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are indicated by black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward step-out of shelf carbonates (gray-shaded intervals), and west-dipping clinoforms in the interpreted Waterways D unit. Gray arrows next to the 8-13-27-3w5 well in the Stewart-Twining channel mark allochthonous lithofacies (AL1, AL2, AL3; see Fig. 4) noted in the core that were probably derived from Swan Hills shelf carbonates making up the BHL2 sequence adjacent to the channel (see Fig. 1).

By some accounts, the more argillaceous Waterways units were formed by clays delivered to the Western Canada Basin from external sources (Sheasby 1971; Stoakes 1980, 1992; Campbell 1992b; Schneider et al. 2013) and deposited with a component of east-to-west asymmetry due to circulation patterns in the basin. The most argillaceous intervals higher GR) were posited to indicate base-level rises, when the Waterways Basin became less restricted due to flooding of the Eastern Shelf carbonate complexes (Wendte and Uyeno 2005). In this paradigm, log-based correlations between the Eastern Shelf area and the Waterways Basin are based on the premise that west- to southwest-dipping shaly clinoforms provided support for subsequent renewed carbonate deposition in the shelf areas during base-level falls. The basinal equivalents of shallow shelf carbonates are less argillaceous Waterways Formation log units (lower GR) in which the clay content was supposedly diluted by carbonate production from the shelf. An alternative to this scenario is that the fine clays were pushed further basinward during shallowing and exposure of the shelf areas, and that log units with high GR represent regressions or lowstands.

Figures 2022 suggest that there are no regional flat surfaces ideally suited to be used as a correlation datum within the Beaverhill Lake Group because of basin floor topography, or because of regional variations in topography (clinoforms and carbonate-shale facies changes) during deposition. The base of the Beaverhill Lake Group (BHL1.1 sequence boundary) is precluded because of basin-floor topographic relief approaching the Swan Hills complex, likely an extension of the West Alberta ridge that formed a residual high following Elk Point Group/Watt Mountain Formation deposition (Moore 1989, Campbell 1992a). Setting the datum on the BHL1.1 sequence boundary causes all of the Waterways units to look like westward-dipping clinoform units because of the greater overall sedimentary thickness in the Eastern Shelf area (e.g., Sheasby 1971, Campbell 1992b, Wendte and Uyeno 2005). The BHL1 sequence is ~50–60 m thick in the Eastern Shelf area (Table 1), with a deeper basin setting indicated by the laterally continuous shales and argillaceous limestones in the Waterways A and B units. Regional comparisons of the BHL1 sequence along the margin of the Swan Hills complex is more problematic because of its variable thickness as indicated by core data (~30–65 m; Table 1), but the presence of aggradational shallow-water limestones and a generally thinner interval can be attributed to both an elevated basin floor coupled with an eastward-deepening depositional profile corresponding to the general carbonate to shale transition. Some of the clinoform onlap/downlap relationships in Wendte and Uyeno (2005) in the Waterways A and B units are convincing however, especially toward the base of the interval. Interpretation of westward-directed depositional topography on shale clinoforms at these stratigraphic levels to localize the basal Waterways C carbonates in the Eastern Shelf is also reasonable. It seems possible that an interpretation of low-angle, west-dipping clinoforms in the Waterways A and B units that helped to initialize subsequent shallow carbonate deposition in a deeper basin setting at the same time as progradation and exposure at the top of the lower atoll stage in an elevated Swan Hills complex reconciles the depositional geometries and facies distributions apparent in the BHL1 and BHL2 sequences.

The Waterways D lowstand interval is part of a regression that appears to be detached from its correlative unconformity; however, this perception is largely a function of well spacing. Well-log correlations between the Eastern Shelf area and the Waterways Basin are aided by facies changes that occur over longer distances commensurate with the average well spacing in that area. On the other hand, facies changes and stratal relationships between the eastern margin of the Swan Hills complex and the Waterways units occur over distances much shorter than the well spacing, and consequently the details of these relationships are generally not apparent or only partially apparent. When positioning the BHL3.1 sequence boundary in the Eastern Shelf area, the application of local criteria such as subaerial exposure surfaces, scoured surfaces, abrupt shallow-over-deep facies juxtapositions, and whether or not the correlative conformity merges with the basinward terminus of an unconformity can lead to different interpretations in light of both the well data limitations along the margin of the Swan Hills complex and the possibility of an extended hiatus associated with the ISHU.

Unlike the ISHU, which is generally easily recognized within the Swan Hills complex, the BHL3.1 sequence boundary is more difficult to trace laterally in the Eastern Shelf area, regardless of its interpreted stratigraphic position. Its most consistent expression in this study is separation of deeper or less restricted facies below from shallow, evaporitic facies above in the relatively shallow settings of the Eastern Shelf, but this juxtaposition is not always associated with an exposure surface. On the other hand, Figure 19 indicates multiple possible exposure surfaces, green shales, and/or GR log markers representing shale or GSB at various levels within the BHL2 regression (upper Waterways C) and BHL3 lowstand (Waterways D). The deepest GR log markers occur near the base of the Waterways C regression, one corresponding to a 1-m-thick green shale and argillaceous mudstone interval in the 6-4-37-16w4 well, another associated with a GSB interval within a porous marginal succession (lithofacies RA3–RM1, stromatoporoid rudstone with large fragments) at 10-30-36-18w5, and yet another signifying a zone of abnormally argillaceous lagoon and tidal flat mudstone with fenestral voids (lithofacies LTF4a) at 10-34-35-17w4. Additional GR markers representing thin GSB intervals are noted from the upper Waterways D and lower Waterways E intervals associated with flooding successions. In general for the Eastern Shelf area, GR log markers are not always reliable indicators of GSB intervals and do not always form regionally persistent markers. The presence of multiple exposure surfaces that are only locally correlative mixed with intermittent open-marine intervals suggests a sedimentary system that was still responding to finer-scale sea-level fluctuations while Carson Creek North and the rest of the Swan Hills complex were continuously exposed during the ISHU hiatus.

While regional definition of the BHL1, BHL2, and BHL3 sequences remains incompletely resolved, the correlation framework provided in Wendte and Uyeno (2005) contains a comprehensive template of regionally tied log-based units constrained by biostratigraphic data that can serve as a basis for future stratigraphic investigations of the Beaverhill Lake Group. The cross sections in Figures 21 and 22 are examples of application of alternate correlation schemes to this template. The base of an undifferentiated MN3-4 conodont zone noted by Wendte and Uyeno (2005) in the 1-7-36-18w4 core (as annotated in the cross sections) occurs below the top of the Waterways D unit, consistent with the timing for the lowstand interval shown in Figure 3. All other biostratigraphic markers posted in Figures 21 and 22 are consistent with the ISHU lowstand interval occupying parts of the MN2 and MN3 biozones, except for an apparent anomaly at the 5-15-28-3w5 location in Figure 22. In this well, the base of the MN3-4 zone occurs in the Waterways C unit, which conflicts not only with the correlations portrayed in Figure 22, but also with those in Wendte and Uyeno (2005). If the results from 5-15 were honored in either set of correlations, they would tend to invalidate the use of GR logs as correlation tools, not to mention imply that most of the Waterways Formation postdates the ISHU.

The interval above the Waterways D lowstand features eastward-retreating carbonate shelves and isolated buildups in the bottom portion of the Waterways E unit, and it includes the backstepping ramp cycles of the Carson Creek North shoal stage. The Swan Hills complex and Eastern Shelf areas both flooded and were accumulating shale toward the end of Beaverhill Lake deposition; however, using the top of the Beaverhill Lake Group as a datum gives an impression that the basin was filled, or at least had a flat floor. Residual depositional topography probably still existed during deposition of the upper Waterways unit, including drape over the Swan Hills shelf complex and buildups, an underfilled low within the Waterways Basin (Figs. 20E and 21), and low-angle, westward-dipping clinoforms that may have played a role in aligning shallow-water carbonates in the overlying Woodbend Group with the Eastern Shelf margins in the Beaverhill Lake Group (see Potma et al. 2002, their Fig. 2).

With enough well data, the finer-scale Waterways D clinoforms are apparent regardless of which regional marker is used as a cross-section datum. The Waterways D unit is also notably more argillaceous on GR logs than most of the Waterways C unit or the overlying Waterways E unit, favoring the model in which lowstand conditions and regressive episodes tended to promote increased flux of fine clastics into the western Waterways Basin. The local regressive architecture associated with the Waterways D interval may also be expressed more regionally within the Western Canada Basin. The appearance of shallow-water stromatoporoids and other fauna near the base of the Moberly Member noted by Schneider et al. (2013) in the Waterways Formation north of the study area is perhaps worth comparing with the Waterways D lowstand interval in future studies of the Beaverhill Lake Group sequences.

SUMMARY AND CONCLUSIONS

The intrabasin shelf lowstand and equivalent basinal succession comprising the Waterways D unit reconcile divergent transgressive–regressive architecture and stratigraphic thickness patterns in the Swan Hills and Eastern Shelf areas of the Beaverhill Lake Group in the southern Western Canada Basin, by incorporating elements of approximately east–west clinoform geometries and active differential subsidence into well-log correlations. The modified Beaverhill Lake Group sequences identified in this study allow for the existence of clinoforms related to both fine- and regional-scale carbonate–shale sedimentary processes at specific stratigraphic levels, within a framework of original basin topography, transient depositional topography, and regional subsidence variations during deposition. As a result, however, no single regional surface within the Beaverhill Lake Group can provide a satisfactory correlation datum within the study area to correctly portray the depositional history of the interval, a situation that contributes to ambiguity regarding the number of regional sequences and the location of their boundaries. Recognition of a possible ISHU hiatus and a correlative lowstand interval in the Eastern Shelf area provides an additional guideline for regional intrabasin correlation in the Beaverhill Lake Group of western Canada, as well as documenting a potentially significant Frasnian sea-level event that perhaps could be recognized elsewhere. Specific conclusions presented in this study include the following:

  • (1) A high-resolution stratigraphic framework developed within the Carson Creek North Swan Hills buildup demonstrates that the ISHU is associated with at least ~13 m of local paleotopographic relief that influenced the distribution of depositional environments at the base of the overlying unit (shoal stage). The hiatus associated with the ISHU is contemporaneous with deposition of a regional intrabasin lowstand interval represented mainly within the Waterways D unit of the Beaverhill Lake Group. The lowstand interval is part of a detached regressive system in the Eastern Shelf area that is absent across the ISHU at Carson Creek North.

  • (2) Three regional Beaverhill Lake Group sequences (BHL1, BHL2, and BHL3, modified after Potma et al., 2001, 2002) are present in the Carson Creek North buildup and in shallow carbonate facies of the Eastern Shelf area. The top of the BHL2 sequence (BHL3.1 sequence boundary) is formed by the ISHU. In the Eastern Shelf area and Waterways Basin, the BHL3.1 sequence boundary occurs at the base of the Waterways D unit, marked by a pronounced basinward shift in the position of the shelf margin coincident with abrupt development of restricted and evaporitic shelf interior deposits in the Eastern Shelf area. The associated lowstand interval is contemporaneous with westward clinoform progradation into the Waterways Basin.

  • (3) The differences in stratigraphic architecture between Carson Creek North and the rest of the Swan Hills complex, and the Eastern Shelf area reflect a combination of original basin-floor topography, and regional differential subsidence. Leading up to the ISHU, the distributions of the Swan Hills platform member and lower atoll stage representing the BHL1 sequence at Carson Creek North, and the Waterways A and B units representing the BHL1 sequence in the Eastern Shelf area were influenced mainly by an eastward-deepening basin-floor profile. The BHL1 sequence filled in some of the original basin-floor topography, but positive relief persisted in the Swan Hills area, allowing a regressive interval to form at the top of the BHL2 sequence in the Eastern Shelf area. An increase in accommodation in the Waterways Basin and Eastern Shelf areas allowed the BHL3 lowstand to accumulate around the Swan Hills complex, while prolonged subaerial exposure resulted in the formation of paleotopography on the ISHU at Carson Creek North. The remainder of the BHL3 sequence (Waterways E, F, and upper Waterways units) is associated with regional flooding of the lowstand shelf areas, more or less contemporaneous with deposition of backstepping ramp cycles in the Carson Creek North shoal stage.

ACKNOWLEDGMENTS

The author acknowledges ExxonMobil Development Company (Houston) and ExxonMobil Canada (Calgary) for their assistance with permission to publish these data. Charles Kerans, Steve Bachtel, Murray Gilhooly, and Ken Potma are thanked for critiques of an early manuscript draft that helped greatly to focus and improve the final version. Toni Simo is to be credited for his review of a subsequent draft, and a special debt of gratitude is owed to Ted Playton for his many helpful suggestions throughout the editing process.

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Figures & Tables

Fig. 1.

—Index map showing the distribution of carbonates in the Swan Hills shelf complex and the Eastern Shelf area, including wells and cores used or referenced in the study. Regional well-log cross sections that appear in Figures 21 and 22 are indicated. The Waterways Formation downlap trends are from Wendte and Uyeno (2005). The westward extent of the Firebag Member is a depositional marker indicating positive basin-floor relief under the Swan Hills complex (see Figs. 21 and 22). Well names represent their locations within the Alberta grid system. The grid system consists of square townships (6 by 6 miles, 9.7 by 9.7 km), each containing 36 sections, with each section divided into 16 legal subdivisions (LSD). Township numbers increase in a northward direction, with their longitudinal positions indicated by westward-increasing range numbers referenced to north-south meridians (w4 and w5). The well name “1-1-50-20w4,” for example, specifies the LSD, section, township number, range number, and reference meridian containing the well.

Fig. 1.

—Index map showing the distribution of carbonates in the Swan Hills shelf complex and the Eastern Shelf area, including wells and cores used or referenced in the study. Regional well-log cross sections that appear in Figures 21 and 22 are indicated. The Waterways Formation downlap trends are from Wendte and Uyeno (2005). The westward extent of the Firebag Member is a depositional marker indicating positive basin-floor relief under the Swan Hills complex (see Figs. 21 and 22). Well names represent their locations within the Alberta grid system. The grid system consists of square townships (6 by 6 miles, 9.7 by 9.7 km), each containing 36 sections, with each section divided into 16 legal subdivisions (LSD). Township numbers increase in a northward direction, with their longitudinal positions indicated by westward-increasing range numbers referenced to north-south meridians (w4 and w5). The well name “1-1-50-20w4,” for example, specifies the LSD, section, township number, range number, and reference meridian containing the well.

Fig. 2.

—Beaverhill Lake Group sequences, Waterways Formation correlation units, and Carson Creek North reef stages, including the interpreted intrabasin lowstand interval corresponding to the Swan Hills ISHU.

Fig. 2.

—Beaverhill Lake Group sequences, Waterways Formation correlation units, and Carson Creek North reef stages, including the interpreted intrabasin lowstand interval corresponding to the Swan Hills ISHU.

FIG. 3.

—Stratigraphic chart showing relationships among the Swan Hills units in this study, members and correlation units of the Waterways Formation, and the architecture of the Eastern Shelf carbonates. The Frasnian Montaigne Noire (MN) standard conodont zonation referenced in the text is shown, as applied to the Beaverhill Lake Group by Wendte and Uyeno (2005). Absolute ages and sea-level data scaled to the conodont zones are from Becker et al. (2012). Beaverhill Lake Group sequence terminology is derived from Potma et al. (2001, 2002) and Wong et al. (2016). Note the eustatic sea-level falls approximately coincident with the BHL2.1 and 3.1 sequence boundaries.

FIG. 3.

—Stratigraphic chart showing relationships among the Swan Hills units in this study, members and correlation units of the Waterways Formation, and the architecture of the Eastern Shelf carbonates. The Frasnian Montaigne Noire (MN) standard conodont zonation referenced in the text is shown, as applied to the Beaverhill Lake Group by Wendte and Uyeno (2005). Absolute ages and sea-level data scaled to the conodont zones are from Becker et al. (2012). Beaverhill Lake Group sequence terminology is derived from Potma et al. (2001, 2002) and Wong et al. (2016). Note the eustatic sea-level falls approximately coincident with the BHL2.1 and 3.1 sequence boundaries.

FIG. 4.

—Symbols and color legend for Swan Hills and Beaverhill Lake lithofacies and environments used throughout this study.

FIG. 4.

—Symbols and color legend for Swan Hills and Beaverhill Lake lithofacies and environments used throughout this study.

Fig. 5.

—Lithofacies and environments in the Swan Hills complex and Eastern Shelf area. A) Dark-colored Amphipora floatstone (lithofacies LTF1) representing subtidal lagoons; 9-10-62-12w5, 2668.2 m. B) Light-colored lagoonal grainstone with Amphipora, lithofacies LTF2, 6-9-62-12w5, 8582 ft. (2615.8 m) C) Bedded reef flat grainstone with stromatoporoid fragments (lithofacies RM1), 6-31-61-11w5, 8656 ft. (2638.3 m) D) Massive stromatoporoid boundstone, reef margin (lithofacies RM2), 16-6-62-11w5, 8777 ft. (2675.2 m) E) Lithofacies RM2 boundstone with branching and tabular stromatoporoids, 6-31-61-11w5, 8710 ft. (2654.8 m) F) Upper foreslope boundstone (lithofacies FS1) with tabular stromatoporoids (t) and brachiopods (b), 16-6-62-11w5, 8811 ft. (2685.6 m) G) Floatstone with bulbous and cylindrical stromatoporoids (lithofacies FS2) and micrite matrix, lower to middle foreslope, 6-36-61-12w5, 8653 ft. (2637.4 m) H) Nodular skeletal wackestone–packstone (lithofacies SB3) with gastropods (g), Thamnopora (th), and brachiopod shell fragments (b), 16-36-41-23w4, 2142 m. I) Basinal nodular mudstone–wackestone (lithofacies SB2) with brachiopods (b), 9-15-62-12w5, 8719 ft. (2657.6 m) J) Vuggy ramp margin rudstone with stromatoporoid fragments (s) and grainstone matrix (lithofacies RA2), 16-11-62-12w5, 8747 ft. (2666.1 m) Some stromatoporoids (arrows) have light-colored micrite or oncolite coatings. K) Lithofacies (RA3) in low-energy ramp environments containing rounded stromatoporoid fragments (s), Amphipora (a), and cylindrical stromatoporoids (c), 8-2-62-12w5, 2637.9 m. L) Stromatolitic mudstone (lithofacies SI3) with authigenic anhydrite (an) from a restricted environment in the Eastern Shelf area, 10-30-36-16w4, 6095 ft. (1857.8 m) M) Green shale breccia (GSB) from the ISHU, 9-10-62-12w5, 2638.0 m. N) Submarine scour (lithofacies SS1) with rounded gray intraclasts matching the lithology below the scour surface (dashed line), 10-34-35-17w4, 6080 ft. (1853.2 m) All scale bars = 2 cm.

Fig. 5.

—Lithofacies and environments in the Swan Hills complex and Eastern Shelf area. A) Dark-colored Amphipora floatstone (lithofacies LTF1) representing subtidal lagoons; 9-10-62-12w5, 2668.2 m. B) Light-colored lagoonal grainstone with Amphipora, lithofacies LTF2, 6-9-62-12w5, 8582 ft. (2615.8 m) C) Bedded reef flat grainstone with stromatoporoid fragments (lithofacies RM1), 6-31-61-11w5, 8656 ft. (2638.3 m) D) Massive stromatoporoid boundstone, reef margin (lithofacies RM2), 16-6-62-11w5, 8777 ft. (2675.2 m) E) Lithofacies RM2 boundstone with branching and tabular stromatoporoids, 6-31-61-11w5, 8710 ft. (2654.8 m) F) Upper foreslope boundstone (lithofacies FS1) with tabular stromatoporoids (t) and brachiopods (b), 16-6-62-11w5, 8811 ft. (2685.6 m) G) Floatstone with bulbous and cylindrical stromatoporoids (lithofacies FS2) and micrite matrix, lower to middle foreslope, 6-36-61-12w5, 8653 ft. (2637.4 m) H) Nodular skeletal wackestone–packstone (lithofacies SB3) with gastropods (g), Thamnopora (th), and brachiopod shell fragments (b), 16-36-41-23w4, 2142 m. I) Basinal nodular mudstone–wackestone (lithofacies SB2) with brachiopods (b), 9-15-62-12w5, 8719 ft. (2657.6 m) J) Vuggy ramp margin rudstone with stromatoporoid fragments (s) and grainstone matrix (lithofacies RA2), 16-11-62-12w5, 8747 ft. (2666.1 m) Some stromatoporoids (arrows) have light-colored micrite or oncolite coatings. K) Lithofacies (RA3) in low-energy ramp environments containing rounded stromatoporoid fragments (s), Amphipora (a), and cylindrical stromatoporoids (c), 8-2-62-12w5, 2637.9 m. L) Stromatolitic mudstone (lithofacies SI3) with authigenic anhydrite (an) from a restricted environment in the Eastern Shelf area, 10-30-36-16w4, 6095 ft. (1857.8 m) M) Green shale breccia (GSB) from the ISHU, 9-10-62-12w5, 2638.0 m. N) Submarine scour (lithofacies SS1) with rounded gray intraclasts matching the lithology below the scour surface (dashed line), 10-34-35-17w4, 6080 ft. (1853.2 m) All scale bars = 2 cm.

Fig. 6.

—Upper atoll and shoal stages in the Carson Creek North Swan Hills A- and B-pools, based on available well and core control at the time of the study, including the lines of cross sections shown in Figures 9 and 10.

Fig. 6.

—Upper atoll and shoal stages in the Carson Creek North Swan Hills A- and B-pools, based on available well and core control at the time of the study, including the lines of cross sections shown in Figures 9 and 10.

FIG. 7

—A) Schematic diagram showing stratigraphic units and lithofacies distribution (see Fig. 4 for legend) in the Carson Creek North buildup, including surfaces for which subaerial exposure has been observed or interpreted from cores. The ISHU is exposed across the buildup, whereas local exposure is indicated schematically for multiple levels in the lower atoll stage. B) Stratigraphic model of the Judy Creek buildup showing the R4 and R0.5 markers discussed in the text, modified from an illustration in Wendte and Uyeno (2005) to match the vertical scale of Carson Creek in A). Double vertical lines indicate well control, and black bars indicate core control for the Judy Creek model. Note similarities between the Judy Creek R0, R0.5, and R4 markers and the Carson Creek North M1c, A1c, and ISHU surfaces, respectively, in relationship to the overall stratigraphic architecture of the two buildups.

FIG. 7

—A) Schematic diagram showing stratigraphic units and lithofacies distribution (see Fig. 4 for legend) in the Carson Creek North buildup, including surfaces for which subaerial exposure has been observed or interpreted from cores. The ISHU is exposed across the buildup, whereas local exposure is indicated schematically for multiple levels in the lower atoll stage. B) Stratigraphic model of the Judy Creek buildup showing the R4 and R0.5 markers discussed in the text, modified from an illustration in Wendte and Uyeno (2005) to match the vertical scale of Carson Creek in A). Double vertical lines indicate well control, and black bars indicate core control for the Judy Creek model. Note similarities between the Judy Creek R0, R0.5, and R4 markers and the Carson Creek North M1c, A1c, and ISHU surfaces, respectively, in relationship to the overall stratigraphic architecture of the two buildups.

Fig. 8.

—Stratigraphic nomenclature and Swan Hills reef stages in a Carson Creek North “type well,” illustrating typical facies, GR log, and porosity log (sonic, neutron, or density) patterns in the buildup interior. The Core Phi track contains porosity data from core analysis (scale: 1 division = 2% porosity) used to assist with calibration of core facies to logs and stratigraphic correlation. The stratigraphic framework also shows the interpreted Carson Creek North equivalents of the Judy Creek R0.5 and R4 markers as discussed in the text. The facies legend for symbols is given in Figure 4.

Fig. 8.

—Stratigraphic nomenclature and Swan Hills reef stages in a Carson Creek North “type well,” illustrating typical facies, GR log, and porosity log (sonic, neutron, or density) patterns in the buildup interior. The Core Phi track contains porosity data from core analysis (scale: 1 division = 2% porosity) used to assist with calibration of core facies to logs and stratigraphic correlation. The stratigraphic framework also shows the interpreted Carson Creek North equivalents of the Judy Creek R0.5 and R4 markers as discussed in the text. The facies legend for symbols is given in Figure 4.

Fig. 9.

—A) East-west stratigraphic correlation across the eastern margin of the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Fig. 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The cross section is continued in Figure 9B, with the 14-1 and 16-1 wells repeated for cross-reference. B) Continuation of the cross section in A). See Figure 6 for the line of section location.

Fig. 9.

—A) East-west stratigraphic correlation across the eastern margin of the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Fig. 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The cross section is continued in Figure 9B, with the 14-1 and 16-1 wells repeated for cross-reference. B) Continuation of the cross section in A). See Figure 6 for the line of section location.

Fig. 10.

—North-south stratigraphic correlation across the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Figure 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The 8-3 well is also included in Figure 9A. See Figure 6 for the line of section location.

Fig. 10.

—North-south stratigraphic correlation across the Carson Creek North A-pool using logs of core porosity data (core PHI) and GR logs (GR). The top of the A1c unit is used as an approximate datum. Vertical well depths are in meters (m, posted every 10 m) or feet (ft, posted every 50 ft) as indicated. Lithofacies described from core (see Figure 4 for legend) are plotted next to the depth track. Horizontal arrows indicate features in core associated with shallowing and possible exposure, as discussed in the text. The 8-3 well is also included in Figure 9A. See Figure 6 for the line of section location.

Fig. 11.

—Cored example of a buildup interior shallowing-upward cycle in the Carson Creek North upper atoll stage (1-18-62-12w5, ~2727 m), showing thin vuggy zones and pendant cements approaching the cycle tops. Cycle tops (indicated by solid lines) are overlain by subtidal lagoons indicated by lithofacies LTF1. Other lithofacies variations (see legend in Fig. 4) are indicated by dashed lines. The core “up–direction” is shown by the large arrow and scale bar.

Fig. 11.

—Cored example of a buildup interior shallowing-upward cycle in the Carson Creek North upper atoll stage (1-18-62-12w5, ~2727 m), showing thin vuggy zones and pendant cements approaching the cycle tops. Cycle tops (indicated by solid lines) are overlain by subtidal lagoons indicated by lithofacies LTF1. Other lithofacies variations (see legend in Fig. 4) are indicated by dashed lines. The core “up–direction” is shown by the large arrow and scale bar.

Fig. 12.

—A) Type A reef interior cycle in core (16-35-61-12w5, ~8680 ft (2645.7 m)), showing subtidal lagoons indicated by nonporous, dark brown Amphipora floatstone (lithofacies LTF1). B) Cored example of a type B interior cycle with porous grainstone in lithofacies RA2 and LTF3a (6-9-62-12w5, ~8568 ft (2611.5 m)). Most of the visible macrofossils are Amphipora, mixed with a few larger stromatoporoid fragments (small arrows). Note that subtidal lithofacies (LTF1–LTF2) are absent at the base of the cycle, resulting in more continuous porosity across cycles. Cycle tops are indicated in both examples by solid white lines, with lithofacies changes indicated by dashed lines. Yellow markers were used during core description and have no particular meaning. The core “up-direction” is shown by the large arrows.

Fig. 12.

—A) Type A reef interior cycle in core (16-35-61-12w5, ~8680 ft (2645.7 m)), showing subtidal lagoons indicated by nonporous, dark brown Amphipora floatstone (lithofacies LTF1). B) Cored example of a type B interior cycle with porous grainstone in lithofacies RA2 and LTF3a (6-9-62-12w5, ~8568 ft (2611.5 m)). Most of the visible macrofossils are Amphipora, mixed with a few larger stromatoporoid fragments (small arrows). Note that subtidal lithofacies (LTF1–LTF2) are absent at the base of the cycle, resulting in more continuous porosity across cycles. Cycle tops are indicated in both examples by solid white lines, with lithofacies changes indicated by dashed lines. Yellow markers were used during core description and have no particular meaning. The core “up-direction” is shown by the large arrows.

Fig. 13.

—Lithofacies (legend in Fig. 4) and log porosity profiles associated with type A and Type B shallowing-upward cycles in the Carson Creek North buildup interior, illustrating how repetitions of each cycle type manifest as distinct vertical porosity patterns. Type B cycles are dominated by interparticle pores in grainstone facies, while type A cycles often feature a thin vuggy interval toward the top of the cycle. In type B cycles, reduced porosity is associated with thin muddy tidal flats (lithofacies LTF4a and LTF4b) at the top of the cycle, or with thin subtidal lagoons (lithofacies LTF2) at the base.

Fig. 13.

—Lithofacies (legend in Fig. 4) and log porosity profiles associated with type A and Type B shallowing-upward cycles in the Carson Creek North buildup interior, illustrating how repetitions of each cycle type manifest as distinct vertical porosity patterns. Type B cycles are dominated by interparticle pores in grainstone facies, while type A cycles often feature a thin vuggy interval toward the top of the cycle. In type B cycles, reduced porosity is associated with thin muddy tidal flats (lithofacies LTF4a and LTF4b) at the top of the cycle, or with thin subtidal lagoons (lithofacies LTF2) at the base.

FIG. 14.

—Depositional environments interpreted for the A3 megacycle (upper atoll stage), based on the dominant lithofacies comprising interior cycles in the bottom portion of the megacycle.

FIG. 14.

—Depositional environments interpreted for the A3 megacycle (upper atoll stage), based on the dominant lithofacies comprising interior cycles in the bottom portion of the megacycle.

FIG. 15.

—Depositional and diagenetic features associated with the ISHU at Carson Creek North. A) Green shale breccia with cream-colored limestone clasts in a matrix of green shale and dark brown micrite; 16-10-62-12w5, 8689 ft. (2648.4 m) B) Cored example of the “cream unit” with elongate cemented voids (arrows) often developed in lithofacies LTF4b (see legend in Fig. 4), localized oxidation (red-brown coloration), and cement-filled shell molds (s); 6-9-62-12w5, 8493 ft. (2586.7 m) C) Matrix dissolution voids filled with coarse calcite cements from the cream unit, with Amphipora (arrows) and larger stromatoporoid fragments (s) indicated; 1-18-62-12w5, 2694.7 m. D) Thin beds of green shale breccia, abundant stylolites, and gray-green clay seams associated with the ISHU in lithofacies LTF4a (see legend Fig. 4); 6-1-62-12w5, 8806 ft. (2684.1 m) E) Reef margin stromatoporoid rudstone below the ISHU with pendant cements (arrows), oxide minerals (red-brown color), and dissolution voids layered with light green micrite (m), fine-grained dolomite (d), and coarse calcite cement (c); 6-34-61-12w5, 8488 ft. (2587.1 m) F) Close-up photo stained with Alizarin Red-S to show the distribution of dolomite. G) Reef flat to reef margin stromatoporoid rudstone with geopetal sediment and cement in irregular dissolution voids below the ISHU. Stromatoporoid fragments (s) are rounded, and some are associated with thin pendant cement (not visible in the photo); 9-9-62-12w5, 2608.9 m. All scale bars = 2 cm.

FIG. 15.

—Depositional and diagenetic features associated with the ISHU at Carson Creek North. A) Green shale breccia with cream-colored limestone clasts in a matrix of green shale and dark brown micrite; 16-10-62-12w5, 8689 ft. (2648.4 m) B) Cored example of the “cream unit” with elongate cemented voids (arrows) often developed in lithofacies LTF4b (see legend in Fig. 4), localized oxidation (red-brown coloration), and cement-filled shell molds (s); 6-9-62-12w5, 8493 ft. (2586.7 m) C) Matrix dissolution voids filled with coarse calcite cements from the cream unit, with Amphipora (arrows) and larger stromatoporoid fragments (s) indicated; 1-18-62-12w5, 2694.7 m. D) Thin beds of green shale breccia, abundant stylolites, and gray-green clay seams associated with the ISHU in lithofacies LTF4a (see legend Fig. 4); 6-1-62-12w5, 8806 ft. (2684.1 m) E) Reef margin stromatoporoid rudstone below the ISHU with pendant cements (arrows), oxide minerals (red-brown color), and dissolution voids layered with light green micrite (m), fine-grained dolomite (d), and coarse calcite cement (c); 6-34-61-12w5, 8488 ft. (2587.1 m) F) Close-up photo stained with Alizarin Red-S to show the distribution of dolomite. G) Reef flat to reef margin stromatoporoid rudstone with geopetal sediment and cement in irregular dissolution voids below the ISHU. Stromatoporoid fragments (s) are rounded, and some are associated with thin pendant cement (not visible in the photo); 9-9-62-12w5, 2608.9 m. All scale bars = 2 cm.

Fig. 16.

—Cored examples of the S1L unit and the Carson Creek North shoal stage. A) Subtidal lagoon in the interior of the S1L unit, represented by dark brown, nodular Amphipora floatstone (lithofacies LTF1), 14-16-62-12w5, 2603.2 m. B) Dark-colored, fine-laminated mudstone and light brown stromatolitic tidal flat mudstone overlying the ISHU (marked by the green shale breccia at the bottom of the photo), 8-2-62-12w5, 2642.5 m. C) Stromatoporoid rudstone (lithofacies LTF3b-RM1) with pendant cements (arrows) below a sharp surface at the top of the S1L unit, 16-11-62-12w5, 8750 ft. (2667.0 m) D) Columnar stromatolite growth (arrows) on top of a cemented Amphipora rudstone (lithofacies LTF3b) at the top of the S1L unit, 6-9-62-12w5, 8470 ft. (2581.7 m) E) Sharp contact between the “cream unit” and the basal shoal stage ramp cycle along the ISHU (solid white line) in 8-12-62-13w5, 2744.0 m. The S1L unit is missing at this location, and the ISHU is encrusted with coarse-laminated micrite (m, dashed line). The overlying ramp cycle contains grainstone and stromatoporoid fragments (lithofacies RA2). F) Sharp contact at the top of the shoal stage encrusted with pyrite (p) and overlain by dark gray-brown basinal shale in the Waterways Formation; 9-10-62-12w5, 2621.0 m. All scale bars = 2 cm. See Figure 4 for lithofacies descriptions.

Fig. 16.

—Cored examples of the S1L unit and the Carson Creek North shoal stage. A) Subtidal lagoon in the interior of the S1L unit, represented by dark brown, nodular Amphipora floatstone (lithofacies LTF1), 14-16-62-12w5, 2603.2 m. B) Dark-colored, fine-laminated mudstone and light brown stromatolitic tidal flat mudstone overlying the ISHU (marked by the green shale breccia at the bottom of the photo), 8-2-62-12w5, 2642.5 m. C) Stromatoporoid rudstone (lithofacies LTF3b-RM1) with pendant cements (arrows) below a sharp surface at the top of the S1L unit, 16-11-62-12w5, 8750 ft. (2667.0 m) D) Columnar stromatolite growth (arrows) on top of a cemented Amphipora rudstone (lithofacies LTF3b) at the top of the S1L unit, 6-9-62-12w5, 8470 ft. (2581.7 m) E) Sharp contact between the “cream unit” and the basal shoal stage ramp cycle along the ISHU (solid white line) in 8-12-62-13w5, 2744.0 m. The S1L unit is missing at this location, and the ISHU is encrusted with coarse-laminated micrite (m, dashed line). The overlying ramp cycle contains grainstone and stromatoporoid fragments (lithofacies RA2). F) Sharp contact at the top of the shoal stage encrusted with pyrite (p) and overlain by dark gray-brown basinal shale in the Waterways Formation; 9-10-62-12w5, 2621.0 m. All scale bars = 2 cm. See Figure 4 for lithofacies descriptions.

FIG. 17

.—A) Thickness of the upper atoll stage above the A2 megacycle, used to demonstrate ISHU paleotopography. The map does not precisely represent the BHL2 sequence, but for a reliable map, the top of the A2 megacycle is the best reference log marker below the ISHU. Note that the depositional limit of the shoal stage is aligned with remnant thicknesses in the upper atoll stage. B) Lithofacies in the A5 megacycle below the ISHU, demonstrating the absence of subtidal lagoons. C) Depositional environments in the S1L unit at the base of the shoal stage, showing the absence of a reef margin. Lateral continuity of the reef flat area is broken up by absences of the S1L unit along the northern and eastern margins of the south buildup.

FIG. 17

.—A) Thickness of the upper atoll stage above the A2 megacycle, used to demonstrate ISHU paleotopography. The map does not precisely represent the BHL2 sequence, but for a reliable map, the top of the A2 megacycle is the best reference log marker below the ISHU. Note that the depositional limit of the shoal stage is aligned with remnant thicknesses in the upper atoll stage. B) Lithofacies in the A5 megacycle below the ISHU, demonstrating the absence of subtidal lagoons. C) Depositional environments in the S1L unit at the base of the shoal stage, showing the absence of a reef margin. Lateral continuity of the reef flat area is broken up by absences of the S1L unit along the northern and eastern margins of the south buildup.

Fig. 18.

—Schematic diagrams illustrating the development of paleotopography on the ISHU at Carson Creek North. A) Deposition and highstand lagoon fill at the end of the upper atoll stage A5 megacycle. B) sea-level fall and denudation/erosion to form the ISHU. C) Subsequent sea-level rise and deposition of the S1L unit, with facies and environments controlled by ISHU paleotopography. D) Continued transgression and deposition of shoal stage high-energy ramp cycles.

Fig. 18.

—Schematic diagrams illustrating the development of paleotopography on the ISHU at Carson Creek North. A) Deposition and highstand lagoon fill at the end of the upper atoll stage A5 megacycle. B) sea-level fall and denudation/erosion to form the ISHU. C) Subsequent sea-level rise and deposition of the S1L unit, with facies and environments controlled by ISHU paleotopography. D) Continued transgression and deposition of shoal stage high-energy ramp cycles.

Fig. 19.

—Stratigraphic section illustrating the transgressive–regressive architecture associated with the Eastern Shelf. Well logs shown are sonic (SON) for porosity and gamma-ray (GR). The top of the Waterways D unit is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Lithofacies described from core (see legend in Fig. 4) are plotted next to the depth track. Horizontal red arrows indicate features in core associated with possible subaerial exposure, as described in the text. See Figure 1 for well locations.

Fig. 19.

—Stratigraphic section illustrating the transgressive–regressive architecture associated with the Eastern Shelf. Well logs shown are sonic (SON) for porosity and gamma-ray (GR). The top of the Waterways D unit is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Lithofacies described from core (see legend in Fig. 4) are plotted next to the depth track. Horizontal red arrows indicate features in core associated with possible subaerial exposure, as described in the text. See Figure 1 for well locations.

Fig. 20.

—Schematic evolution of two-dimensional (2D) basin-floor topography and sea level, accounting for regional thickness trends and carbonate-shale facies patterns in the Beaverhill Lake Group using example wells from Carson Creek North (CCN), the Waterways Basin (WB), and the Eastern Shelf area (ESA). A) Elevated basin floor underlying the BHL1 sequence in the Carson Creek area, with sea-level fall and exposure of the Swan Hills complex promoting initiation of carbonate deposition above west-dipping basinal clinoforms in ESA. B) sea-level rise combined with possible basin-floor subsidence in the ESA, resulting in Swan Hills backstepping in the CCN BHL2 sequence coincident with near-drowning of the ESA carbonates. Depositional bathymetry increased in the WB as a result. C) sea-level fall combined with possible uplift along the West Alberta Ridge, resulting in exposure of the BHL2 sequence and formation of the ISHU, coincident with maximum westward development of shelf carbonates in the ESA at the base of the lowstand. D) Increased basin-floor subsidence in the ESA combined with rising sea level and initial transgression of the ISHU after deposition of the Waterways D unit lowstand. E) Regional flooding at CCN and the ESA resulting in termination of carbonate deposition during the BHL3 sequence, with depositional bathymetry due to drape over CCN and a residual low in the WB caused by low-angle, west-dipping basinal clinoforms. Accommodation in both areas may have been provided by rapid sea-level rise following the ISHU combined with onset of a more regional rapid subsidence regime.

Fig. 20.

—Schematic evolution of two-dimensional (2D) basin-floor topography and sea level, accounting for regional thickness trends and carbonate-shale facies patterns in the Beaverhill Lake Group using example wells from Carson Creek North (CCN), the Waterways Basin (WB), and the Eastern Shelf area (ESA). A) Elevated basin floor underlying the BHL1 sequence in the Carson Creek area, with sea-level fall and exposure of the Swan Hills complex promoting initiation of carbonate deposition above west-dipping basinal clinoforms in ESA. B) sea-level rise combined with possible basin-floor subsidence in the ESA, resulting in Swan Hills backstepping in the CCN BHL2 sequence coincident with near-drowning of the ESA carbonates. Depositional bathymetry increased in the WB as a result. C) sea-level fall combined with possible uplift along the West Alberta Ridge, resulting in exposure of the BHL2 sequence and formation of the ISHU, coincident with maximum westward development of shelf carbonates in the ESA at the base of the lowstand. D) Increased basin-floor subsidence in the ESA combined with rising sea level and initial transgression of the ISHU after deposition of the Waterways D unit lowstand. E) Regional flooding at CCN and the ESA resulting in termination of carbonate deposition during the BHL3 sequence, with depositional bathymetry due to drape over CCN and a residual low in the WB caused by low-angle, west-dipping basinal clinoforms. Accommodation in both areas may have been provided by rapid sea-level rise following the ISHU combined with onset of a more regional rapid subsidence regime.

Fig. 21.

—Stratigraphic correlation using GR logs between the Eastern Shelf area and Carson Creek North, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and Carson Creek North shown in B). The 6-15-48-3w5 and 15-35-44-2w5 wells are repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum, except at Carson Creek North, where the top of the Beaverhill Lake Group is used, positioned so that the Carson Creek North ISHU is approximately level with the top of the Waterways D unit in the Eastern Shelf area. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are shown with black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward shelf margin step-out and west-dipping clinoforms in the Waterways D lowstand unit.

Fig. 21.

—Stratigraphic correlation using GR logs between the Eastern Shelf area and Carson Creek North, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and Carson Creek North shown in B). The 6-15-48-3w5 and 15-35-44-2w5 wells are repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum, except at Carson Creek North, where the top of the Beaverhill Lake Group is used, positioned so that the Carson Creek North ISHU is approximately level with the top of the Waterways D unit in the Eastern Shelf area. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are shown with black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward shelf margin step-out and west-dipping clinoforms in the Waterways D lowstand unit.

Fig. 22.

—Stratigraphic correlation using GR logs between the Eastern Shelf and the Stewart-Twining channel, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and the Stewart-Twining area shown in B). The 16-36 well location is repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are indicated by black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward step-out of shelf carbonates (gray-shaded intervals), and west-dipping clinoforms in the interpreted Waterways D unit. Gray arrows next to the 8-13-27-3w5 well in the Stewart-Twining channel mark allochthonous lithofacies (AL1, AL2, AL3; see Fig. 4) noted in the core that were probably derived from Swan Hills shelf carbonates making up the BHL2 sequence adjacent to the channel (see Fig. 1).

Fig. 22.

—Stratigraphic correlation using GR logs between the Eastern Shelf and the Stewart-Twining channel, across the Waterways Basin (see Fig. 1 for location of the section line). The section is split, with the Eastern Shelf wells from Figure 19 shown in A) and the Stewart-Twining area shown in B). The 16-36 well location is repeated for cross-reference. The BHL2.1 sequence boundary (top of the Waterways B unit) is used as an approximate datum. Vertical well depths are given in meters (m) or feet (ft) as indicated. Cored intervals are indicated by black bars, including biostratigraphic markers from Wendte and Uyeno (2005) as described in the text. Note the basinward step-out of shelf carbonates (gray-shaded intervals), and west-dipping clinoforms in the interpreted Waterways D unit. Gray arrows next to the 8-13-27-3w5 well in the Stewart-Twining channel mark allochthonous lithofacies (AL1, AL2, AL3; see Fig. 4) noted in the core that were probably derived from Swan Hills shelf carbonates making up the BHL2 sequence adjacent to the channel (see Fig. 1).

Table 1.

—Stratigraphic tops data and examined core intervals for regional wells penetrating the Elk Point/Watt Mountain Formations. Well locations are plotted in Figure 1. Interval tops are listed as measured depths, in feet (ft) or meters (m) according to the well’s original measurement units in column 3. Thickness values for the Beaverhill Lake (BHL) Group, and the BHL1, BHL2, and BHL3 regional sequences were calculated from the data in the columns on the right.

Well location Area Units Beaverhill Lake Gp. Swan Hills Fm. ISHU (BHL3.1) Lower atoll stage (BHL2.1) Platform member Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lower atoll stage 
Swan Hills Complex 
6-1-62-12w5 CCN A-Pool ft 8667 8754 8809 8905 8993 9125 8804–9000 140 43 29 67 27 
6-3-62-12w5 CCN A-Pool ft 8506 8547 8607 8720 8797 8935 8550–8823 131 31 34 66 23 
16-11-62-12w5 CCN A-Pool ft 8660 8734 8756 8883 8953 9070 8710–8973 125 29 39 57 21 
6-17-62-12w5 CCN B-Pool ft 8572 8610 8675 8816 8890 9030 8634–8834 140 31 43 65 23 
13-14-52-17w5 Edson 3648 3650 3701 3729 3742 3773 3742–3769 125 53 28 44 13 
9-3-52-17w5 Edson 3728 3738 3785 3816 3830 3859 3760–3835 131 57 31 43 14 
11-33-51-16w5 Edson ft 11,965 12,060 12,092 12,172 12,220 12,340 12,025–12,198 114 39 24 51 15 
6-30-46-17w5 Hanlan N ft 15,265 15,360 15,430 15,515 15,555 15,635 15,438–15,498 113 50 26 37 12 
11-8-47-17w5 Hanlan N ft 15,040 15,100 15,145 15,234 15,270 15,340 15,115–15,252 91 32 27 32 11 
11-19-47-17w5 Hanlan N ft 14,940 15,024 15,071 15,145 15,167 15,235 15,054–15,246 90 40 23 27 
11-22-44-16w5 Hanlan S 4673 4714 4714 4741 4752 4777 4726–4778 104 41 27 36 11 
6-20-44-16w5 Hanlan S 4764 4776 4801 4832 NP 4846 4794–4835 82 37 31 14  
2-15-36-9w5 Strachan 4573 4596 4603 4640 4655 4687 4590–4608 114 30 37 47 15 
10-33-36-10w5 Strachan ft 15,607 15,670 15,725 15,825 15,880 15,950 NC/NE 105 36 30 38 17 
6-12-38-10w5 Strachan 4351 4388 4409 4431 4445 4482 4399–4418 131 58 22 51 14 
6-20-33-4w5 Caroline 3580 3637 3648 3669 3680 3712 3625–3676 132 68 21 43 11 
6-13-33-5w5 Caroline 3698 3736 3757 3782 3791 3825 3730–3797 127 59 25 43 
7-18-34-4w5 Caroline 3586 3645 3657 3680 3691 3723 3655–3708 137 71 23 43 11 
6-29-34-5w5 Caroline 3737 3770 3798 3823 3834 3864 3775–3873 127 61 25 41 11 
10-33-34-5w5 Caroline 3684 3731 3747 3770 3782 3815 3725–3824 131 63 23 45 12 
3-10-35-5w5 Caroline 3652 3705 3716 3743 3758 3792 3710–3766 140 64 27 49 15 
10-15-35-5w5 Caroline 3653 3721 3721 3745 3762 3794 3723–3758 141 68 24 49 17 
5-32-35-5w5 Caroline 3650 3717 3718 3746 3762 3791 3723–3762 141 68 28 45 16 
10-24-31-4w5 Twining 3605 3655 3660 3691 3707 3738 3670–3699 133 55 31 47 16 
15-16-31-5w5 Twining 3959 3998 4015 4046 4058 4088 3995–4023 129 56 31 42 12 
7-32-32-28w4 Stewart 2864 2915 2944 2968 2978 3022 2940–2976 158 80 24 54 10 
Well location Area Units Beaverhill
Lake Gp. 
WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl2.1) Elk Point/Watt Mtn. Core interval BHL Gp. BHL3 BHL2 BHL1 Lowstand 
Waterways Basin 
8-13-27-3w5 WWYS Basin 3635 3666 3675 3696 3743 3761 3696–3753 126 61 47 18 21 
5-15-28-3w5 WWYS Basin 3659 3690 3698 3731 3764 3786 3710–3746 127 72 33 22 33 
6-6-29-2w5 WWYS Basin 3462 3507 3518 3560 3593 3611 NC/NE 149 98 33 18 42 
6-14-33-2w5 WWYS Basin 3115 3153 3162 3207 3228 3266 3138–3157 151 92 21 38 45 
9-17-34-25w4 WWYS Basin ft 7980 8149 8190 8310 8403 8575 8071–8088 181 101 28 52 37 
16-16-37-25w4 WWYS Basin ft 8016 8181 8215 8348 8450 8630 NC/NE 187 101 31 55 41 
6-16-38-3w5 WWYS Basin 3048 3086 3109 3142 3161 3215 NC/NE 167 94 19 54 33 
13-36-39-25w4 WWYS Basin ft 7390 7555 7590 7695 7838 8046 NC/NE 200 93 44 63 32 
Well location Area Units Beaverhill Lake Gp. WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl3.1) Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lowstand 
6-11-40-21w4 WWYS Basin 1940 1991 2004 2031 2080 2146 NC/NE 206 91 49 66 27 
12-14-40-26w4 WWYS Basin 2378 2424 2435 2474 2508 2571 NC/NE 193 96 34 63 39 
9-23-41-9w5 WWYS Basin ft 12,435 12,520 12,550 12,695 12,800 12,944 NC/NE 155 79 32 44 44 
6-20-41-24w4 WWYS Basin ft 7260 7420 7460 7556 7707 7922 NC/NE 202 90 46 66 29 
6-4-41-1w5 WWYS Basin ft 8682 8790 8830 8975 9060 9258 NC/NE 176 89 26 60 44 
7-1-41-3w5 WWYS Basin 2888 2923 2944 2979 3006 3063 NC/NE 175 91 27 57 35 
16-16-42-2w5 WWYS Basin 2695 2730 2753 2789 2819 2877 NC/NE 182 94 30 58 36 
15-35-44-2w5 WWYS Basin 2559 2591 2606 2648 2682 2746 NC/NE 187 89 34 64 42 
10-25-46-2w5 WWYS Basin ft 8162 8270 8340 8450 8575 8789 NC/NE 191 88 38 65 34 
6-15-48-3w5 WWYS Basin ft 8078 8178 8232 8358 8480 8697 NC/NE 189 85 37 66 38 
9-18-49-9w5 WWYS Basin 3065 3103 3125 3153 3174 3227 NC/NE 162 88 21 53 28 
8-30-52-5w5 WWYS Basin 2408 2445 2454 2483 2523 2585 NC/NE 177 75 40 62 29 
6-3-51-7w5 WWYS Basin ft 8645 8765 8824 8923 9005 9208 NC/NE 172 85 25 62 30 
3-18-52-8w5 WWYS Basin ft 9000 9130 9205 9292 9360 9545 NC/NE 166 89 21 56 27 
16-5-55-9w5 WWYS Basin ft 8628 8733 8798 8890 8950 9140 NC/NE 156 80 18 58 28 
10-26-59-9w5 WWYS Basin ft 7550 7651 7718 7795 7897 8042 NC/NE 150 75 31 44 23 
6-29-61-9w5 WWYS Basin ft 7464 7570 7642 7722 7844 7947 NC/NE 147 79 37 31 24 
Eastern Shelf area 
6-5-30-21w4 Stewart 2069 2108 2121 2136 2195 2250 2201–2257 181 67 59 55 15 
8-33-31-22w4 Stewart 2149 2184 2198 2233 2279 2326 2245–2277 177 84 46 47 35 
10-30-32-23w4 Stewart 2295 2338 2354 2370 2430 2482 2372–2426 187 75 60 52 16 
16-27-32-24w4 Stewart 2346 2386 2410 2435 2480 2531 2414–2443 185 89 45 51 25 
6-11-33-24w4 Stewart 2343 2380 2402 2420 2470 2516 2422–2425 173 77 50 46 18 
16-36-41-23w4 Eastern Shelf 2040 2092 2103 2134 2191 2250 2132–2185 210 94 57 59 31 
1-7-36-18w4 Eastern Shelf 1881 1923 1940 1971 2020 2080 1944–1962 199 90 49 60 31 
6-4-37-16w4 Eastern Shelf 1752 1792 1809 1844 1890 1951 1850–1868 199 92 46 61 35 
10-30-36-16w4 Eastern Shelf ft 5806 5946 5985 6095 6304 6465 6060–6180 201 88 64 49 34 
10-34-35-17w4 Eastern Shelf ft 5890 6030 6070 6172 6348 NP 6065–6365  86 54  31 
14-35-34-17w4 Eastern Shelf 1818 1860 1869 1903 1950 2009 NC/NE 191 85 47 59 34 
14-11-39-17w4 Eastern Shelf 1705 1740 1757 1793 1846 1905 NC/NE 200 88 53 59 36 
6-11-40-21W4 Eastern Shelf 1940 1990 2005 2030 2080 2146 NC/NE 206 90 50 66 25 
6-10-41-19W4 Eastern Shelf ft 5820 5985 6012 6130 6280 6500 NC/NE 207 94 46 67 36 
7-9-41-15W4 Eastern Shelf ft 4915 5040 5090 5212 5380 5590 NC/NE 206 91 51 64 37 
2-2-42-14w4 Eastern Shelf 1390 1428 1449 1484 1545 1585 NC/NE 195 94 61 40 35 
8-23-43-16w4 Eastern Shelf 1487 1520 1544 1580 1638 1698 NC/NE 211 93 58 60 36 
6-35-46-15w4 Eastern Shelf 1322 1374 1390 1420 1477 1540 NC/NE 218 98 57 63 30 
Well location Area Units Beaverhill Lake Gp. Swan Hills Fm. ISHU (BHL3.1) Lower atoll stage (BHL2.1) Platform member Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lower atoll stage 
Swan Hills Complex 
6-1-62-12w5 CCN A-Pool ft 8667 8754 8809 8905 8993 9125 8804–9000 140 43 29 67 27 
6-3-62-12w5 CCN A-Pool ft 8506 8547 8607 8720 8797 8935 8550–8823 131 31 34 66 23 
16-11-62-12w5 CCN A-Pool ft 8660 8734 8756 8883 8953 9070 8710–8973 125 29 39 57 21 
6-17-62-12w5 CCN B-Pool ft 8572 8610 8675 8816 8890 9030 8634–8834 140 31 43 65 23 
13-14-52-17w5 Edson 3648 3650 3701 3729 3742 3773 3742–3769 125 53 28 44 13 
9-3-52-17w5 Edson 3728 3738 3785 3816 3830 3859 3760–3835 131 57 31 43 14 
11-33-51-16w5 Edson ft 11,965 12,060 12,092 12,172 12,220 12,340 12,025–12,198 114 39 24 51 15 
6-30-46-17w5 Hanlan N ft 15,265 15,360 15,430 15,515 15,555 15,635 15,438–15,498 113 50 26 37 12 
11-8-47-17w5 Hanlan N ft 15,040 15,100 15,145 15,234 15,270 15,340 15,115–15,252 91 32 27 32 11 
11-19-47-17w5 Hanlan N ft 14,940 15,024 15,071 15,145 15,167 15,235 15,054–15,246 90 40 23 27 
11-22-44-16w5 Hanlan S 4673 4714 4714 4741 4752 4777 4726–4778 104 41 27 36 11 
6-20-44-16w5 Hanlan S 4764 4776 4801 4832 NP 4846 4794–4835 82 37 31 14  
2-15-36-9w5 Strachan 4573 4596 4603 4640 4655 4687 4590–4608 114 30 37 47 15 
10-33-36-10w5 Strachan ft 15,607 15,670 15,725 15,825 15,880 15,950 NC/NE 105 36 30 38 17 
6-12-38-10w5 Strachan 4351 4388 4409 4431 4445 4482 4399–4418 131 58 22 51 14 
6-20-33-4w5 Caroline 3580 3637 3648 3669 3680 3712 3625–3676 132 68 21 43 11 
6-13-33-5w5 Caroline 3698 3736 3757 3782 3791 3825 3730–3797 127 59 25 43 
7-18-34-4w5 Caroline 3586 3645 3657 3680 3691 3723 3655–3708 137 71 23 43 11 
6-29-34-5w5 Caroline 3737 3770 3798 3823 3834 3864 3775–3873 127 61 25 41 11 
10-33-34-5w5 Caroline 3684 3731 3747 3770 3782 3815 3725–3824 131 63 23 45 12 
3-10-35-5w5 Caroline 3652 3705 3716 3743 3758 3792 3710–3766 140 64 27 49 15 
10-15-35-5w5 Caroline 3653 3721 3721 3745 3762 3794 3723–3758 141 68 24 49 17 
5-32-35-5w5 Caroline 3650 3717 3718 3746 3762 3791 3723–3762 141 68 28 45 16 
10-24-31-4w5 Twining 3605 3655 3660 3691 3707 3738 3670–3699 133 55 31 47 16 
15-16-31-5w5 Twining 3959 3998 4015 4046 4058 4088 3995–4023 129 56 31 42 12 
7-32-32-28w4 Stewart 2864 2915 2944 2968 2978 3022 2940–2976 158 80 24 54 10 
Well location Area Units Beaverhill
Lake Gp. 
WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl2.1) Elk Point/Watt Mtn. Core interval BHL Gp. BHL3 BHL2 BHL1 Lowstand 
Waterways Basin 
8-13-27-3w5 WWYS Basin 3635 3666 3675 3696 3743 3761 3696–3753 126 61 47 18 21 
5-15-28-3w5 WWYS Basin 3659 3690 3698 3731 3764 3786 3710–3746 127 72 33 22 33 
6-6-29-2w5 WWYS Basin 3462 3507 3518 3560 3593 3611 NC/NE 149 98 33 18 42 
6-14-33-2w5 WWYS Basin 3115 3153 3162 3207 3228 3266 3138–3157 151 92 21 38 45 
9-17-34-25w4 WWYS Basin ft 7980 8149 8190 8310 8403 8575 8071–8088 181 101 28 52 37 
16-16-37-25w4 WWYS Basin ft 8016 8181 8215 8348 8450 8630 NC/NE 187 101 31 55 41 
6-16-38-3w5 WWYS Basin 3048 3086 3109 3142 3161 3215 NC/NE 167 94 19 54 33 
13-36-39-25w4 WWYS Basin ft 7390 7555 7590 7695 7838 8046 NC/NE 200 93 44 63 32 
Well location Area Units Beaverhill Lake Gp. WWYS E WWYS D WWYS C (Bhl3.1) WWYS B (Bhl3.1) Elk Point/Watt Mtn. Core interval examined Interval thickness 
BHL
Gp. 
BHL3 BHL2 BHL1 Lowstand 
6-11-40-21w4 WWYS Basin 1940 1991 2004 2031 2080 2146 NC/NE 206 91 49 66 27 
12-14-40-26w4 WWYS Basin 2378 2424 2435 2474 2508 2571 NC/NE 193 96 34 63 39 
9-23-41-9w5 WWYS Basin ft 12,435 12,520 12,550 12,695 12,800 12,944 NC/NE 155 79 32 44 44 
6-20-41-24w4 WWYS Basin ft 7260 7420 7460 7556 7707 7922 NC/NE 202 90 46 66 29 
6-4-41-1w5 WWYS Basin ft 8682 8790 8830 8975 9060 9258 NC/NE 176 89 26 60 44 
7-1-41-3w5 WWYS Basin 2888 2923 2944 2979 3006 3063 NC/NE 175 91 27 57 35 
16-16-42-2w5 WWYS Basin 2695 2730 2753 2789 2819 2877 NC/NE 182 94 30 58 36 
15-35-44-2w5 WWYS Basin 2559 2591 2606 2648 2682 2746 NC/NE 187 89 34 64 42 
10-25-46-2w5 WWYS Basin ft 8162 8270 8340 8450 8575 8789 NC/NE 191 88 38 65 34 
6-15-48-3w5 WWYS Basin ft 8078 8178 8232 8358 8480 8697 NC/NE 189 85 37 66 38 
9-18-49-9w5 WWYS Basin 3065 3103 3125 3153 3174 3227 NC/NE 162 88 21 53 28 
8-30-52-5w5 WWYS Basin 2408 2445 2454 2483 2523 2585 NC/NE 177 75 40 62 29 
6-3-51-7w5 WWYS Basin ft 8645 8765 8824 8923 9005 9208 NC/NE 172 85 25 62 30 
3-18-52-8w5 WWYS Basin ft 9000 9130 9205 9292 9360 9545 NC/NE 166 89 21 56 27 
16-5-55-9w5 WWYS Basin ft 8628 8733 8798 8890 8950 9140 NC/NE 156 80 18 58 28 
10-26-59-9w5 WWYS Basin ft 7550 7651 7718 7795 7897 8042 NC/NE 150 75 31 44 23 
6-29-61-9w5 WWYS Basin ft 7464 7570 7642 7722 7844 7947 NC/NE 147 79 37 31 24 
Eastern Shelf area 
6-5-30-21w4 Stewart 2069 2108 2121 2136 2195 2250 2201–2257 181 67 59 55 15 
8-33-31-22w4 Stewart 2149 2184 2198 2233 2279 2326 2245–2277 177 84 46 47 35 
10-30-32-23w4 Stewart 2295 2338 2354 2370 2430 2482 2372–2426 187 75 60 52 16 
16-27-32-24w4 Stewart 2346 2386 2410 2435 2480 2531 2414–2443 185 89 45 51 25 
6-11-33-24w4 Stewart 2343 2380 2402 2420 2470 2516 2422–2425 173 77 50 46 18 
16-36-41-23w4 Eastern Shelf 2040 2092 2103 2134 2191 2250 2132–2185 210 94 57 59 31 
1-7-36-18w4 Eastern Shelf 1881 1923 1940 1971 2020 2080 1944–1962 199 90 49 60 31 
6-4-37-16w4 Eastern Shelf 1752 1792 1809 1844 1890 1951 1850–1868 199 92 46 61 35 
10-30-36-16w4 Eastern Shelf ft 5806 5946 5985 6095 6304 6465 6060–6180 201 88 64 49 34 
10-34-35-17w4 Eastern Shelf ft 5890 6030 6070 6172 6348 NP 6065–6365  86 54  31 
14-35-34-17w4 Eastern Shelf 1818 1860 1869 1903 1950 2009 NC/NE 191 85 47 59 34 
14-11-39-17w4 Eastern Shelf 1705 1740 1757 1793 1846 1905 NC/NE 200 88 53 59 36 
6-11-40-21W4 Eastern Shelf 1940 1990 2005 2030 2080 2146 NC/NE 206 90 50 66 25 
6-10-41-19W4 Eastern Shelf ft 5820 5985 6012 6130 6280 6500 NC/NE 207 94 46 67 36 
7-9-41-15W4 Eastern Shelf ft 4915 5040 5090 5212 5380 5590 NC/NE 206 91 51 64 37 
2-2-42-14w4 Eastern Shelf 1390 1428 1449 1484 1545 1585 NC/NE 195 94 61 40 35 
8-23-43-16w4 Eastern Shelf 1487 1520 1544 1580 1638 1698 NC/NE 211 93 58 60 36 
6-35-46-15w4 Eastern Shelf 1322 1374 1390 1420 1477 1540 NC/NE 218 98 57 63 30 

NC = no core available; NE = not examined; NP = not picked; WWYS = Waterways Formation.

Contents

Society for Sedimentary Geology

NEWADVANCES IN DEVONIAN CARBONATES: OUTCROP ANALOGS, RESERVOIRS AND CHRONOSTRATIGRAPHY

Ted E. Playton
Ted E. Playton
Tengizchevroil, Atyrau 060011, Kazakhstan
Search for other works by this author on:
Charles Kerans
Charles Kerans
Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA
Search for other works by this author on:
John A.W. Weissenberger
John A.W. Weissenberger
ATW Associates, Calgary, Alberta, T3E 7M8, Canada
Search for other works by this author on:
Society for Sedimentary Geology
Volume
107
ISBN electronic:
9781565763456
Publication date:
January 01, 2017

GeoRef

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