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Abstract

The Helena and Wallace formations, currently of the “middle Belt carbonate”, were deposited in the block–fault Belt basin, within the Proterozoic Columbia continent, which filled from about 1480 to 1400 Ma. Dolomitic argillite–capped cycles of the Helena Formation were thought to represent a marine carbonate shelf deposit along the eastern margin of the Belt basin. Siliciclastic and calcitic rocks of the Wallace Formation were considered to be the western facies of the middle Belt carbonate, deposited in deeper water. This study shows that the Helena–type cycles form a unit across most of the Belt basin that is disconformably overlain by Wallace–type rocks. The Helena and Wallace formations are here revised to reflect the stacked stratigraphic relations. Both are inferred to be deposits of broad, shallow lakes. The Helena and Wallace are assigned to the resurrected and revised Piegan Group.

The revised Helena Formation is characterized by cycles one to 10 m thick. The lower half-cycles are composed of light gray, thin, graded, siliciclastic layers 0.3 to 10 cm thick. Some continue upward and become mixed with tan-weathering dolomite in the upper half-cycles. In other cycles siliciclastic graded layers thin and fine upward but remain siliciclastic. The Helena Formation can be divided into lower, middle, and upper informal members. The lower and upper members have centimeter-scale bedded cycles, but the middle membercontains cycles with dark-gray, decimeter-scale hummocky cross-stratified arenite beds. The Grinnell Glacier section of Glacier National Park is selected as the revised Helena reference section. It is 500 m thick and contains 363 thin-bedded, dolomite-capped cycles, averaging 1.4 m thick. The Helena thins to 100 m on eastern thrust plates of the Front Range, thickens to 800 m north of Plains, Montana, but thins to 250 m in the Coeur d'Alene Mining District.

Based principally on scattered halite casts, the crosscutting of the siliciclastic lithofacies by the dolomitic cycle caps, and the absence of significant scour at the cycle bases, the Helena Formation is interpreted to have been deposited in an underfilled, periodically hypersaline, broad, shallow lake. Its flat lake floor was everywhere above storm wave base. Stacking patterns of small-scale cycles indicate the Helena represents a large-scale expanding and contracting lake sequence.

The revised Wallace Formation is characterized across northwestern Montana by gray-weathering siliciclastic upward-fining andthinning cycles, mostly 2 to 5 m thick. It can be subdivided into the following six members: (1) oolitic member, (2) molartooth member, (3) Baicalia member, (4) pinch-and-swell member, (5) microcouplet member, and (6) the full-cycle member. Across northern Idaho the Wallace members and cyclic patterns merge into a continuous unit of medium-gray arenite lenses in dark-gray argillite. Thinly laminated black argillite and dolomite units previously assigned to the upper Wallace in Idaho along with dolomitic argillite beds of the upper part of the Helena in Montana are here assigned to the lower Missoula Group. The Clark Fork section of northern Idaho is designated the Wallace reference section. It is 400 m thick and has 47 cycles. The Wallace Formation thickens eastward to more than 1,000 m in the Mission Range and thins to 300 m in Glacier National Park.

Siliciclastic cycles of the Wallace Formation are similar to those of the Helena Formation. Cycle boundaries lack evidence of significant exposure and erosion, but only the lower and upper Wallace cycles have dolomite caps. For these reasons the Wallace Formation is interpreted to represent an underfilled and balanced-fill lake deposit that expanded and contracted, forming a genetic sequence. Widespread hummocky arenaceous beds indicate that the Wallace lake expanded westward, but its floor was flat and everywhere above storm wave base.

Introduction

The Helena and Wallace formations together currently constitute the informal “middle Belt carbonate”, a stratigraphic unit of group scale that reaches more than 2,000 m thick and contains siliciclastic and carbonate–bearing cycles that were deposited in the Belt block–fault basin within the Proterozoic Columbia continent (Sears and Price, 2002) from approximately 1,460 Ma to 1,450 Ma (Fig. 1A,B). The basin was split by late Proterozoic rifting, leaving the one–sided Belt basin attached to Laurentia. Geologists have questioned whether, with most of the siliciclastic source terrane gone, the subaqueous cyclic deposits of the middle Belt carbonate were connected on the north, west, or south to the open Proterozoic ocean and are thus marine, or whether the basin was fully enclosed, and the cycles were therefore lacustrine. The Helena and Wallace formations exemplify the difficulty in differentiating marine from lacustrine deposits in the Proterozoic. The lacustrine interpretation proposed here provides a model to solve that problem.

The revised Helena Formation is characterized by cycles one to 10 m thick. The lower half–cycles are composed of light gray, thin, graded, siliciclastic layers 0.3 to 10 cm thick. Some continue upward and become mixed with tan–weathering dolomite in the upper half–cycles. In other cycles siliciclastic graded layers thin and fine upward but remain siliciclastic. The Helena Formation can be divided into lower, middle, and upper informal members. The lower and upper members have centimeter–scale bedded cycles, but the middle member contains cycles with dark–gray, decimeter–scale hummocky cross–stratified arenite beds. The Grinnell Glacier section of Glacier National Park is selected as the revised Helena reference section. It is 500 m thick and contains 363 thin–bedded, dolomite–capped cycles, averaging 1.4 m thick. The Helena thins to 100 m on eastern thrust plates of the Front Range, thickens to 800 m north of Plains, Montana, but thins to 250 m in the Coeur d’Alene Mining District.

Based principally on scattered halite casts, the crosscutting of the siliciclastic lithofacies by the dolomitic cycle caps, and the absence of significant scour at the cycle bases, the Helena Formation is interpreted to have been deposited in an underfilled, periodically hypersaline, broad, shallow lake. Its flat lake floor was everywhere above storm wave base. Stacking patterns of small–scale cycles indicate the Helena represents a large–scale expanding and contracting lake sequence.

The revised Wallace Formation is characterized across northwestern Montana by gray–weathering siliciclastic upward–fining and–thinning cycles, mostly 2 to 5 m thick. It can be subdivided into the following six members: (1) oolitic member, (2) molartooth member, (3) Baicalia member, (4) pinch–and–swell member, (5) microcouplet member, and (6) the full–cycle member. Across northern Idaho the Wallace members and cyclic patterns merge into a continuous unit of medium–gray arenite lenses in dark–gray argillite. Thinly laminated black argillite and dolomite units previously assigned to the upper Wallace in Idaho along with dolomitic argillite beds of the upper part of the Helena in Montana are here assigned to the lower Missoula Group. The Clark Fork section of northern Idaho is designated the Wallace reference section. It is 400 m thick and has 47 cycles. The Wallace Formation thickens eastward to more than 1,000 m in the Mission Range and thins to 300 m in Glacier National Park.

Siliciclastic cycles of the Wallace Formation are similar to those of the Helena Formation. Cycle boundaries lack evidence of significant exposure and erosion, but only the lower and upper Wallace cycles have dolomite caps. For these reasons the Wallace Formation is interpreted to represent an underfilled and balanced–fill lake deposit that expanded and contracted, forming a genetic sequence. Widespread hummocky arenaceous beds indicate that the Wallace lake expanded westward, but its floor was flat and everywhere above storm wave base.

Fig. 1.—

A)Generalized geologic map of the Belt basin. Inset locates maps in Figures 1B, 6, and 17.

Fig. 1.—

A)Generalized geologic map of the Belt basin. Inset locates maps in Figures 1B, 6, and 17.

Fig. 1.—

B)Location map of localities and features described in the text.

Fig. 1.—

B)Location map of localities and features described in the text.

Data in this paper come from 23 full and partial stratigraphic sections in the Helena Formation and 21 full and partial sections in the Wallace Formation. Sections were measured with a 5 ft. (1.5 m) Jacob’s Staff and sketched and described in the field at a scale of 10 ft. (3.05 m) to 1 inch (2.54 cm). GPS locations of the measured sections are shown in Table 1. Analysis of the sections has led to revision of the Helena and Wallace formations.

Table 1.—

GPS coordinates of measured sections.

Helena Formation Measured Section Locations
#NameStart of SectionEnd of Section
1Crater Mountain46°54'25.89N 114°53'31.58W46°54'48.49N 114°54'11.06W
2St. Regis Lakes47°25'48.52N 115°44'46.92W47°25'43.54N 115°44'59.69W
3Placer Creek47°27'03.79N 115°56'03.44W47°27'07.90N 115°55'51.37W
4Galena Mine47°28'48.24N 115°57'59.35W47°29'08.44N 115°57'37.45W
5Steamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
6Clark Fork48°11'03.35N 116°14'06.76W48°10'43.15N 116°13'45.04W
7Big Hole Peak Weeksville Road Round Mtn.47°32'21.90N 115°04'33.92W 47°31'27.41N 115°59'54.45W 47°32'05.13N 115°03'13.88W47°32'38.64N 115°04'10.78W 47°31'29.81N 115°00'15.06W 47°32'06.92N 115°03'12.47W
8Bull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
9West Libby Dam48°24'38.04N 115°19'23.49W48°24'01.74N 115°19'26.98W
10St. Mary Lake47°15'50.42N 113°55'06.88W47°16'09.17N 113°54'59.07W
11Mollman Pass Eagle Pass47°28'59.50N 113°57'25.15W 47°26'50.75N 113°56'44.63W47°29'26.25N 113°57'05.56W 47°26'53.73N 113°56'38.36W
12Red Mountain47°04'32.55N 112°45'49.73W47°06'26.49N 112°44'34.27W
13Rogers Pass47°06'00.50N 112°21'16.34W47°05'32.54N 112°21'50.91W
14Little Skunk Creek47°12'20.75N 112°24'47.25W47°12'21.54N 112°25'03.68W
15Dearborn River47°16'20.79N 112°30'52.71W47°16'14.16N 112°31'05.26W
16Wood Creek Hogback47°25'06.12N 112°47'21.93W47°25'00.56N 112°47'21.82W
17Grinnell Glacier Big Bend48°46'02.12N 113°43'10.64W 48°43'30.63N 113°43'24.91W48°46'09.05N 113°43'49.56W 48°42'18.57N 113°43'02.97W
Wallace Formation Measured Section Locations
I.D.NameStart of SectionEnd of Section
SCSteamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
CFClark Fork48°10'43.15N 116°13'45.04W48°10'38.08N 116°13'29.18W
BRBull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
LDEast Libby Dam48°22'28.62N 115°18'55.29W48°22'08.66N 115°19'02.83W
GWGrey Wolf47°16'10.97N 113°53'21.45W47°16'11.87N 113°52'49.28W
SPSpider Lake47°26'21.51N 113°52'36.91W47°26'21.59N 113°52'34.72W
SMSunday Mountain47°20'38.63N 113°29'08.52W47°20'43.97N 113°28'52.09W
SLHolland Lake47°27'40.83N 113°34'09.38W47°28'03.33N 113°33'39.83W
NTNapa Trail47°47'18.55N 113°40'51.04W47°47'22.18N 113°40'52.19W
GRGrant Ridge48°19'57.20N 113°45'06.23W48°20'09.08N 113°44'45.51W
OCOusel Creek48°29'41.35N 113°53'36.71W48°29'49.38N 113°53'26.13W
LLLower Loop48°44'59.80N 113°47'07.48W48°45'10.86N 113°47'54.16W
ULUpper Loop48°45'12.59N 113°47'44.88W48°45'05.54N 113°46'59.51W
LPLogan Pass48°42'12.58N 113°42'16.23W48°42'20.58N 113°42'16.94W
SDSouth Drywood Canyon49°14'46.40N 114°04'56.03W49°14'49.93N 114°05'11.01W
Helena Formation Measured Section Locations
#NameStart of SectionEnd of Section
1Crater Mountain46°54'25.89N 114°53'31.58W46°54'48.49N 114°54'11.06W
2St. Regis Lakes47°25'48.52N 115°44'46.92W47°25'43.54N 115°44'59.69W
3Placer Creek47°27'03.79N 115°56'03.44W47°27'07.90N 115°55'51.37W
4Galena Mine47°28'48.24N 115°57'59.35W47°29'08.44N 115°57'37.45W
5Steamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
6Clark Fork48°11'03.35N 116°14'06.76W48°10'43.15N 116°13'45.04W
7Big Hole Peak Weeksville Road Round Mtn.47°32'21.90N 115°04'33.92W 47°31'27.41N 115°59'54.45W 47°32'05.13N 115°03'13.88W47°32'38.64N 115°04'10.78W 47°31'29.81N 115°00'15.06W 47°32'06.92N 115°03'12.47W
8Bull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
9West Libby Dam48°24'38.04N 115°19'23.49W48°24'01.74N 115°19'26.98W
10St. Mary Lake47°15'50.42N 113°55'06.88W47°16'09.17N 113°54'59.07W
11Mollman Pass Eagle Pass47°28'59.50N 113°57'25.15W 47°26'50.75N 113°56'44.63W47°29'26.25N 113°57'05.56W 47°26'53.73N 113°56'38.36W
12Red Mountain47°04'32.55N 112°45'49.73W47°06'26.49N 112°44'34.27W
13Rogers Pass47°06'00.50N 112°21'16.34W47°05'32.54N 112°21'50.91W
14Little Skunk Creek47°12'20.75N 112°24'47.25W47°12'21.54N 112°25'03.68W
15Dearborn River47°16'20.79N 112°30'52.71W47°16'14.16N 112°31'05.26W
16Wood Creek Hogback47°25'06.12N 112°47'21.93W47°25'00.56N 112°47'21.82W
17Grinnell Glacier Big Bend48°46'02.12N 113°43'10.64W 48°43'30.63N 113°43'24.91W48°46'09.05N 113°43'49.56W 48°42'18.57N 113°43'02.97W
Wallace Formation Measured Section Locations
I.D.NameStart of SectionEnd of Section
SCSteamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
CFClark Fork48°10'43.15N 116°13'45.04W48°10'38.08N 116°13'29.18W
BRBull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
LDEast Libby Dam48°22'28.62N 115°18'55.29W48°22'08.66N 115°19'02.83W
GWGrey Wolf47°16'10.97N 113°53'21.45W47°16'11.87N 113°52'49.28W
SPSpider Lake47°26'21.51N 113°52'36.91W47°26'21.59N 113°52'34.72W
SMSunday Mountain47°20'38.63N 113°29'08.52W47°20'43.97N 113°28'52.09W
SLHolland Lake47°27'40.83N 113°34'09.38W47°28'03.33N 113°33'39.83W
NTNapa Trail47°47'18.55N 113°40'51.04W47°47'22.18N 113°40'52.19W
GRGrant Ridge48°19'57.20N 113°45'06.23W48°20'09.08N 113°44'45.51W
OCOusel Creek48°29'41.35N 113°53'36.71W48°29'49.38N 113°53'26.13W
LLLower Loop48°44'59.80N 113°47'07.48W48°45'10.86N 113°47'54.16W
ULUpper Loop48°45'12.59N 113°47'44.88W48°45'05.54N 113°46'59.51W
LPLogan Pass48°42'12.58N 113°42'16.23W48°42'20.58N 113°42'16.94W
SDSouth Drywood Canyon49°14'46.40N 114°04'56.03W49°14'49.93N 114°05'11.01W

Geologic Setting

The geologic setting of the Belt basin is well described in Ross and Villeneuve (2003). Sediments that filled the Belt basin during deposition of the Helena and Wallace formations came principally from the west and east. To the east lay the crystalline terrane of the Laurentian craton, mantled by fine– to mostly coarse–grained, quartz sand of the Neihart Formation. Well– rounded, coarse quartz sand grains with fewer lithic grains were reworked into the Belt basin and formed a distinctive, but minor, grain population, mostly limited to the eastern side of the Belt basin. By far most of the Belt sediment came from the west side of the basin in the form of fine–grained, feldspathic quartz sand, silt, and clay. Rocks of the Helena and Wallace carry the detrital non–North American grain population (Ross and Villeneuve, 2003; Link et al., this volume), confirming the western sediment source of continental proportions (Frost and Winston, 1987). Moores (1991) proposed that Australia presently represents that Proterozoic continent in the longstanding SWEAT hypothesis. More recently, however, Sears and Price (2000, 2003), Sears et al. (2004), and Sears (2006) have presented convincing evidence that the western continent is now Siberia, not Australia.

Winston (1986c) hypothesized that major crustal blocks of the Belt basin were separated by three east–west fault zones, the Perry, Garnet, and Jocko lines (Fig. 1A). Germane to this discussion is the Jocko Line, south of which the Wallace Formation thickens, indicating syn–depositional down–to–the–south movement during deposition of the Wallace Formation. The Jocko Line also bounds the south edge of the Noxon arch of White (2000), proposed on the basis of a thin Revett section north of the line. The Noxon arch lacks evidence of Proterozoic compression, and is here referred to as the Noxon block to conform to other Protero–zoic tectonic blocks of the Belt basin (Fig. 1B). The Revett, Helena,and Wallace formations all thin over the Noxon block, but they thicken to the east near the trace of the Hope fault (Fig. 1B), suggesting that it approximates the eastern edge of the Noxon block. The northern and western limits have not been determined.

Despite rifting and formation of the western edge of North America late in the Proterozoic, the Belt basin remained essentially quiescent through the rest of the Proterozoic and Paleozoic. Mesozoic compression formed western and eastern thrust belts across western Montana (Winston, 1986c) separated by an area of folded Belt rocks (Fig. 1A,B). The western thrust belt extends northward into the Libby thrust belt (Fig. 1B). The Helena and Wallace formations thin and are close to their eastern deposi– tional limits in sections on the Steinbach thrust plate of the eastern thrust belt (Fig. 2). Extension during the Tertiary cut Belt rocks into horst and graben blocks. Thus all the stratigraphic sections in this study have been tectonically displaced. Nevertheless, Belt stratigraphic sections can be confidently correlated, and the depositional history of the Helena and Wallace can be analyzed and comprehended.

History of Helena And Wallace Nomenclature

The development of Helena and Wallace stratigraphic nomenclature is rooted in the early days of Belt stratigraphic exploration when U.S. Geological Survey geologists, chiefly under the leadership of C.D. Walcott, began the Belt saga by studying three separate parts of the Belt basin near the beginning of the 20th century. Not knowing the Belt correlations, researchers erected suites of new formations in each area. Walcott (1899, 1906) named the Helena limestone for tan–weathering dolomitic beds that alternate with thinner, dark argillite beds in the hills above Helena, Montana (Fig. 2). In the type area the Helena Formation crops out in patches, and Walcott did not describe a type section. Knopf (1950) redefined the Helena limestone as the Helena dolomite. Walcott stipulated that the Helena overlies green argillite beds of the Empire shales and underlies red argillite beds of the Marsh shales (now the Snowslip Formation). Walcott dispatched Bailey Willis to Glacier National Park, and Willis (1902) named the Siyeh limestone for a dark, thick–bedded limestone and dolomite with argillaceous interbeds that rests on mudcracked Grinnell redbeds and continues up to the base of the Purcell lava (Fig. 2). In terms of this paper, the Siyeh includes green argillite of the Empire Formation of the Ravalli Group, continues up through the Helena and Wallace formations, and extends through most of the red and green argillite of the Snowslip Formation of the Missoula Group (Fig. 2). Walcott also sent Ransome and Calkins to the Coeur d’Alene Mining District of northern Idaho, where Ransome (1905) named, and Ransome and Calkins (1908) more fully described, the Wallace Formation as a thick heterogeneous unit characterized by thin–bedded, calcareous quartzites, impure limestones, and shales. They overlie mudcracked red argillite of the St. Regis Formation of the Ravalli Group and underlie purple arenite of the Striped Peak Formation (Fig. 2). Ransome and Calkins recognized within the Wallace a lower unit of green argillite or slate, a middle unit of light gray quartzite and dark argillite interbeds, and an upper unit of thinly laminated black argillite and tan–weathering dolomite. The lower unit represents a thinned wedge of the Empire Formation, which Hobbs et al. (1965) placed in the upper part of the St. Regis Formation. The middle unit includes the revised Helena and Wallace formations, and the upper unit correlates with the Snowslip and Shepard formations of the Missoula Group in Montana (Winston, 1986a; Lemoine and Winston, 1986; Winston and Link, 1993) (Fig. 2).

Fig. 2.—

Correlation diagram showing the history of Helena and Wallace stratigraphic nomenclature

Fig. 2.—

Correlation diagram showing the history of Helena and Wallace stratigraphic nomenclature

For the first part of the 20th Century various authors (e.g., Clapp and Deiss, 1931; Fenton and Fenton, 1937; Ross, 1963) mismatched and miscorrelated carbonate rocks of the Helena, Siyeh, and Wallace formations with the Newland Limestone of the lower Belt and with the Shepard Formation of the Missoula Group (Winston, 1986a), resulting in a chaotic Belt stratigraphic framework. During this time Fenton and Fenton (1937) gathered the Wallace, Siyeh, and Helena formations together into their Piegan Group, in which they also included units now assigned to the Newland and Shepard formations (Fig. 2).

Finally, in their successful synthesis of Belt stratigraphy, Smith and Barnes (1966) correlated the Siyeh Formation of Glacier National Park with the middle part of the Wallace Formation of northern Idaho and with the upper part of the Kitchener Formation of British Columbia (Fig. 2). They also correlated these three formations with the Helena Formation. They revised the Piegan Group, placing the lower boundary where red and green argillite of the Ravalli Group (St. Regis, Spokane, and Empire formations) pass up into carbonate–rich rocks of the Helena, Siyeh, and Wallace formations. Across the eastern and central part of the Belt basin, they placed the top of the Piegan Group where carbonate–bearing beds of the Helena, Siyeh, and middle Wallace are overlain by noncalcareous reddish, purplish, and greenish beds of the Missoula Group, chiefly the Snowslip Formation. However, in northwestern Montana, where the Striped Peak Formation forms the basal redbeds of the upper Belt, they placed the top of the Piegan Group at the top of the Shepard Formation, realizing that the upper Wallace of northwest Montana interfingered with the Snowslip and Shepard formations of the Missoula Group farther east (Fig. 2).

Smith and Barnes (1966) also pointed out lateral facies changes in the Piegan Group, showing that the Piegan Group thickens from the eastern exposures westward into the basin and that the carbonate–rich, stromatolitic and dolomitic rocks of the Siyeh Formation and correlative rocks of the eastern Belt pass westward to calcitic and more terrigenous rocks of the middle Wallace. From this they inferred that the dolomitic rocks of the eastern Piegan Group were deposited in shallow water, and that the basin deepened westward into the middle Wallace. This idea was supported by McKelvey (1968) and further developed by the recognition of dolomitic shallow–water cycles along the eastern edge of the basin (Mudge, 1972; Eby, 1977).

With Harrison’s (1972) overview of Belt stratigraphy and tectonics, the concept of an eastern shallow–water marine shelf facies with dolomitic cycles, embodied by the Helena Formation, and a thicker, deeper–water, more siliciclastic western facies embodied by the Wallace became well established. Harrison also replaced the Piegan Group with the informal term “middle Belt carbonate”, representing a “lithostratigraphic zone” (Fig. 2). He synonymized the Siyeh Formation with the Helena Formation, thus leaving two formations within his middle Belt carbonate: the Helena Dolomite on the east and the Wallace Formation on the west (Harrison, 1984).

Harrison et al. (1986) carried the name Helena Formation from the eastern side of the Belt Basin westward across the Wallace 1° x 2° quadrangle to the Clark Fork River and the east limb of the Paradise anticlinorium (Fig. 1B). In the Helena they described cycles with lower gray to green argillite and siltite capped by orange–weathering dolomite. But in the upper Helena there were thick intervals of black argillite and dolomitic siltite similar to the middle Wallace. West of the Paradise anticlinorium and into Idaho, Harrison et al. (1986) mapped the middle Belt carbonate unit as the Wallace Formation and recognized three members: (1) a lower member containing interlaminae of siltite and dolomitic argillite, similar in many respects to the Helena, (2) a middle member characterized by very fine–grained gray quartzite layers interstratified with black argillite layers, and (3) an upper member represented by a succession of laminated black argillite, green argillite, tan– weathering dolomite, and black argillite (Fig. 2). The lower Wallace of Harrison et al. (1986) is here placed in the Helena Formation. The middle Wallace of Harrison et al. (1986) here becomes the Wallace Formation. And the upper Wallace is now recognized to correlate with the lower Missoula Group (Winston, 1986a; Lemoine and Winston, 1986) (Fig. 2).

In a section at Big Hole Peak, north of Plains, Montana, Grotzinger (1981, 1986a) described in the lower Wallace member thin–bedded siliciclastic–to–dolomite cycles that closely resemble Helena cycles. These are overlain by gray quartzite and dark gray argillite characteristic of the middle Wallace member. The lower cyclic member became more quartzitic in the Clark Fork section.

A different view of the Helena–Wallace relationship emerged near Libby Dam (Fig. 1B) in the Kalispell 1° x 2° quadrangle, where Harrison et al. (1992) discovered and mapped a unit of Helena–type cycles that clearly lies beneath middle Wallace–like quartzite and dark argillite. They assigned the Helena–like cycles to the Helena Formation and the overlying Wallace–like rocks to the Wallace Formation. Harrison and Cressman (1993) further described the Helena–Wallace relationships, proposing informal lower–body and main–body members of the Helena. They proposed that green to tan argillite and dolomite of the lower Helena member interfinger with green argillitic siltite and calcitic and dolomitic argillite of the lower Wallace to the west. They also proposed that the main cyclic body of the Helena interfingers westward with the middle Wallace but that everywhere in the Libby thrust belt the main body of the Helena is overlain by the middle Wallace.

Analysis of the Helena and Wallace presented here shows that the stratigraphic relations of the Helena beneath the Wallace, described by Harrison and Cressman (1993) at Libby Dam, extend eastward across the Belt basin, and also extend westward with some modification into northern Idaho. The concept of an eastern cyclic dolomitic Helena Formation and a western gray quartzite and dark argillite Wallace Formation no longer applies. Therefore, as recommended by Winston (2003), the Helena and Wallace formations need to be revised. This revision retains the lithologic concepts of the Helena Formation as a cyclic unit with dolomitic caps and Wallace Formation as calcitic gray quartzitic and black argillitic unit with cycles. It will permit more detailed mapping of the Helena and Wallace, and, because the two are separated by a disconformity, more faithfully reflects the strati– graphic development of the basin. The Piegan Group as delineated by Smith and Barnes (1966) is resurrected with modification at the top to include the Helena and Wallace Formations.

Revision of the Helena Formation

The Helena Formation is revised here to represent a lithostrati– graphic unit characterized by sedimentary cycles, mostly ranging from a meter to 10 m thick, with light–gray–weathering, quartzitic and argillitic siliciclastic lower half–cycles and tan–weathering, mixed siliciclastic and dolomitic, upper half–cycles. The Helena Formation extends from the Steinbach thrust plate of the eastern part of the Belt basin (Fig. 1B), where it is 100 m thick, to the central part of the Belt basin, where it thickens to more than 800 m thick. It thins again to 250 m to the west across the Noxon block. It has previously been included in the lower part of the Helena Formation in the eastern part of the Belt basin and in much of the lower member of the Wallace Formation in the western part of the basin.

The Helena Formation has eastern and western facies. The eastern facies of the Helena Formation is composed of cycles with lower light–gray–weathering, thin, graded siltstone–to–mudstone layers that are capped by graded siltstone–to–mudstone layers mixed with tan–weathering dolomite. The western facies of Helena in northwestern Montana and northern Idaho contains lower, middle, and upper members. The lower and upper members, like those of the Helena to the east, are thin bedded, but dolomite, marking their cycle tops diminishes westward to the western part of the Coeur d’Alene Mining District (Fig. 1B). The middle Helena contains a wedge of thicker cycles with thicker–bedded gray quartzite lenses in the lower half–cycles. The upper half–cycles of the wedge are composed of dark argillite and thin dolomitic layers. This wedge thickens in northern Idaho and resembles the Wallace Formation, but it retains dolomite at the tops of most cycles.

Across most of the Belt basin the base of the Helena is here placed at the lowest significant, mappable dolomite bed or at the base of a cycle with a dolomitic cap. It overlies green and purple argillite of the Empire Formation or green beds of the St. Regis Formation. This boundary does not differ significantly from the bases of the Helena and Wallace formations as previously mapped. The upper boundary of the Helena Formation across western Montana is redrawn at the top of thin cycles with thin– bedded siliciclastic lower half–cycles and tan–weathering dolo– mitic thinly layered upper half–cycles. They lie below basal oolite beds of the Wallace Formation. Where oolite beds are absent at the base of the Wallace, such as in parts of northern Idaho, the thin, silty, siliciclastic and dolomitic graded layers of the uppermost Helena are sharply overlain by thicker, sandier, lenticular graded layers of the lowermost Wallace.

The Helena Formation thins to the west. It extends into the Purcell Supergroup of Alberta, where it correlates with the lower Siyeh Formation, and into British Columbia, where it correlates with the lower Kitchener Formation (McMechan, 1981). It probably correlates with the lower part of the Apple Creek Formation of the Lemhi Group in east–central Idaho.

Because no complete stratigraphic section of the Helena Formation crops out near Helena, Montana, a suitable reference section must be sought elsewhere. The Grinnell Glacier section in Glacier National Park (Fig. 1B) exposes a full, unfaulted section of the Helena Formation containing well developed, thin, siliciclas– tic–to–dolomite cycles characteristic of the eastern facies of the formation. It is accessible and is chosen as the new representative section of the Helena. A field description of the section is on file at the Montana Bureau of Mines and Geology, Butte. It is 383 m thick and contains 158 cycles, which average 2.5 m thick. The measured section begins in the uppermost part of the Grinnell Formation (GPS coordinates 48°46’ 02.12 N, 113°43’ 10.64 W) along the Grinnell Glacier trail. Much of the Empire is covered, but the basal beds of the Helena crop out above the Empire, and the rest of the Helena is well exposed in continuous ledges up to the oolite beds at the base of the Wallace Formation (GPS coordinates 48°46’ 09.05 N 113°43’ 49.56 W). It is probably the most complete section of the eastern Helena facies. Horodyski (1983, his fig. 8) also illustrated this section.

Revision of the Wallace Formation

As revised here, the Wallace Formation constitutes a widespread lithostratigraphic unit characterized by dark–gray–weathering siliciclastic and calcitic graded beds with lower arenite lenses that range from a decimeter or more thick down to a few centimeters thick that are capped by more continuous dark–gray to black argillite layers. Also common in the Wallace are thinner layers of alternating silt and argillite laminae millimeters thick. Its lateral extent is here expanded from the western part of the Belt basin eastward across the basin to the Rocky Mountain Front (Fig. 1A). It disconformably overlies the Helena Formation and underlies thin, even and rippled argillitic and dolomitic beds of the lowest Missoula Group.

The Wallace has western and eastern facies. In the type area of the Coeur d’Alene Mining District (Fig. 1B), the western Wallace facies is characterized by a unit, probably hundreds of meters thick, of gray arenite lenses capped by black argillite, characteristic of the middle Wallace member of Harrison et al. (1986). Within the western Wallace is an interval or intervals up to 60 m thick of undulating, decimeter–thick, greenish–gray quartzite beds. The boundary between the western and eastern facies approximates the Libby thrust belt (Fig. 1B). The eastern facies of the Wallace stretches from northern Idaho across western Montana to the Rocky Mountain Front. It is also characterized by siliciclastic and calcitic arenite lenses and dark argillite beds that impart a gray–weathering outcrop to most of the Wallace, but is more calcitic. The eastern Wallace facies is characterized by upward–fining siliciclastic cycles and is subdivided into six widespread informal members: the oolitic, molartooth, Baicalia, pinch–and–swell, microlamina, and full–cycle members, named for principal lithofacies characteristic of each member. Cycle caps of the lower two members and the uppermost part of the full cycle member are dolomitic.

The base of the western Wallace from the Coeur d’Alene Mining District to northern Idaho is placed where thick, graded, siliciclastic beds, up to a decimeter thick, of medium gray quartz– ite lenses and black argillite sharply overlie thin, dolomitic fine arenite and green argillite beds of the uppermost Helena. The base of the eastern Wallace facies is marked in most places by an oolite bed or beds up to a decimeter to two thick. Where oolite is missing, the base of the Wallace is placed where thick, molartooth– bearing, arenite lenses of the Wallace sharply overlie the more even, thinner beds of the Helena.

In northern Idaho the revised top of the Wallace marks the easily identified and long–recognized boundary between the middle and upper Wallace members (Ransome and Calkins, 1908; Harrison et al., 1986; Harrison et al., 1992; Lamoine and Winston 1986; Lewis et al., 2002). Here quartzitic lenses and interbedded black argillite of the former middle Wallace are sharply overlain by very thin–bedded black argillite of the former upper Wallace, here assigned to the Missoula Group. Across Montana this boundary can be identified as the top of cycle 17 in the full–cycle member of the uppermost Wallace. A domal stromatolite bed forms the base of this cycle in many places. Above the gray quartzite and dark argillite beds of the upper part of the Wallace in the central part of the Belt basin are thin, tan–weathering dolomitic and green argillite and quartzite beds that were formerly included in the Helena Formation. They are here reassigned to the Snowslip Formation.

The Wallace Formation probably correlates homotaxially with the lower part of the Siyeh Formation of Alberta, and its lower part is represented by outcrops of the upper Kitchener Formation along the southeast side of Moyie Lake in British Columbia. It probably correlates with the upper part of the Apple Creek Formation in the Salmon River Mountains of east–central Idaho (Winston et al., 1999).

The Clark Fork section, north of Clark Fork, Idaho, just west of the Hope fault (Fig. 2B) is chosen as the new reference section for the Wallace Formation. It is well exposed in roadcuts, represents the lithology of the former middle Wallace, and contains the five members of the eastern Wallace facies. It is easily accessible at the intersection of Idaho State Highway 200 and the Denton Curves road. The base of the Wallace crops out along the Denton Curves road, about 100 m north of its intersection with Highway 200, and is marked by an intraclast bed bounded by stromatolites. It lies 10 m below a more prominent bed of domal stromatolites. The section continues south and eastward through high cliff road cuts, where all the members of the Wallace can be identified, and is capped by the stromatolite–based cycle #17, below green argillite beds assigned to the Snowslip. The section is 400 m thick and contains 47 cycles, which average 8.5 m thick. A field description of the section is on file at the Montana Bureau of Mines and Geology, Butte.

Revision of the Piegan Group

The Piegan Group is resurrected to simply include the revised Helena and Wallace formations of western Montana and northern Idaho. The lower boundary of the revised Piegan Group coincides with the base of the revised Helena Formation, and the top of the Piegan Group coincides with the revised top of the Wallace Formation.

Sediment types of the Helena and Wallace Formations

Revision of the Helena and Wallace formations lays the foundation for interpreting the sedimentologic processes, deposi– tional environments, and history of the Piegan Group. Litholo– gies of the Helena and Wallace are here classified on the basis of recurrent sediment types. Sediment types are similar to lithofa– cies of many authors, but are expressed as sediments to separate them from lithified rock, which in the Belt includes diagenetic and metamorphic overprint. Belt sediment types are described on the basis of four semi–independent attributes. They are: (1) exquisitely preserved sedimentary structures, (2) grain size, (3) mineral composition, and (4) color. Among these, sedimentary structures are the most striking and stratigraphically significant, and they are the principal attribute upon which sediment types are defined. Sediment types are the basic building blocks of Belt stratigraphy and sedimentology (e.g., Winston, 1986b; Winston and Link, 1993). Sediment types are easy to grasp and are essential in comprehending modern Belt stratigraphy and sedimentology. Helena and Wallace sediment types are described in Table 2.

Table 2.—

Descriptions and interpretations of the ten principal Piegan Group sediment types.

graphicDescription: Beds up to 20 centimeters thick ranging from coarse- to medium-grained, crossbedded quartzose, oolitic sand, to mudchips and molartooth intraclasts. Forms bases of many cycles in the Helena and Wallace formations.Interpretation: Traction transport of coarse- and medium-grained sand and coarse clasts, together with winnowing of muddy sediment, over wind-setup surfaces during expansion of the Belt lake and on cycle-highstand beaches.
graphicDescription: Well sorted, clean, fine-grained, light gray sand with low-angle hummocky cross-stratification, horizontal stratification, or poorly defined stratification. Most are stacked into cosets more than 10 cm thick and extend continuously across individual outcrops. Form the bases of cycles in the Helena and Wallace formations.Interpretation: Deposition by strong oscillatory storm currents, which suspended silt and clay and concentrated find sand on the Belt lake floor. Accumulated on maximum flooding surfaces at the bases of Helena and Wallace cycles.
graphicDescription: Size-graded fine-sand-to-mud layers more than 3 cm thick with sharp, commonly guttered and loaded bases below hummocky, flat-laminated, or poorly laminated, light to medium gray sand lenses that grade up to dark gray mud. Capping mud layers are commonly cut by sand-filled cracks and gutters or are loaded below sand of overlying couples. Common in lower parts of cycles in the Wallace formation and combined with pinch-and-swell couplets in the Helena descriptions.Interpretation: Hummocks record traction transport and deposition by large storm waves. Flat laminae may record combined flow, and poorly laminated layers may reflect rapid accumulation of storm-suspended sand with little reworking. Capping mud layers probably accumulated as storms waned and sediment suspension exceeded transport capacity, followed by settleout in still water. Solitary waves generated sand- filled cracks (Winston and Smith,1997).
graphicDescription: Similar in form to pinch-and-swell couples, but thinner, .3 to 3 cm-thick couplet-scale graded layers, result in smaller very fine sand hummocks, gutters, loads, and thinner dark gray capping mud. Common above pinch-and-swell couples or at the bases of cycles in the Wallace Formation and middle member of the Helena Formation.Interpretation: Sand layers record smaller, possibly depth-limited storm waves than those of pinch-and-swell couples, but processes are similar.
graphicDescription: Graded silt-to-clay couplets with sharp-based, light gray silty and very fine-sand laminae that gently rise and fall across outcrops. Capping clay layers are light gray or dolomitic. Form the bases of many Helena cycles.Interpretation: Commonly interlayered with pinch-and-swell couplets on one hand and with lenticular couplets on the other, suggesting they formed by intermediate-size oscillatory waves. Wave size may have been depth-limited.
graphicDescription: Even, continuous, non-cracked graded fine sand and silt-to-mud couplets that stretch across outcrops for more than a few meters.Interpretation: Deposited from suspension in contrast to traction deposition of undulating couplets. Sediment suspension clouds were probably generated either by storms or from influx of terrestrial sheetfloods.
graphicDescription: Light gray, flat-based, straight-crested, fine-grained sandy symmetric-ripple lenses beneath gray argillitic to tan-weathering dolomitic mud caps, with or without desiccation cracks in the Helena and Wallace formations. Commonly interlayered with undulating couplets.Interpretation: Fine sand reworked by fair-weather waves in shallow water, followed by suspension settleout in still water.
graphicDescription: Silt laminae, commonly only a few grains thick, sharply capped by dark gray or tan-weathering, dolomitic clay. Range from even to tightly folded and from non-cracked to desiccation cracked. Some with small-oscillation-ripple fine sand lenses. Commonly forms near shore from undulating and even couplets in the lower half-cycles in the Wallace formation.Interpretation: Concentration of microcouplets in the landward facies-tract margins and at the tops of cycles, together with occasional desiccation cracks, indicate shallow-water accumulation, probably from storm suspension in very shallow water (Hill and Nedeau,1989).
graphicDescription: Concentrations of fine carbonate silt peloids in fine sparry to compacted micritic matrix, commonly mixed with clay and scattered quartz silt grains. Poorly stratified to undulating couplet layers. Dolomitized Helena upper half-cycles.Interpretation: Precipitated as calcitic peloids, probably induced by bacterial metabolism in the water column, with high calcium, magnesium, and carbonate concentrations during periods of lake contraction.
graphicDescription: Mostly domal stromatolite beds, commonly on transgressive ravinement surfaces at the bases of Helena and Wallace cycles. Include biostromes of Baicalia and Conophyton growth forms in the Baicalia member of the Wallace Formation.Interpretation: Domal stromatolites grew on eroded pinnacles of ravinement surfaces during expansion of the Belt lake. Calcitic Baicalia and Conophyton biostromes may indicate freshening of a balanced-fill lake during deposition of the Baicalia member of the Wallace Formation.
graphicDescription: Beds up to 20 centimeters thick ranging from coarse- to medium-grained, crossbedded quartzose, oolitic sand, to mudchips and molartooth intraclasts. Forms bases of many cycles in the Helena and Wallace formations.Interpretation: Traction transport of coarse- and medium-grained sand and coarse clasts, together with winnowing of muddy sediment, over wind-setup surfaces during expansion of the Belt lake and on cycle-highstand beaches.
graphicDescription: Well sorted, clean, fine-grained, light gray sand with low-angle hummocky cross-stratification, horizontal stratification, or poorly defined stratification. Most are stacked into cosets more than 10 cm thick and extend continuously across individual outcrops. Form the bases of cycles in the Helena and Wallace formations.Interpretation: Deposition by strong oscillatory storm currents, which suspended silt and clay and concentrated find sand on the Belt lake floor. Accumulated on maximum flooding surfaces at the bases of Helena and Wallace cycles.
graphicDescription: Size-graded fine-sand-to-mud layers more than 3 cm thick with sharp, commonly guttered and loaded bases below hummocky, flat-laminated, or poorly laminated, light to medium gray sand lenses that grade up to dark gray mud. Capping mud layers are commonly cut by sand-filled cracks and gutters or are loaded below sand of overlying couples. Common in lower parts of cycles in the Wallace formation and combined with pinch-and-swell couplets in the Helena descriptions.Interpretation: Hummocks record traction transport and deposition by large storm waves. Flat laminae may record combined flow, and poorly laminated layers may reflect rapid accumulation of storm-suspended sand with little reworking. Capping mud layers probably accumulated as storms waned and sediment suspension exceeded transport capacity, followed by settleout in still water. Solitary waves generated sand- filled cracks (Winston and Smith,1997).
graphicDescription: Similar in form to pinch-and-swell couples, but thinner, .3 to 3 cm-thick couplet-scale graded layers, result in smaller very fine sand hummocks, gutters, loads, and thinner dark gray capping mud. Common above pinch-and-swell couples or at the bases of cycles in the Wallace Formation and middle member of the Helena Formation.Interpretation: Sand layers record smaller, possibly depth-limited storm waves than those of pinch-and-swell couples, but processes are similar.
graphicDescription: Graded silt-to-clay couplets with sharp-based, light gray silty and very fine-sand laminae that gently rise and fall across outcrops. Capping clay layers are light gray or dolomitic. Form the bases of many Helena cycles.Interpretation: Commonly interlayered with pinch-and-swell couplets on one hand and with lenticular couplets on the other, suggesting they formed by intermediate-size oscillatory waves. Wave size may have been depth-limited.
graphicDescription: Even, continuous, non-cracked graded fine sand and silt-to-mud couplets that stretch across outcrops for more than a few meters.Interpretation: Deposited from suspension in contrast to traction deposition of undulating couplets. Sediment suspension clouds were probably generated either by storms or from influx of terrestrial sheetfloods.
graphicDescription: Light gray, flat-based, straight-crested, fine-grained sandy symmetric-ripple lenses beneath gray argillitic to tan-weathering dolomitic mud caps, with or without desiccation cracks in the Helena and Wallace formations. Commonly interlayered with undulating couplets.Interpretation: Fine sand reworked by fair-weather waves in shallow water, followed by suspension settleout in still water.
graphicDescription: Silt laminae, commonly only a few grains thick, sharply capped by dark gray or tan-weathering, dolomitic clay. Range from even to tightly folded and from non-cracked to desiccation cracked. Some with small-oscillation-ripple fine sand lenses. Commonly forms near shore from undulating and even couplets in the lower half-cycles in the Wallace formation.Interpretation: Concentration of microcouplets in the landward facies-tract margins and at the tops of cycles, together with occasional desiccation cracks, indicate shallow-water accumulation, probably from storm suspension in very shallow water (Hill and Nedeau,1989).
graphicDescription: Concentrations of fine carbonate silt peloids in fine sparry to compacted micritic matrix, commonly mixed with clay and scattered quartz silt grains. Poorly stratified to undulating couplet layers. Dolomitized Helena upper half-cycles.Interpretation: Precipitated as calcitic peloids, probably induced by bacterial metabolism in the water column, with high calcium, magnesium, and carbonate concentrations during periods of lake contraction.
graphicDescription: Mostly domal stromatolite beds, commonly on transgressive ravinement surfaces at the bases of Helena and Wallace cycles. Include biostromes of Baicalia and Conophyton growth forms in the Baicalia member of the Wallace Formation.Interpretation: Domal stromatolites grew on eroded pinnacles of ravinement surfaces during expansion of the Belt lake. Calcitic Baicalia and Conophyton biostromes may indicate freshening of a balanced-fill lake during deposition of the Baicalia member of the Wallace Formation.

Event graded layers are the most pervasive sedimentary structures in Belt rocks, and the grain size and thicknesses of the graded layers form the basis of the sediment–type classification scheme. Sharp–based graded layers 0.3 to 3 cm thick are called couplets. Graded layers 3 to 10 cm, and locally thicker, are here called couples, and thin graded layers less than 0.3 cm thick are here called microcouplets. Within the graded layers are additional sedimentary structures, such as crossbeds and mudcracks. They provide additional criteria for describing and classifying the array of siliciclastic sediment types in the Helena and Wallace.

Whereas siliciclastic sediment types reflect transport and deposition of terrigenous sediments, carbonate sediments are formed in place and differ genetically from siliciclastic sediment types. Therefore carbonate sediment types are separated from siliciclastic sediment types by their mineralogy. However, carbonate sediments in the Helena and Wallace are commonly mixed with siliciclastic sediment types, where they become modifiers of the siliciclastic sediment type.

Color of Belt rocks largely reflects diagenesis, but through diagenesis some colors reflect original depositional conditions and environments. Therefore they play a role in description of Belt rocks. For example, dark muddy layers largely record preserved organic material in subaqueous sediments; light gray arenite mostly reflects clean sand; green beds commonly record reduced ferrous iron in chloritic subaqueous sediments; and hematitic red beds mostly, but not everywhere, record oxidizing subaerial exposure. Historically color has played a very important role in defining Belt formations. Consequently, some mapped formations reflect diagenesis and do not everywhere coincide with stratigraphic units based on original sediment types.

Although Winston and Link (1993) recognize nineteen sediment types in the Belt Supergroup, sediment types in the Helena and Wallace formations are limited to the following ten (Table 2).

  1. The coarse–sand–and–intraclast sediment type is composed of intraclasts, quartz sand, and oolite (Fig. 3C,D). It mostly represents thin lag deposits on flat ravinement surfaces at the bases of the cycles, wind–setup flats, and beach deposits of cycle highstands.

  2. The hummocky–couple sediment type is composed of tabular, stacked fine sand beds more than 3 cm thick, with internal flat laminae or low–angle hummocky cross laminae (Fig. 3E,F). It records storm–deposited sand beds from which silt and clay have been winnowed.

  3. The pinch–and–swell–couple sediment type contains hummocky and faintly laminated sand lenses more than 3 cm thick that grade up into more continuous thin beds of dark–gray mud (Fig. 3G). Bases of the sand layers bow down into the mud layers below as gutter casts and load casts. It records hummocky sand deposition by storms followed by suspension settleout of dark mud (Johnson, 1990).

  4. The pinch–and–swell–couplet sediment type is similar to the pinch–and–swell–couple type but differs in having beds 0.3 cm to 3 cm thick (Figs. 3H, 4A). It also records storm deposition, but by smaller waves than the pinch–and–swell–couple type.

  5. The undulating–couplet sediment type has thin, fine sand and silt layers with gently undulating internal laminae that grade up to mud, forming couplets 0.3 to 3 cm thick (Fig. 4B,C). It is inferred to record traction transport and deposition by small storm waves, probably limited in size by shallow water, followed by suspension settleout.

  6. The even–couplet type of couplets 0.3 to 3 cm thick has evenly laminated lower fine sand and silt layers that grade up to mud. It records settleout of silt and clay suspended by storms or introduced by terrestrial sheetfloods (Fig. 4B,F).

  7. The lenticular–couplet sediment type contains oscillation sand ripple lenses in the lower layers that are capped by mud at the couplet tops (Fig. 4D). It records sand rippled by fair–weather waves followed by suspension settleout in very shallow water.

  8. The microcouplet sediment type has very thin alternating laminae of silt and clay, forming paired layers less than 0.3 cm thick (Fig. 4E,F,G). It represents episodic influx of suspended silt and clay and settleout in shallow, still water.

  9. The carbonate–silt sediment type is composed of silt–size carbonate peloids, probably induced by cyanobacteria in water with high bicarbonate concentrations (Ambrose, 2005). It forms dolomite beds in the Helena in the absence of silici– clastic input. Carbonate silt grains are also mixed with silici– clastic sediment types in the dolomitic cycle caps of the Helena and the lowermost and uppermost Wallace.

  10. The stromatolite sediment type includes domal stromatolites that grew on wave–agitated ravinement surfaces of the Helena and Wallace cycle bases and on highstand strands. It also includes the Baicalia and Conophyton forms that flourished in the Baicalia cycles of the Wallace Formation (Fig. 4H).

Fig. 3.—

Helena cycles, coarse sand and intraclast, hummocky couple, pinch–and–swell couple, and couplet sediment types. A) Three siliciclastic–to–dolomite cycles and the base of a fourth in the lower member of the Helena Formation at Ravalli Hill. Smooth, light yellowish gray beds are mostly siliciclastic undulating couplets of the lower half–cycles. Blocky tan–weathering beds are dolomitic undulating and even couplets of the upper half–cycles. B) A siliciclastic–to–dolomite cycle 5 m thick, underlain and overlain by cycles of the middle Helena member in the lower part of the Bull River section (Fig. 6, Table 1). C) Coarse quartz, round intraclast and oolitic grainstone of the coarse sand and intraclast sediment type. Scale in centimeters. D) Tan dolomite intraclasts and dark– gray molartooth ribbon intraclasts of the coarse sand and intraclast sediment type. E) Flat–laminated fine arenite bed overlain by a hummocky crossbed, both of the hummocky couple sediment type from the Helena member at Big Hole Peak (Fig. 6, Table 1). F) Tabular beds of fine arenite of the hummocky couple sediment type. G) Thick and thin medium–gray arenite lenses with gutters capped by black argillite of the pinch–and–swell couple sediment type. Argillite beds are cut by sand–filled cracks attributed to solitary windgenerated waves (Winston and Smith, 1997).H) Pinch–and– swell couplets with hummocky cross–laminated fine light gray arenite capped by dark gray mud.

Fig. 3.—

Helena cycles, coarse sand and intraclast, hummocky couple, pinch–and–swell couple, and couplet sediment types. A) Three siliciclastic–to–dolomite cycles and the base of a fourth in the lower member of the Helena Formation at Ravalli Hill. Smooth, light yellowish gray beds are mostly siliciclastic undulating couplets of the lower half–cycles. Blocky tan–weathering beds are dolomitic undulating and even couplets of the upper half–cycles. B) A siliciclastic–to–dolomite cycle 5 m thick, underlain and overlain by cycles of the middle Helena member in the lower part of the Bull River section (Fig. 6, Table 1). C) Coarse quartz, round intraclast and oolitic grainstone of the coarse sand and intraclast sediment type. Scale in centimeters. D) Tan dolomite intraclasts and dark– gray molartooth ribbon intraclasts of the coarse sand and intraclast sediment type. E) Flat–laminated fine arenite bed overlain by a hummocky crossbed, both of the hummocky couple sediment type from the Helena member at Big Hole Peak (Fig. 6, Table 1). F) Tabular beds of fine arenite of the hummocky couple sediment type. G) Thick and thin medium–gray arenite lenses with gutters capped by black argillite of the pinch–and–swell couple sediment type. Argillite beds are cut by sand–filled cracks attributed to solitary windgenerated waves (Winston and Smith, 1997).H) Pinch–and– swell couplets with hummocky cross–laminated fine light gray arenite capped by dark gray mud.

Fig. 4.—

Field photos of pinch–and–swell couplet, undulating couplet, even couplet, microcouplet, and stromatolite sediment types. A) Pinch–and–swell couplets with fine light gray, yellow–weathering, arenite lenses capped by black argillite layers. B) Cycle boundary in the Helena Formation. Tan weathering, dolomitic even–couplet beds at the top of the lower cycle are cut by rills that are filled with molartooth intraclast fragments beneath siliciclastic light gray and dark gray even and undulating couplets in the lower siliciclastic part of the overlying cycle. C) Undulating couplets of darker gray very fine arenite and silt, overlain by lighter gray mud. D) Lenticular couplet sediment type with dark greenish gray oscillation–ripple arenite lenses capped by more continuous yellowish gray mud. E) Tan–weathering dolomitic microcouplets. F) Dolomitic even couplets. (G) Fine silty and clayey, tan–weathering dolomitic microcouplets and even couplets. Darker tan silt layers are capped by lighter tan mud laminae.

Fig. 4.—

Field photos of pinch–and–swell couplet, undulating couplet, even couplet, microcouplet, and stromatolite sediment types. A) Pinch–and–swell couplets with fine light gray, yellow–weathering, arenite lenses capped by black argillite layers. B) Cycle boundary in the Helena Formation. Tan weathering, dolomitic even–couplet beds at the top of the lower cycle are cut by rills that are filled with molartooth intraclast fragments beneath siliciclastic light gray and dark gray even and undulating couplets in the lower siliciclastic part of the overlying cycle. C) Undulating couplets of darker gray very fine arenite and silt, overlain by lighter gray mud. D) Lenticular couplet sediment type with dark greenish gray oscillation–ripple arenite lenses capped by more continuous yellowish gray mud. E) Tan–weathering dolomitic microcouplets. F) Dolomitic even couplets. (G) Fine silty and clayey, tan–weathering dolomitic microcouplets and even couplets. Darker tan silt layers are capped by lighter tan mud laminae.

Additional important sedimentary structures in the Piegan Group include: (1) desiccation cracks, most common in the microcouplet sediment type as far west as Libby Dam, (2) compacted sand–filled cracks in the mud caps of pinch–and–swell layers, formed by solitary storm waves passing through fluid mud (Winston and Smith, 1997), (3) calcite pods of early carbonate cement, (4) molartooth blobs and ribbons formed by gas bubbles and cracks that were filled by vaterite that has inverted to calcite (Furniss et al., 1998; Gellatly and Winston, 1998), and (5) scattered halite casts. Sediment types as described and interpreted in Table 2 are similar to previously described Helena and Wallace sediment types (Winston, 1986b, 1986c; Winston and Lyons, 1993; Winston and Link, 1993; Winston, 1997, 2003) with minor modification.

stratigraphy and sedimentology of the Helena Formation

Helena Cycle Patterns

More than 90% of the Helena Formation is composed of sedimentary cycles with siliciclastic lower half–cycles and dolo– mitic upper half–cycles, most of which fine and thin upward.Some have been described by Winston and Lyons (1993). Figure 5A and B illustrates the principal vertical sediment–type configurations in the Helena, some of which are shown in Figure 3A and B. Some intervals are totally siliciclastic and range from those that are uniform and noncyclic to those that fine and thin upward through a succession of sediment types. Dolomite–capped cycles are more abundant and range from those in which a single sediment type is mixed with dolomite in the upper half–cycles, to those in which an upward–fining and –thinning succession of siliciclastic sediment types is overprinted by dolomite in the upper half–cycles. Cycles are classified into hummocky–couple, pinch–and–swell, undulating–couplet, even–couplet, lenticular– couplet, microcouplet, and carbonate–silt cycle styles, named for the sediment type in the lower part of each cycle, which sets the stage for the cycle succession. Lithostratigraphy of the Helena Formation is described below in terms of the cycle types.

Fig. 5.—

A) Diagram illustrating the principal styles of vertical successions of hummocky couple, pinch–and–swell, undulating couplet, even couplet, microcouplet, lenticular couplet, and carbonate silt sediment types in Helena cycles.

Fig. 5.—

A) Diagram illustrating the principal styles of vertical successions of hummocky couple, pinch–and–swell, undulating couplet, even couplet, microcouplet, lenticular couplet, and carbonate silt sediment types in Helena cycles.

Fig. 5.—

B) Symbols for siliciclastic and dolomitic sediment types shown in Figure 5A.

Fig. 5.—

B) Symbols for siliciclastic and dolomitic sediment types shown in Figure 5A.

Helena Cyclic Stratigraphy

The stratigraphy of the Helena Formation is based on measured sections located in Figure 6 and in Table 1 from which the cross sections in Figures 711 are constructed. Shown in the cross sections are the dominant cycle types, which provide a basis for subdividing the Helena into lower, middle, and upper informal members (Figs. 714). The lower Helena member is characterized by light gray undulating–couplet and lenticular–couplet cycles (Figs. 711). The middle member contains two wedges of hum– mocky–couple and dark gray pinch–and–swell cycles in the west that pass eastward and southwestward to undulating–couplet cycles (Figs. 7, 8, 10). The upper member marks the return of light gray undulating–couplet cycles across the basin.

Fig. 6.—

Map of Figure 1A inset showing Piegan Group outcrops, principal faults, location of Helena measured sections, and cross sections in Figures 711. Numbered sections are as follows: 1 = Crater Mountain; 2 = St. Regis Lakes; 3 = Placer Creek; 4 = Galena Mine; 5 = Steamboat Creek; 6 = Clark Fork; 7 = West Libby Dam; 8 = Bull River; 9 = Big Hole Peak, Weeksville Road, and Round Horn composite; 10 = St. Mary Lake; 11 = Mollman Pass and Eagle Pass; 12 = Red Mountain; 13 = Rogers Pass; 14 = Little Skunk; 15 = Dearborn River; 16 = Wood Creek Hogback; 17 = Grinnell Glacier and Big Bend

Fig. 6.—

Map of Figure 1A inset showing Piegan Group outcrops, principal faults, location of Helena measured sections, and cross sections in Figures 711. Numbered sections are as follows: 1 = Crater Mountain; 2 = St. Regis Lakes; 3 = Placer Creek; 4 = Galena Mine; 5 = Steamboat Creek; 6 = Clark Fork; 7 = West Libby Dam; 8 = Bull River; 9 = Big Hole Peak, Weeksville Road, and Round Horn composite; 10 = St. Mary Lake; 11 = Mollman Pass and Eagle Pass; 12 = Red Mountain; 13 = Rogers Pass; 14 = Little Skunk; 15 = Dearborn River; 16 = Wood Creek Hogback; 17 = Grinnell Glacier and Big Bend

Fig. 7.—

North–south cross section A–A’ showing the principal styles of Helena cycles through the western part of the Belt basin, where the Helena thins over the Noxon block.

Fig. 7.—

North–south cross section A–A’ showing the principal styles of Helena cycles through the western part of the Belt basin, where the Helena thins over the Noxon block.

Helena sections reach more than 750 m thick in the central part of the basin and thin both westward and eastward. In the west, the Helena thins to about 250 m in the Placer Creek and the Galena Mine and Steamboat Creek sections on the Noxon block. There, dolomite marking the cycle caps diminishes in the lower Helena member and in units A and B of the middle member, resulting in a continuous interval of green siliciclastic undulating couplets that resemble the Empire Formation (Fig. 7). The Helena thins to the east by its lower and middle members passing laterally into the Empire Formation, with only 30 to 100 m of upper Helena extending onto the Steinbach plate (Fig. 9). In the following discussion the main basin refers to the thick part of the Helena Formation bounded by the Clark Fork, St. Regis Lakes, and Crater Mountain sections on the west and the Red Mountain and Grinnell Glacier sections on the east.

Lower Helena Member.—

Across most of the basin the lower Helena member can be subdivided into a lower unit A characterized by interstratified undulating–couplet and lenticular–couplet cycles, and an upper unit B with mostly undulating–couplet cycles (Figs. 711, 12A,B). Undulating–couplet cycles are widespread across the northwestern part of the Belt basin in unit A, where they surround a lens of green, siliciclastic undulating couplets, which is probably a tongue of the Empire Formation farther east (Figs. 8, 10). They pass southwestward to even–couplet cycles in the St. Regis Lakes section and in turn to lenticular–couplet cycles at Crater Mountain (Fig. 8). Undulating–couplet cycles of the northwestern part of the basin also pass eastward to lenticular–couplet and even– couplet cycles, and into the Empire Formation at the Grinnell Glacier section (Figs. 9, 10).

Fig. 8.—

North–south cross section B–B’ showing the principal styles of Helena cycles through the thick western part of the main Helena basin.

Fig. 8.—

North–south cross section B–B’ showing the principal styles of Helena cycles through the thick western part of the main Helena basin.

Fig. 9.—

North–south cross section C–C’ showing the principal types of Helena cycles along the eastern part of the Belt Basin including sections on the Steinbach thrust plate.

Fig. 9.—

North–south cross section C–C’ showing the principal types of Helena cycles along the eastern part of the Belt Basin including sections on the Steinbach thrust plate.

Fig. 10.—

East–west cross section D–D’ across the northern part of the Belt basin.

Fig. 10.—

East–west cross section D–D’ across the northern part of the Belt basin.

Hummocky couple and pinch–and–swell cycles mark the base of Unit B of the Helena lower member in the west and east (Figs. 710, 12B). These are overlain by undulating–couplet cycles, which spread eastward across the basin and interfinger with even–couplet and lenticular–couplet cycles toward the southwest.

Middle Helena Member.—

The middle member of the Helena Formation is characterized by hummocky–couple cycles in the western part of the main basin and by pinch–and–swell cycles that stretch eastward far across the central part of the basin. The middle Helena member can be subdivided into a lower unit A, with hummocky–sand cycles in the western part of the main basin, unit B with mostly pinch–and– swell cycles in the west, and unit C, which again has hummocky–couple cycles in the northern and western part of the main basin (Figs. 711, 13).

Fig. 11.—

East–west cross section E–E’ across the central part of the Belt basin.

Fig. 11.—

East–west cross section E–E’ across the central part of the Belt basin.

Fig. 12.—

A) Sediment–type successions in noncyclic and cyclic intervals the lower Helena member, unit A. B) Sediment type successions in the lower Helena member, unit B.

Fig. 12.—

A) Sediment–type successions in noncyclic and cyclic intervals the lower Helena member, unit A. B) Sediment type successions in the lower Helena member, unit B.

The base of member A is marked by the abrupt expansion of hummocky–couple and pinch–and–swell cycles across the western and central parts of the main basin (Figs. 711). Undulating–couplet cycles, so common in the lower Helena, are limited to the easternmost and westernmost sections. Even–couplet and microcouplet cycles are also rare. Well developed is the vertical succession from hummocky–couple beds passing up into pinch–and–swell beds, some of which continue into dolomite of the cycle caps. Elsewhere dolomitic cycle caps are mostly composed of undulating–couplet and even–couplet sediment types. Member A thins westward and passes into an interval of noncyclic undulating couplets on the Noxon block (Fig. 7).

Middle Helena member B is marked by the virtual disappearance of hummocky–couple cycles and the sharp limitation of pinch–and–swell cycles to Clark Fork, Bull River, and west Libby Dam sections (Figs. 711, 13B). In their absence, undulating– couplet cycles extend widely across the basin and become interbedded with even–couplet and microcouplet cycles toward the east and southwest. Pinch–and–swell couplets and undulating couplets continue upward into some dolomitic cycle caps. Elsewhere even couplets and microcouplets form the dolomitic cycle caps. Wholly siliciclastic cycles occur in the St. Regis Lakes section, and noncyclic undulating even couplets and undulating couplets crop out in the Steamboat and Galena Mine sections on the Noxon block (Fig. 7).

Fig. 13.—

A) Sediment types in noncyclic intervals and vertical sediment–type successions in the middle Helena member, unit A. B)> Sediment types in noncyclic intervals and vertical sediment–type successions in the middle Helena member, unit B. C) Sediment types and vertical sediment–type successions in the middle Helena member, unit C. Symbols are shown in Figure 12.

Fig. 13.—

A) Sediment types in noncyclic intervals and vertical sediment–type successions in the middle Helena member, unit A. B)> Sediment types in noncyclic intervals and vertical sediment–type successions in the middle Helena member, unit B. C) Sediment types and vertical sediment–type successions in the middle Helena member, unit C. Symbols are shown in Figure 12.

Middle Helena unit C is the thickest unit of the middle Helena member and has a great variety of sediment–type patterns within the cycles (Fig. 13C). These illustrate the progressive eastward fining and thinning of the cycle types from hummocky sand in the west, to pinch–and–swell, to undulating, to even–couplet, to microcouplet cycle types toward the east (Figs. 713). The base of the unit is marked by the abrupt expansion of hummocky–couple cycles across the western part of the main basin accompanied by shifting of pinch–and–swell cycles into the central part of the basin (Figs. 711). Undulating–couplet and even–couplet cycles also occur in the central, southeastern, and eastern parts of the basin, and microcouplet cycles are limited to the easternmost sections. Within the cycles hummocky–couple beds rarely continue up into the dolomitic caps, and more commonly they are overlain by pinch–and–swell beds and undulating couplets, which also continue up into dolomite cycle caps in several sections. Elsewhere dolomitic undulating couplets and even couplets form the cycle caps. Wholly siliciclastic hummocky–couple, pinch–and–swell, and undulating–couplet cycles also occur, mostly in the west. Noncyclic pinch–and–swell intervals also occur at the St. Regis Lakes and Galena Mine sections (Fig. 7).

Upper Helena Member.—

The upper member of the Helena Formation records the reestablishment of cycle patterns similar to those of the lower Helena. Undulating–couplet cycles extend across the central, southwestern, and eastern parts of the main basin (Figs. 711, 14). As in the lower Helena, siliciclastic undulating couplets resembling the Empire Formation were deposited across the Noxon block from the Steamboat Creek, Placer Creek, and Galena Mine sections and southeast to the St. Regis Lakes section (Fig. 7). These pass southward to greenish microcouplet cycles with occasional mudcracks in the Crater Mountain section. Undulating–couplet cycles of the main basin also pass laterally to microcouplet and carbonate silt cycles on the Steinbach plate (Figs. 9, 11).

Fig. 14.—

Sediment types in noncyclic intervals and sediment–type successions in cycles of the upper Helena member. Symbols are shown in Figure 12.

Fig. 14.—

Sediment types in noncyclic intervals and sediment–type successions in cycles of the upper Helena member. Symbols are shown in Figure 12.

Summary of Helena Cycle Facies Patterns

The cycle types that are generalized in the stratigraphic cross sections are illustrated in more detail in Figures 1214. They show a remarkable regularity in the sediment–type stacking patterns within the Helena that can be generalized into the following points.

  1. Some Helena intervals are noncyclic and are composed of single sediment types up to 65 m thick (Figs. 12A, 13A,B,C, 14).

  2. Many cycles, like the noncyclic intervals, are composed of only a single siliciclastic sediment type that is overprinted by dolomite, forming the upper half–cycles.

  3. Most cycles in the Helena are asymmetric, with a succession of siliciclastic sediment types. Bases of the cycles are sharp and are overlain by a stepwise succession of the following silici– clastic sediment types: a) hummocky couple, b) pinch–and– swell couple and couplet, c) undulating couplet, d) even couplet, e) lenticular couplet, and f) microcouplet.

  4. Among these, some cycles are wholly siliciclastic (Fig. 5).

  5. Most cycles display the above succession of siliciclastic sediment types, which are sharply overprinted by dolomite in the upper half–cycles. Fine dolomite crystals have replaced silt– size carbonate grains of the carbonate–silt sediment type that are mixed with siliciclastic sediments in the upper halves of most cycles. In some cycles the base of the dolomite lies within a single siliciclastic sediment type; in other cycles dolomite coincides with a change in siliciclastic sediment types.

  6. Some dolomitic cycle caps are composed wholly of the car– bonate–silt sediment type (Fig. 5).

  7. The vertical succession of siliciclastic sediment types in the cycles is replicated laterally in the Helena members. For example, coarser–grained, thicker sediment types that form the bases of cycles in the north and west characteristically pinch out eastward and southwestward, so that the next overlying sediment types form the adjacent cycle bases. Nevertheless, where sediment types at cycle bases are omitted, the general vertical sediment–type sequences within the cycles are maintained. Sediment types in the upper parts of the cycles are also cut out in a systematic way. Sediment types at the tops of cycles in the east step down westward from the carbonate silt progressively down to hummocky– couple sediment types.

In addition to the above depositional patterns, the nondepositional surfaces are also important. The dolomitic tops of many cycles are flat and are simply overlain conformably by the siliciclastic sediment type at the base of the overlying cycle. These are mostly concentrated in the center of the basin. Other cycle bases are flat and mantled by thin, centimeter–scale layers of molartooth intraclasts of the coarse sand and intraclast sediment type. They indicate limited scour with little erosional relief. Most of these lie shoreward of the conformable cycle bases. Still other cycle bases are more deeply scoured with up to 10 cm of relief. Molartooth intraclasts commonly fill the scour pockets, and occasional domal stromatolites envelop the pinnacles. These cycle bases lie along the eastern basin margin. No cycle bases reflect deep erosion or exposure accompanied with diagenetic alteration.

The above points are summarized and synthesized into the conceptual model shown in Figure 15, where the vertical succession of sediment types in the cycles on the right is configured in plan view in the conceptual block diagram on the left. The shoreward–fining and –thinning facies–tract pattern of the silici– clastic sediment types is shown in the block diagram, and alongside them is the dolomitic facies–tract counterpart. Most of the observed vertical and lateral sediment–type successions observed in the Helena can be illustrated in this model, which forms the basis for interpreting the depositional processes and environments of the Helena Formation.

Depositional Constraints Inferred from the Helena Cycle Model

The Helena sediment–type model (Fig. 15) and the topography of the Helena basin place important constraints on the interpretation of Helena sedimentology. Four of these constraints are enumerated below.

  1. The asymmetric cycles record abrupt expansion of the Helena water body across nearly level, flat flooding surfaces to high– stand, followed by progressive contraction and exposure around the basin margin. The siliciclastic sediment types either aggraded to the tops of the cycles or prograded only one sediment type basinward.

  2. The dolomite cycle caps that commonly crosscut the siliciclas– tic sediment types show that the dolomite behaved independently from the siliciclastic facies tract of Figure 15. The siliciclastic sediment types of the facies–tract model record the array of depositional processes and environments, whereas the dolomite probably represents induced chemical precipitation of carbonate silt grains that mixed with siliciclastic grains even beneath the conformable cycle boundaries. Therefore the dolomite does not represent a prograding carbonate depositional environment.

  3. The flat, only locally scoured cycle bases and the lateral continuity of the facies patterns within the cycles indicate that the depositional surfaces of the Helena basin were exceedingly flat and virtually level. Hummocky–couple, pinch–and–swell, and undulating–couplet beds at the cycle bases show that the Helena basin floor was everywhere above storm wave base. There are no sub–wave–base clinothem slope deposits in the Helena, and therefore no adjoining shelf. The basin was shallow and flat. In addition, the absence of erosional incision indicates that base level never dropped below the flat, level depositional surface of the basin.

  4. During highstand, wave turbulence was greatest in the northern and western parts of the main Helena basin and diminished eastward and southwestward toward the inferred basin margins.

Lacustrine Interpretation of The Helena Cycles

I have previously proposed that the Helena Formation was deposited in a lake (Winston, 1986b, 1991, 1994, 1997, 2003), and I continue to hold this view. The above constraints and additional points enumerated below strengthen the lake interpretation and narrow the Helena to the deposit of a broad, shallow underfilled lake deposit (Bohacs et al., 2000).

  1. The dolomite precipitation across the siliciclastic facies tract during cycle regression indicates that contraction of an enclosed, isolated water body induced carbonate supersaturation and precipitation.

  2. Cross sections in Figures 711 show that the Helena basin was bordered on the east by Laurentia and may have been bordered along the southern part of the Noxon block by an adjacent western shore line.

  3. Common halite casts in dolomitic beds of the eastern part of the basin (Eby, 1977) and less common large halite casts in the pinch–and–swell beds in the western part of the basin (Grotzinger, 1981) indicate that, at times at least, waters were hypersaline across the whole Helena basin.

  4. The upward progression from cyclic ephemeral continental playa–lake deposition in redbeds of the Ravalli Group into more prolonged cyclic lacustrine deposition in green beds of Empire, up into similar Helena rocks, supports the continuity of lake deposition from the Ravalli Group into the Helena (Winston and Lyons, 1993). Siliciclastic sediment types and their patterns in the lower Helena are similar to those in the upper Empire. At no point in the Helena succession is there evidence for replacement of lacustrine conditions by marine processes.

  5. Finally, the oxygen–isotope δ18O values, ranging from –12% to – 9% in the Helena parts of the Piegan sections reported by Frank et al. (1997), can be explained by evaporative fractionation of ocean waters and atmospheric transport and precipitation to an inland basin. Frank and others demonstrated the consistency and reliability of isotope data in the Belt, and Kah (2000) supported the reliability of oxygen isotopes in Proterozoic rocks.

Lacustrine Deposition of Helena Cycles

Depositional processes of the Helena cycles are recorded in the lateral arrangements of the sediment types (Table 2), and the history of the Helena cycles is recorded in the vertical stacking patterns of the sediment types within the cycles (Figs. 711). The sediment–type stacking patterns are interpreted to record repeated expansions and contractions of the Helena lake and can be expressed in terms of shifts of the general facies tract illustrated in Figure 15 from lowstand, to highstand, to contraction, and back to lowstand.

Lake Lowstand.—

Lake lowstands are recorded at the cycle bases. Cycle bases that lack scour and evidence of exposure conformably overlie dolomite of the underlying cycle and mark subaqueous low– stands across the lake floor. Cycle bases mantled by intraclasts were probably exposed and represent flat, nearly horizontal flooding surfaces of the overlying cycles. Exposure of the cycle bases is also recorded in desiccation cracks that underlie the cycle bases along the lake margins.

Highstand Lake Facies Tract.—

The minor scour and thin intraclast lenses indicate that the lake expanded across the flat–level flooding surfaces to high– stand without depositing a transgressive facies tract. The conformable highstand cycle bases record expansion of the lake and a shift from chemical saturation of the mixed siliciclastic and dolomite sediments of the restricted lake below to undersaturation above the cycle boundaries accompanying fresh–water inflow and freshening of the expanded lake. Flooding surfaces of the highstand tract were scoured either during transgression or as ravinement surfaces developed during lake highstand. Nonlithified carbonate mud on the flooding surfaces was swept away, and intraclasts and rigid molartooth ribbons and blobs (O’Connor, 1972) collected in scour pockets and in thin patches on the flooding surfaces (Fig. 4B). Dolomitic pinnacles of the underlying scoured cycle tops projected above the fine accumulating sediment and some were colonized by domal stromatolites.

The deposits of highstand facies tracts are represented by the siliciclastic facies tract illustrated in Figure 15 and reflect decreasing wave turbulence from the hummocky–couple sediment type at the basin center toward finer and thinner–bedded sediment types at the basin margins. Wind fetch and water depth were sufficient to form large storm waves in the center of the basin. Fine sand and mud derived from the western craton were worked and reworked by storms, which winnowed the mud and concentrated the sand, forming the sandy hummocks of the siliciclastic hummocky couple sediment type.

Fig. 15.—

Conceptual facies-tract model at cycle boundary illustrating the lateral shoreward-fining and -thinning arrangement of sediment types. Both the siliciclastic sediment types and their mixed dolomitic and siliciclastic counterparts are shown side-by-side. Jagged lines at the block diagram edges represent exposed and scouredsurfaces at the cycle boundaries. Smooth surfaces illustrate conformable cycle boundaries where precipitated carbonate was commonly mixed with siliciclasticsediments. Vertical succession of siliciclastic sediment types in Helena cycles on the right. Symbols are shown in Figure 12.

Fig. 15.—

Conceptual facies-tract model at cycle boundary illustrating the lateral shoreward-fining and -thinning arrangement of sediment types. Both the siliciclastic sediment types and their mixed dolomitic and siliciclastic counterparts are shown side-by-side. Jagged lines at the block diagram edges represent exposed and scouredsurfaces at the cycle boundaries. Smooth surfaces illustrate conformable cycle boundaries where precipitated carbonate was commonly mixed with siliciclasticsediments. Vertical succession of siliciclastic sediment types in Helena cycles on the right. Symbols are shown in Figure 12.

The hummocky couples passed laterally to the hummocky sand lenses of the pinch–and–swell couple and couplet sediment types, in which the sand was also reworked by storm waves into hummocks and formed loads and gutters in the subjacent mud layers (Fig. 3G,H). However, in the pinch–and– swell sediment types mud remained and settled as the storms waned and died. The mud lay in a nearly fluid state on the lake floor (Johnson, 1999) and was cracked by solitary waves of the next storm. The cracks were filled with sand and compacted, forming the zigzag cracks so distinctive of the pinch–and–swell sediment type (Winston and Smith, 1997) (Figs. 3G, 4A). Compared to the hummocky–couple sediment type, the smaller hummocks and retention of mud in the pinch–and–swell couplet sediment type implies smaller or less effective waves. Wave strength may have been diminished by fluid mud of the sediment–water interface, similarly to the way fluid–mud banks along the Louisiana Coast attenuate onshore waves there (Smith and Winston, 1997).

Pinch–and–swell beds passed landward to the undulating– couplet sediment type, in which the thinner beds and finer grains reflect less turbulence (Fig. 4B,C). Yet the gently rising and falling laminae in the silt layers have the form of small hummocks, which they probably are. Similar laminae have been described by de Raaf et al. (1977) and attributed to waves. The small hummocks probably reflect reduced wave turbulence and size. Perhaps the most satisfactory explanation for these small hummocks is that the sizes of the storm waves were limited and attenuated by shallow water depths, as they are along the modern northwest coastline of Florida (Tanner, 1967).

Landward from the undulating couplets are even couplets (Fig. 15), which probably record deposition from suspension in still water rather than from traction transport. Fine sediment may have been suspended by storms or brought by sheetfloods into the basin, where it settled out in shallow, still environments without being reworked by waves.

The even couplets pass to lenticular couplets deposited in very shallow water where fine sand and silt were reworked into oscillation ripples, probably by fair–weather waves close to the highstand lake margin (Fig. 4D). Mud settled out during calm periods and was locally exposed and desiccation–cracked along the lake margins.

The thinner microcouplet sediment type, also near the basin margin, records episodic accumulation of minute amounts of even finer sediment, most probably from suspension in shallow water (Fig. 4E,F,G). It is similar to shallow–water microlaminated sediments in the Beaufort Sea (Hill and Nedeau, 1989). Mud of the microcouplet sediment type may have inhibited traction transport and the formation of oscillation ripples. Mudcracks in some microcouplet intervals indicate that they too were intermittently exposed and desiccated.

In contrast to the siliciclastic sediment types, the carbonate silt type records precipitation of silt–size carbonate peloids, probably induced by cyanobacteria, in the virtual absence of influx of siliciclastic sediment. During lake highstand it was limited to the easternmost parts of the basin, farthest from supply of siliciclastic sediment.

Contracting–Lake Phase.—

As lake waters evaporated faster than they were replenished, the lake level fell, and dissolved minerals, chiefly dolomite or a precursor form, and occasionally halite, were deposited across the lake floor and mixed with the siliciclastic sediments. In some cycles, particularly toward the basin center, precipitated carbonate abruptly mixed with the siliciclastic sediment grains without otherwise affecting the depositional processes. In other cycles, precipitation of dolomite was accompanied by basinward shift of an adjacent finer–grained, shallower sediment type. Margins of the contracting lake were exposed and desiccated, forming the mudcracks that are mostly restricted to the lenticular and microcouplet sediment types of the lake margins. Some siliciclastic cycles near or on the Noxon block lack dolomitic caps. Here fresh waters, seeping into the west side of the basin from the Siberian continent, may have kept lake waters below the carbonate precipitation threshold.

Stacking Patterns of Cycle Types in the Helena Formation.—

In a broad way, cycle stacking patterns in the Helena (Figs. 713) record a single large–scale transgressive–regressive sequence. Lenticular couplet–based cycles low in the Helena indicate continuity of shallow lacustrine conditions from the Empire up into the Helena. Their upward passage to cycles with undulating– couplet bases implies slight lake deepening during lower Helena deposition. The two wedges of hummocky crossbedded sand and pinch–and–swell beds of the middle Helena member in the western part of the basin record two periods of deepening and increasing fetch separated by a shallowing interval. Return to cycles with mostly undulating couplets in the upper Helena records shallowing of the basin waters. Concomitant with shallowing was the eastward expansion of the Helena lake margin onto the Steinbach plate. Exposure at the top of the Helena marks a sequence boundary beneath the overlying Wallace Formation.

Comparative Lacustrine Examples.—

Additional support for a Helena lake comes from Bohacs et al. (2000), who, in their synthesis of lake systems, pointed out that shorelines in shallow, broad lakes shift rapidly, leaving thin, poorly developed strandline deposits. Water chemistry in lakes may also change greatly over short stratigraphic intervals. They also emphasized that lake sequences differ from marine sequences in that water and sediment supply are linked in determining lake level, and both fill potential accommodation space of lake basins. The interaction of sediment and water supply with potential accommodation space determines whether lakes are open and have an outlet, or whether they are closed. Based on these principles Bohacs et al. classified lake sequences into overfilled (those lakes with an outlet), balanced–fill (those lakes that alternate between open and closed), and underfilled (those lakes with interior drainage). Underfilled–lake sequences are additionally characterized by asymmetric cycles with rapid lake expansion followed by evaporation and contraction. This results in aggradational sediment filling accompanied by chemical precipitation, which culminates in exposure marked by desiccation with minimal erosion. All of the above attributes are hallmarks of Helena cycles, strongly supporting the interpretation that the Helena was deposited in an underfilled lake basin. Comparable Phanerozoic underfilled–lake deposits include the Carboniferous Horton Bluff Formation of Nova Scotia (Martel and Gibling, 1991, the Upper Triassic Lockatong Formation of the Newark basin (Van Houten, 1962), the lower Jurassic East Berlin Formation, Hartford basin, Connecticut (Demicco and Gierlowski–Kordesh, 1986), and the Eocene Wilkins Peak Member of the Green River Formation, Wyoming (Smoot, 1983). Among these the Helena Formations represents an especially sandy and silty succession deposited across an unusually broad, flat, shallow, lake floor.

Contrasts with Marine Cycles.—

Helena cycles differ from both marine siliciclastic cycles and marine carbonate cycles in important ways. Marine parasequen– ces are comparable in scale to marine siliciclastic parasequences. Some marine stratigraphic sequences have Type II sequence boundaries with little erosion, and are overlain by abrupt transgressions, leading to highstand facies tracts (e.g., Van Wagoner et al., 1988). In these respects they are similar to Helena cycles. However, in shallowing–upward marine siliciclastic highstand tracts, fine–grained offshore muds are overlain by prograding coarser shoreface deposits and tidal–flat deposits. Coarsening– upward, prograding shoreface deposits contrast with Helena cycles, which fine upward and do not prograde to the extent that marine siliciclastic parasequences do. Absence of tidal–channel and current–rippled tidal–sandflat deposits in the Helena further militates against a marine parasequence interpretation.

Carbonate Helena cycles also differ from marine carbonate cycles. Grotzinger (1989) has shown that the processes and deposits of marine carbonate shelves extend far back into the Proterozoic, and, although the biota of carbonate shelves has changed, the environments and dynamics of carbonate rimed shelves and ramps have generally persisted since the Protero– zoic. Clearly, the Helena was not deposited in a rimmed carbonate shelf or ramp environment, leaving only tidal flats and lagoons as possible Helena analogues. Again carbonate tidal flats can be ruled out in the Helena by the absence of channels, or any evidence of sustained unidirectional flow. This limits Helena marine environments to broad microtidal lagoons, which commonly do deposit meter–scale cycles. The two generally accepted possible processes for producing marine carbonate cycles are eustatically driven allocyclic processes and autocyclic filling of the available accommodation space by deposits of the carbonate factory (Ginsburg, 1971; Read, 1995; Pratt et al., 1992). Falls in sea level across shallow–water carbonate platforms expose the platforms to erosion and karst, even in the Protero– zoic (Grotzinger, 1986b; Clough and Goldhammer, 2000). Eustatically driven marine cycles in the Helena are rejected on this basis. Although the tops of autocycles build up to sea level and are not necessarily eroded, carbonate facies are linked to depositional environments in marine deposits and prograde at the tops of the cycles. Helena cycles differ in that dolomite distribution in the cycles behaves independently from the siliciclastic depositional environments.

THE Wallace Formation

Wallace Stratigraphy

As in the Helena, cycles are also a dominant theme in Wallace stratigraphy, particularly across western Montana. Wallace cycles are characteristically thicker than those in the Helena, and different cycle styles group into distinctive units. Five units, A–E, were described by Winston (1997) and, to conform to prevailing nomenclature, were assigned to the Helena Formation in the east and to the Wallace on the west. The lowest four units are here included in the revised Wallace Formation, and two additional Wallace units have been discovered below the four described units. The uppermost unit (unit 5 of Winston, 1997) is here placed in the Missoula Group. To distinguish this stratigraphic configuration from that of Winston (1997) the six informal Wallace units have been elevated to informal members and given lithologic names that reflect the most striking characteristic of each member (Fig. 16). This is not to imply that these lithologies do not occur elsewhere, but that they are a distinctive characteristic of each member. The Wallace members are here designated from bottom to top: (1) the oolitic member, (2) the molartooth member, (3) the Baicalia member (unit A of Winston, 1997), (4) the pinch–and–swell member (unit B of Winston, 1997), (5) the microcouplet member (unit C of Winston, 1997), and (6) the full–cycle member (unit D of Winston, 1997). The six Wallace members can be correlated across western Montana (Figs. 16, 17), and individual cycles within the Baicalia, pinch–and–swell, microcouplet, and full–cycle units have been correlated across Montana as well (Winston, 1997) (Figs. 2023). However, approximately where they extend westward into Idaho, the distinctive lithologies of the upper five members blend into a continuous, thick succession of gray pinch–and–swell couples and couplets of the Wallace Formation, and the identity of the members becomes lost from Steamboat Creek south through the Coeur d’Alene Mining District and beyond (Figs. 1623). Only the base of the oolitic member, at the bottom of the Wallace Formation, and the uppermost pinch–and–swell beds at the top of the Wallace, clearly extend far into northern Idaho. Measured sections in the Wallace are located in Figure 17 and in Table 1, and cross sections of the six members in Figures 1823 are plotted from the A–A’ section line in Figure 17.

Fig. 16.—

Generalized cross section of the Wallace Formation along line A–A' of Figure 17, showing the six informal members of the Wallace Formation across western Montana and their gradation into continuous pinch-and-swell beds of the undifferentiated Wallace Formation in northern Idaho.

Fig. 16.—

Generalized cross section of the Wallace Formation along line A–A' of Figure 17, showing the six informal members of the Wallace Formation across western Montana and their gradation into continuous pinch-and-swell beds of the undifferentiated Wallace Formation in northern Idaho.

Fig. 17.—

Generalized geologic map of Piegan Group outcrops and the locations of measured sections projected to line A-A' in Figure 19. Abbreviations are as follows: BM = Baldhead Mountain; BR = Bull River; CF = Clark Fork; GR = Grant Ridge; GW = Graywolf; LD = Libby Dam; LL = lower Loop; LP = Logan Pass; NT = Napa Trail; OC = Ousel Creek; RP = Rogers Pass; SC = Steamboat Creek; SD = South Drywood Creek; SL = Sapphire Lake; SM = Sunday Mountain; SP = Spider Lake; UL = upper Loop.

Fig. 17.—

Generalized geologic map of Piegan Group outcrops and the locations of measured sections projected to line A-A' in Figure 19. Abbreviations are as follows: BM = Baldhead Mountain; BR = Bull River; CF = Clark Fork; GR = Grant Ridge; GW = Graywolf; LD = Libby Dam; LL = lower Loop; LP = Logan Pass; NT = Napa Trail; OC = Ousel Creek; RP = Rogers Pass; SC = Steamboat Creek; SD = South Drywood Creek; SL = Sapphire Lake; SM = Sunday Mountain; SP = Spider Lake; UL = upper Loop.

Oolitic Member.—

The oolitic member takes its name from the oolite bed or beds that mark the base of the redefined Wallace Formation across Montana and parts of northern Idaho (Figs. 16, 18). In the Grinnell and the Big Bend composite section of Glacier National Park, the base of the 43–m–thick oolitic unit is placed at the lowest crossbedded, coarse–grained quartz arenite and oolite bed. It is overlain by additional tabular oolite beds separated by mudcracked undulating silt layers that grade up into dolomitic undulating couplets crowded with molartooth ribbons. The unit is capped by a biostrome of large, 2–m–high stromatolite bioherms with branching Baicalia–form centers wrapped in laminated domal outer layers. This biostrome unit also extends across much of western Montana and into northern Idaho at the Clark Fork section. The multiple oolite beds in the eastern part of the unit pinch out westward near Sunday Mountain into a single basal oolite bed overlain by faint, muddy pinch–and– swell couplets packed with molartooth ribbons beneath the domal stromatolite bioherms. Farther northwest, in the Bull River section (Figs. 16, 18), the basal oolite bed is overlain by molartooth–bearing pinch–and–swell couplets below the stromatolite bed. At Clark Fork, Idaho, the oolite bed is represented by intraclasts coated by stromatolites, and elsewhere in Idaho the oolite bed is replaced by siliciclastic pinch–and–swell couples and couplets that sharply mark the base of the Wallace above pod–bearing, undulating dolomitic couplets of the Helena Formation. The stromatolite bed at the top of the member extends west to Clark Fork.

Fig. 18.—

Cross section of the lowest cycle in the oolitic member of the Wallace Formation from Steamboat Creek to Logan Pass.

Fig. 18.—

Cross section of the lowest cycle in the oolitic member of the Wallace Formation from Steamboat Creek to Logan Pass.

Molartooth Member.—

The molartooth member of the Wallace, as the name implies, contains molartooth ribbons in mostly muddy siliciclastic–to– dolomite cycles containing pinch–and–swell couples and couplets (Figs. 16, 19). In the muddy eastern sections molartooth ribbons so crowd the bedding that it becomes difficult to see stratification in the siliciclastic layers. Near the center of the basin in the Graywolf section and north of the Sunday Mountain section the member reaches 300 m thick, where pinch–and–swell couples in the lower half–cycles become sandier and better defined. West of the Graywolf section, the molartooth member thins and becomes sandier, with well preserved pinch–and–swell couples in the lower half–cycles and molartooth–bearing muddier pinch–and–swell couplets in the upper half–cycles. At Clark Fork the cycles become totally siliciclastic with pinch–and–swell couples in the lower parts and pinch–and–swell couplets in the upper parts, and the molartooth ribbons nearly disappear. In the Coeur d’Alene Mining District, including the Galena Mine and Steamboat Creek sections, the member becomes unrecognizable in the dark–gray pinch–and–swell couples of the type Wallace Formation.

Fig. 19.—

Generalized cross section of a cycle representative of the molartooth unit of the Wallace Formation from Steamboat Creek to Logan Pass.

Fig. 19.—

Generalized cross section of a cycle representative of the molartooth unit of the Wallace Formation from Steamboat Creek to Logan Pass.

Baicalia Member.—

The Baicalia member (unit A of Winston, 1997) contains two cycles 43–80 m thick (Fig. 16). A cross section of the lower cycle is shown in Figure 20. In the central part of the basin, Baicalia bioherms of each cycle are surrounded by broken calcitic Baicalia fragments and white micrite and grade up to black microcouplets with thin molartooth ribbons. To the east in Glacier National Park, the lower Baicalia bioherms of each cycle are overlain by Conophyton bioherms, forming the Baicalia–Conophyton cycles of Horodyski (1983, 1989, 1998). To the southeast at Rogers Pass, the member passes into two intervals of domal stromatolites and poorly developed Baicalia and Conophyton growth forms separated by beds of black microcouplets. The two Baicalia cycles extend west to Libby Dam, but at Clark Fork the Baicalia bioherms are replaced by siliciclastic pinch–and–swell couples and a bed of domal stromatolites at the base of the upper cycle. Only the black microcouplets at the tops of the two Baicalia cycles continue west onto northern Idaho.

Fig. 20.—

Cross section of the lower Baicalia cycle from Steamboat Creek to South Drywood Creek and Baldhead Mountain, modified from Winston (1997, fig. 11).

Fig. 20.—

Cross section of the lower Baicalia cycle from Steamboat Creek to South Drywood Creek and Baldhead Mountain, modified from Winston (1997, fig. 11).

Pinch–and–Swell Member.—

The pinch–and–swell member of the Wallace Formation (unit B of Winston, 1997) (Fig. 16) contains nine cycles from 1 m to 30 m thick. Siliciclastic hummocky–couple beds and pinch–and–swell couples in the lower half–cycles fine and thin upward to pinch–and– swell couplets in the upper half–cycles. Hummocky–couples and pinch–and–swell couples of the nine cycles also thin and fine to pinch–and–swell couplets and even couplets eastward from the basin center. A cross section of cycle 4 is shown in Figure 21. The member blends westward into the undifferentiated pinch–and– swell couples of the Wallace in the Coeur d’Alene Mining District.

Fig. 21.—

Cross section of cycle # 4 in the pinch–and–swell member of the Wallace from Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 12).

Fig. 21.—

Cross section of cycle # 4 in the pinch–and–swell member of the Wallace from Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 12).

Microcouplet Member.—

The microcouplet member (unit C of Winston, 1997) contains 12 cycles from less than 1 m to 30 m thick. It is characterized by comparatively thin intervals of pinch–and–swell couplets in the lower half–cycles and thick intervals of microcouplets in the upper half–cycles (Fig. 22). It can be identified across the central part of the basin, but, like the units immediately below and above, it grades westward into the continuous Wallace beds of northern Idaho. It also grades eastward into green and red argillite assigned to the lower part of the Snowslip Formation at Rogers Pass.

Fig. 22.—

Cross section of cycle # 7 in the microcouplet member of the Wallacefrom Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 13).

Fig. 22.—

Cross section of cycle # 7 in the microcouplet member of the Wallacefrom Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 13).

Full–Cycle Member.—

The full–cycle member (unit D of Winston, 1998) contains 17 siliciclastic cycles from 1 to 37 m thick. Its cycles contain a full complement of upward–fining and –thinning sediment types from pinch–and–swell couples, to pinch–and–swell couplets, to undulating couplets, to microcouplets. A cross section of cycle 11 is shown in Figure 23. The pinch–and–swell beds of the uppermost cycles extend across the basin and form the revised, clearly demarcated upper boundary of the Wallace Formation. They lie below the black microcouplet beds of the former upper Wallace member in Idaho, and below thin–bedded arenite, green argillite, and dolomite in the Snowslip Formation in Montana, both of which are assigned to the Missoula Group. Aside from the top, the rest of the full–cycle member also becomes undefined in the Coeur d’Alene Mining District.

Fig. 23.—

Cross section of cycle # 11 in the full-cycle member of the Wallacefrom Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 14).

Fig. 23.—

Cross section of cycle # 11 in the full-cycle member of the Wallacefrom Steamboat Creek to South Drywood Creek, modified from Winston (1997, fig. 14).

Lacustrine Interpretation of the Wallace Formation

Based principally on the continuity of sediment types from the Helena into the Wallace, and on the similarity of sediment– type successions in the Wallace cycles to those in the Helena cycles, the Wallace is also interpreted to be lacustrine. The dolomite–capped cycles in the oolitic and molartooth members and in the uppermost cycles of the full–cycle member of the Wallace are inferred to have been deposited in an underfilled lake basin. As in the Helena, dolomite in the cycle caps of these members cuts across the siliciclastic sediment types and behaved independently as a chemical precipitate in an enclosed basin. However, the calcitic and siliciclastic cycles in the thick, Baicalia, pinch–and–swell, microcouplet, and lower full–cycle members are interpreted to have been deposited in a balanced– fill lake (Bohacs et al., 2000). Their cycles fine and thin upward through a succession of sediment types which record abrupt expansion and establishment of a highstand facies tract followed by progressive basinward shift of the siliciclastic sediment types without dolomite precipitation. This pattern is characteristic of balanced–fill lakes, in which water flows out of the lake during highstand but may be enclosed below sill height during lowstand (Bohacs et al., 2000). In balanced–fill lake cycles erosion at the bases of the cycles is minimal, flooding surfaces are sharply overlain by expanded lake facies, and siliciclastic facies commonly prograde with lake contraction. Upward fining in the cycles reflects the link between sediment supply and water supply. Inflow during humid periods brings influx of more sediment with coarser grains, whereas arid periods limit the amount of water inflow and coarse–sediment supply. Wallace cycles are consistent with these principles. For much the same reasons as for the Helena cycles, the Wallace cycles are inconsistent with marine shoaling–upward parasequences and marine carbonate cycles. The thicker cycles in the Baicalia, pinch–and– swell, microcouplet, and lower full–cycle members than in the oolitic and molartooth cycles suggests that the cycles in those members expanded and contracted at a lower frequency in a larger lake. Again, the absence of tidal sedimentary structures and prograding marine shoreface sediments makes an alternate marine setting unlikely.

Depositional History of the Wallace Formation

The sedimentary history of the Wallace Formation is recorded in the stacking patterns of cycles in the Wallace members. Cycles from the oolitic member through the pinch–and– swell member record general lake expansion, followed by contraction in the microcouplet member. The Wallace lake expanded again in the full–cycle member before it contracted high in the Wallace below the lowermost Snowslip. The Wallace lake floor remained everywhere above storm wave base and therefore, like the Helena, was exceptionally flat, shallow, and featureless.

The base of the oolite member of the Wallace, marked by abrupt appearance of coarse quartz sand and oolite in the eastern and central parts of the basin and hummocky pinch– and–swell couples in the western part of the basin, records initiation and rapid expansion of the Wallace lake. The thick pinch–and–swell couples probably indicate broad fetch as the lake expanded westward, beyond the former limit of the Helena lake. Fine sand and mud continued to come from the Siberian source terrane along the western side of the basin. The oolite beds are interpreted to record wave–reworked sand on exposed lake wind–setup flats as the lake expanded eastward over the unconformity at the top of the Helena Formation. Eby (1977) discussed similarity of the cerebroid ooliths of the Wallace to ooids of the exposed Great Salt Lake wind–setup flats. Coarse quartz sand from the eastern side of the basin was transported far out into the basin by longshore drift on the exposed wind– setup flats.

Muddy cycles of the molartooth member above the oolitic member record trapping of fluid mud across the shallow Wallace lake floor in the absence of a deep–water mud sink. Mud was continually resuspended by storms and driven shoreward, forming the extremely muddy cycles along the eastern part of the basin and leaving behind sandier cycles in the deeper central and western parts of the basin (Winston, 1997). Carbon dioxide and methane generated in the mud formed the great networks of molartooth ribbons (Furniss et al., 1998).

The Baicalia cycles record a major change in Wallace mineralogy. The abrupt shift to clean, calcitic sediments, nearly devoid of clay, may indicate outflow of suspended clay during open lake highstand and freshening of the lake waters. Clearing of the lake waters may have stimulated photosynthesis and development of the Baicalia and Conophyton biostromes. The Baicalia forms grew under periodically turbulent, probably storm conditions across the central part of the basin. The Conophyton forms grew under calmer conditions in the upper part of the two cycles in Glacier National Park in the protection of Baicalia bioherms (Winston, 1997). The dark microcouplet intervals capping the two Baicalia cycles probably record dropping of the lake level below the spill level and short returns to an underfilled phase of the balanced–fill lake cycle. The westward replacement of the Baicalia bioherms and their calcitic fragmental debris by siliciclastic pinch–and–swell couplets in Idaho reflects influx of sand and mud from the Siberian segment of the Columbian continent, and confirms the continuity of a two–sided basin during Wallace deposition.

The nine cycles of the pinch–and–swell member record expansions and contractions of the Wallace lake during periods when the fetch was broad and sand influx and turbulence were high. At the same time, the lake retreated along the eastern margin, as indicated by the encroachment of siliciclastic even and lenticular green argillite couplets of the Snowslip Formation on the Steinbach thrust plate.

Cycles of the microcouplet member record reduced turbulence, probably as a result of decreased fetch brought on by a period of Wallace lake lowstand or limited influx of sediment and water.

Cycles of the full–cycle Wallace member continued to record abrupt expansions of the Belt lake to the eastern highstand margin, followed by more gradual contraction and reduction of wave turbulence. Evans et al. (2000) obtained a date of 1454 Ma from a volcanic ash bed probably from cycle 13 or 14 high in the full–cycle member, a few meters below the oolitic quartz sand– and stromatolite–based cycles at the top of the Wallace. These sandy, oolitic, and stromatolitic cycles record prograding wind– setup flats at the margins of the shrinking Wallace lake below the rippled, mudcracked, green and purple argillite of the Snowslip Formation in Montana and its western counterpart of black argillite in Idaho.

Summary

The Helena and Wallace formations were thought to have a facies relationship, with the Helena representing a marine dolo– mitic carbonate shelf along the east side of the basin and the Wallace being a deeper–water basin deposit to the west. This study shows that the thin–bedded, dolomite–capped cycles that characterize the Helena Formation instead form a widespread unit from western Montana into northern Idaho. The Helena Formation is revised to encompass this unit. The gray quartzite and dark argillite beds that characterize the Wallace Formation are here shown to form a widespread unit disconformably above the Helena and below the Missoula Group. The Wallace Formation is revised to include this unit. Together they constitute the resurrected Piegan Group.

Ten sediment types in the Piegan Group form the building blocks of Helena and Wallace sedimentology. In both formations, siliciclastic sediment types thin and fine upward, producing shallowing–upward cycles. Dolomite is mixed with siliciclastic sediment types in the upper parts of most Helena cycles and in lower and upper Wallace cycles. The dolomite is interpreted to be a chemical deposit, possibly biologically induced, when calcium, magnesium, and carbonate concentrations were increased during contraction of an enclosed, underfilled lake. Consequently the Helena Formation and lower and uppermost parts of the Wallace Formation are interpreted to be underfilled–lake deposits. Periods of increased influx of water and sediment expanded the Helena and Wallace underfilled lakes, whereas periods of reduced influx of water and sediment contracted them, forming the siliciclastic–to–dolomite cycles. The wholly siliciclastic and calcitic cycles of the middle part of the Wallace Formation are interpreted to record deposition in a balanced–fill lake, which periodically had an outlet.

The criteria proposed here for lacustrine interpretation in the Proterozoic may serve as guideposts for judging other Protero– zoic subaqueous deposits. The sedimentary facies in these broad, shallow, flat–bottomed Piegan Group lakes may also serve as a model for interpreting similar shallow–water Phanerozoic lakes that filled sparsely vegetated basins.

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Acknowledgements

My happy years spent working on the Helena and Wallace were made all the more enjoyable by the camaraderie of my Belt friends, especially the editors of this volume, Paul Link and Reed Lewis, Reed’s cohorts Russ Burmester and Mark McFaddan, U.S.G.S. geologists including the late Jack Harrison, former students who worked on the Piegan Group, John Grotzinger, Mike O’Connor, Jay Johnson, and Alecia Stickney Platt, and my enduring field assistants, Nate Hathaway and Tim Wheeler. Ray Price and Peter Southgate edited the submitted draft of this paper, and their valuable suggestions fortunately resulted in a largely rewritten manuscript, which was re–edited and significantly improved by Paul Link, Reed Lewis, Russ Burmester, and John Southard.

Figures & Tables

Table 1.—

GPS coordinates of measured sections.

Helena Formation Measured Section Locations
#NameStart of SectionEnd of Section
1Crater Mountain46°54'25.89N 114°53'31.58W46°54'48.49N 114°54'11.06W
2St. Regis Lakes47°25'48.52N 115°44'46.92W47°25'43.54N 115°44'59.69W
3Placer Creek47°27'03.79N 115°56'03.44W47°27'07.90N 115°55'51.37W
4Galena Mine47°28'48.24N 115°57'59.35W47°29'08.44N 115°57'37.45W
5Steamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
6Clark Fork48°11'03.35N 116°14'06.76W48°10'43.15N 116°13'45.04W
7Big Hole Peak Weeksville Road Round Mtn.47°32'21.90N 115°04'33.92W 47°31'27.41N 115°59'54.45W 47°32'05.13N 115°03'13.88W47°32'38.64N 115°04'10.78W 47°31'29.81N 115°00'15.06W 47°32'06.92N 115°03'12.47W
8Bull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
9West Libby Dam48°24'38.04N 115°19'23.49W48°24'01.74N 115°19'26.98W
10St. Mary Lake47°15'50.42N 113°55'06.88W47°16'09.17N 113°54'59.07W
11Mollman Pass Eagle Pass47°28'59.50N 113°57'25.15W 47°26'50.75N 113°56'44.63W47°29'26.25N 113°57'05.56W 47°26'53.73N 113°56'38.36W
12Red Mountain47°04'32.55N 112°45'49.73W47°06'26.49N 112°44'34.27W
13Rogers Pass47°06'00.50N 112°21'16.34W47°05'32.54N 112°21'50.91W
14Little Skunk Creek47°12'20.75N 112°24'47.25W47°12'21.54N 112°25'03.68W
15Dearborn River47°16'20.79N 112°30'52.71W47°16'14.16N 112°31'05.26W
16Wood Creek Hogback47°25'06.12N 112°47'21.93W47°25'00.56N 112°47'21.82W
17Grinnell Glacier Big Bend48°46'02.12N 113°43'10.64W 48°43'30.63N 113°43'24.91W48°46'09.05N 113°43'49.56W 48°42'18.57N 113°43'02.97W
Wallace Formation Measured Section Locations
I.D.NameStart of SectionEnd of Section
SCSteamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
CFClark Fork48°10'43.15N 116°13'45.04W48°10'38.08N 116°13'29.18W
BRBull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
LDEast Libby Dam48°22'28.62N 115°18'55.29W48°22'08.66N 115°19'02.83W
GWGrey Wolf47°16'10.97N 113°53'21.45W47°16'11.87N 113°52'49.28W
SPSpider Lake47°26'21.51N 113°52'36.91W47°26'21.59N 113°52'34.72W
SMSunday Mountain47°20'38.63N 113°29'08.52W47°20'43.97N 113°28'52.09W
SLHolland Lake47°27'40.83N 113°34'09.38W47°28'03.33N 113°33'39.83W
NTNapa Trail47°47'18.55N 113°40'51.04W47°47'22.18N 113°40'52.19W
GRGrant Ridge48°19'57.20N 113°45'06.23W48°20'09.08N 113°44'45.51W
OCOusel Creek48°29'41.35N 113°53'36.71W48°29'49.38N 113°53'26.13W
LLLower Loop48°44'59.80N 113°47'07.48W48°45'10.86N 113°47'54.16W
ULUpper Loop48°45'12.59N 113°47'44.88W48°45'05.54N 113°46'59.51W
LPLogan Pass48°42'12.58N 113°42'16.23W48°42'20.58N 113°42'16.94W
SDSouth Drywood Canyon49°14'46.40N 114°04'56.03W49°14'49.93N 114°05'11.01W
Helena Formation Measured Section Locations
#NameStart of SectionEnd of Section
1Crater Mountain46°54'25.89N 114°53'31.58W46°54'48.49N 114°54'11.06W
2St. Regis Lakes47°25'48.52N 115°44'46.92W47°25'43.54N 115°44'59.69W
3Placer Creek47°27'03.79N 115°56'03.44W47°27'07.90N 115°55'51.37W
4Galena Mine47°28'48.24N 115°57'59.35W47°29'08.44N 115°57'37.45W
5Steamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
6Clark Fork48°11'03.35N 116°14'06.76W48°10'43.15N 116°13'45.04W
7Big Hole Peak Weeksville Road Round Mtn.47°32'21.90N 115°04'33.92W 47°31'27.41N 115°59'54.45W 47°32'05.13N 115°03'13.88W47°32'38.64N 115°04'10.78W 47°31'29.81N 115°00'15.06W 47°32'06.92N 115°03'12.47W
8Bull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
9West Libby Dam48°24'38.04N 115°19'23.49W48°24'01.74N 115°19'26.98W
10St. Mary Lake47°15'50.42N 113°55'06.88W47°16'09.17N 113°54'59.07W
11Mollman Pass Eagle Pass47°28'59.50N 113°57'25.15W 47°26'50.75N 113°56'44.63W47°29'26.25N 113°57'05.56W 47°26'53.73N 113°56'38.36W
12Red Mountain47°04'32.55N 112°45'49.73W47°06'26.49N 112°44'34.27W
13Rogers Pass47°06'00.50N 112°21'16.34W47°05'32.54N 112°21'50.91W
14Little Skunk Creek47°12'20.75N 112°24'47.25W47°12'21.54N 112°25'03.68W
15Dearborn River47°16'20.79N 112°30'52.71W47°16'14.16N 112°31'05.26W
16Wood Creek Hogback47°25'06.12N 112°47'21.93W47°25'00.56N 112°47'21.82W
17Grinnell Glacier Big Bend48°46'02.12N 113°43'10.64W 48°43'30.63N 113°43'24.91W48°46'09.05N 113°43'49.56W 48°42'18.57N 113°43'02.97W
Wallace Formation Measured Section Locations
I.D.NameStart of SectionEnd of Section
SCSteamboat Creek47°39'49.03N 116°09'18.76W47°40'08.47N 116°09'24.02W
CFClark Fork48°10'43.15N 116°13'45.04W48°10'38.08N 116°13'29.18W
BRBull River48°03'05.44N 115°50'07.28W48°03'29.48N 115°49'50.16W
LDEast Libby Dam48°22'28.62N 115°18'55.29W48°22'08.66N 115°19'02.83W
GWGrey Wolf47°16'10.97N 113°53'21.45W47°16'11.87N 113°52'49.28W
SPSpider Lake47°26'21.51N 113°52'36.91W47°26'21.59N 113°52'34.72W
SMSunday Mountain47°20'38.63N 113°29'08.52W47°20'43.97N 113°28'52.09W
SLHolland Lake47°27'40.83N 113°34'09.38W47°28'03.33N 113°33'39.83W
NTNapa Trail47°47'18.55N 113°40'51.04W47°47'22.18N 113°40'52.19W
GRGrant Ridge48°19'57.20N 113°45'06.23W48°20'09.08N 113°44'45.51W
OCOusel Creek48°29'41.35N 113°53'36.71W48°29'49.38N 113°53'26.13W
LLLower Loop48°44'59.80N 113°47'07.48W48°45'10.86N 113°47'54.16W
ULUpper Loop48°45'12.59N 113°47'44.88W48°45'05.54N 113°46'59.51W
LPLogan Pass48°42'12.58N 113°42'16.23W48°42'20.58N 113°42'16.94W
SDSouth Drywood Canyon49°14'46.40N 114°04'56.03W49°14'49.93N 114°05'11.01W
Table 2.—

Descriptions and interpretations of the ten principal Piegan Group sediment types.

graphicDescription: Beds up to 20 centimeters thick ranging from coarse- to medium-grained, crossbedded quartzose, oolitic sand, to mudchips and molartooth intraclasts. Forms bases of many cycles in the Helena and Wallace formations.Interpretation: Traction transport of coarse- and medium-grained sand and coarse clasts, together with winnowing of muddy sediment, over wind-setup surfaces during expansion of the Belt lake and on cycle-highstand beaches.
graphicDescription: Well sorted, clean, fine-grained, light gray sand with low-angle hummocky cross-stratification, horizontal stratification, or poorly defined stratification. Most are stacked into cosets more than 10 cm thick and extend continuously across individual outcrops. Form the bases of cycles in the Helena and Wallace formations.Interpretation: Deposition by strong oscillatory storm currents, which suspended silt and clay and concentrated find sand on the Belt lake floor. Accumulated on maximum flooding surfaces at the bases of Helena and Wallace cycles.
graphicDescription: Size-graded fine-sand-to-mud layers more than 3 cm thick with sharp, commonly guttered and loaded bases below hummocky, flat-laminated, or poorly laminated, light to medium gray sand lenses that grade up to dark gray mud. Capping mud layers are commonly cut by sand-filled cracks and gutters or are loaded below sand of overlying couples. Common in lower parts of cycles in the Wallace formation and combined with pinch-and-swell couplets in the Helena descriptions.Interpretation: Hummocks record traction transport and deposition by large storm waves. Flat laminae may record combined flow, and poorly laminated layers may reflect rapid accumulation of storm-suspended sand with little reworking. Capping mud layers probably accumulated as storms waned and sediment suspension exceeded transport capacity, followed by settleout in still water. Solitary waves generated sand- filled cracks (Winston and Smith,1997).
graphicDescription: Similar in form to pinch-and-swell couples, but thinner, .3 to 3 cm-thick couplet-scale graded layers, result in smaller very fine sand hummocks, gutters, loads, and thinner dark gray capping mud. Common above pinch-and-swell couples or at the bases of cycles in the Wallace Formation and middle member of the Helena Formation.Interpretation: Sand layers record smaller, possibly depth-limited storm waves than those of pinch-and-swell couples, but processes are similar.
graphicDescription: Graded silt-to-clay couplets with sharp-based, light gray silty and very fine-sand laminae that gently rise and fall across outcrops. Capping clay layers are light gray or dolomitic. Form the bases of many Helena cycles.Interpretation: Commonly interlayered with pinch-and-swell couplets on one hand and with lenticular couplets on the other, suggesting they formed by intermediate-size oscillatory waves. Wave size may have been depth-limited.
graphicDescription: Even, continuous, non-cracked graded fine sand and silt-to-mud couplets that stretch across outcrops for more than a few meters.Interpretation: Deposited from suspension in contrast to traction deposition of undulating couplets. Sediment suspension clouds were probably generated either by storms or from influx of terrestrial sheetfloods.
graphicDescription: Light gray, flat-based, straight-crested, fine-grained sandy symmetric-ripple lenses beneath gray argillitic to tan-weathering dolomitic mud caps, with or without desiccation cracks in the Helena and Wallace formations. Commonly interlayered with undulating couplets.Interpretation: Fine sand reworked by fair-weather waves in shallow water, followed by suspension settleout in still water.
graphicDescription: Silt laminae, commonly only a few grains thick, sharply capped by dark gray or tan-weathering, dolomitic clay. Range from even to tightly folded and from non-cracked to desiccation cracked. Some with small-oscillation-ripple fine sand lenses. Commonly forms near shore from undulating and even couplets in the lower half-cycles in the Wallace formation.Interpretation: Concentration of microcouplets in the landward facies-tract margins and at the tops of cycles, together with occasional desiccation cracks, indicate shallow-water accumulation, probably from storm suspension in very shallow water (Hill and Nedeau,1989).
graphicDescription: Concentrations of fine carbonate silt peloids in fine sparry to compacted micritic matrix, commonly mixed with clay and scattered quartz silt grains. Poorly stratified to undulating couplet layers. Dolomitized Helena upper half-cycles.Interpretation: Precipitated as calcitic peloids, probably induced by bacterial metabolism in the water column, with high calcium, magnesium, and carbonate concentrations during periods of lake contraction.
graphicDescription: Mostly domal stromatolite beds, commonly on transgressive ravinement surfaces at the bases of Helena and Wallace cycles. Include biostromes of Baicalia and Conophyton growth forms in the Baicalia member of the Wallace Formation.Interpretation: Domal stromatolites grew on eroded pinnacles of ravinement surfaces during expansion of the Belt lake. Calcitic Baicalia and Conophyton biostromes may indicate freshening of a balanced-fill lake during deposition of the Baicalia member of the Wallace Formation.
graphicDescription: Beds up to 20 centimeters thick ranging from coarse- to medium-grained, crossbedded quartzose, oolitic sand, to mudchips and molartooth intraclasts. Forms bases of many cycles in the Helena and Wallace formations.Interpretation: Traction transport of coarse- and medium-grained sand and coarse clasts, together with winnowing of muddy sediment, over wind-setup surfaces during expansion of the Belt lake and on cycle-highstand beaches.
graphicDescription: Well sorted, clean, fine-grained, light gray sand with low-angle hummocky cross-stratification, horizontal stratification, or poorly defined stratification. Most are stacked into cosets more than 10 cm thick and extend continuously across individual outcrops. Form the bases of cycles in the Helena and Wallace formations.Interpretation: Deposition by strong oscillatory storm currents, which suspended silt and clay and concentrated find sand on the Belt lake floor. Accumulated on maximum flooding surfaces at the bases of Helena and Wallace cycles.
graphicDescription: Size-graded fine-sand-to-mud layers more than 3 cm thick with sharp, commonly guttered and loaded bases below hummocky, flat-laminated, or poorly laminated, light to medium gray sand lenses that grade up to dark gray mud. Capping mud layers are commonly cut by sand-filled cracks and gutters or are loaded below sand of overlying couples. Common in lower parts of cycles in the Wallace formation and combined with pinch-and-swell couplets in the Helena descriptions.Interpretation: Hummocks record traction transport and deposition by large storm waves. Flat laminae may record combined flow, and poorly laminated layers may reflect rapid accumulation of storm-suspended sand with little reworking. Capping mud layers probably accumulated as storms waned and sediment suspension exceeded transport capacity, followed by settleout in still water. Solitary waves generated sand- filled cracks (Winston and Smith,1997).
graphicDescription: Similar in form to pinch-and-swell couples, but thinner, .3 to 3 cm-thick couplet-scale graded layers, result in smaller very fine sand hummocks, gutters, loads, and thinner dark gray capping mud. Common above pinch-and-swell couples or at the bases of cycles in the Wallace Formation and middle member of the Helena Formation.Interpretation: Sand layers record smaller, possibly depth-limited storm waves than those of pinch-and-swell couples, but processes are similar.
graphicDescription: Graded silt-to-clay couplets with sharp-based, light gray silty and very fine-sand laminae that gently rise and fall across outcrops. Capping clay layers are light gray or dolomitic. Form the bases of many Helena cycles.Interpretation: Commonly interlayered with pinch-and-swell couplets on one hand and with lenticular couplets on the other, suggesting they formed by intermediate-size oscillatory waves. Wave size may have been depth-limited.
graphicDescription: Even, continuous, non-cracked graded fine sand and silt-to-mud couplets that stretch across outcrops for more than a few meters.Interpretation: Deposited from suspension in contrast to traction deposition of undulating couplets. Sediment suspension clouds were probably generated either by storms or from influx of terrestrial sheetfloods.
graphicDescription: Light gray, flat-based, straight-crested, fine-grained sandy symmetric-ripple lenses beneath gray argillitic to tan-weathering dolomitic mud caps, with or without desiccation cracks in the Helena and Wallace formations. Commonly interlayered with undulating couplets.Interpretation: Fine sand reworked by fair-weather waves in shallow water, followed by suspension settleout in still water.
graphicDescription: Silt laminae, commonly only a few grains thick, sharply capped by dark gray or tan-weathering, dolomitic clay. Range from even to tightly folded and from non-cracked to desiccation cracked. Some with small-oscillation-ripple fine sand lenses. Commonly forms near shore from undulating and even couplets in the lower half-cycles in the Wallace formation.Interpretation: Concentration of microcouplets in the landward facies-tract margins and at the tops of cycles, together with occasional desiccation cracks, indicate shallow-water accumulation, probably from storm suspension in very shallow water (Hill and Nedeau,1989).
graphicDescription: Concentrations of fine carbonate silt peloids in fine sparry to compacted micritic matrix, commonly mixed with clay and scattered quartz silt grains. Poorly stratified to undulating couplet layers. Dolomitized Helena upper half-cycles.Interpretation: Precipitated as calcitic peloids, probably induced by bacterial metabolism in the water column, with high calcium, magnesium, and carbonate concentrations during periods of lake contraction.
graphicDescription: Mostly domal stromatolite beds, commonly on transgressive ravinement surfaces at the bases of Helena and Wallace cycles. Include biostromes of Baicalia and Conophyton growth forms in the Baicalia member of the Wallace Formation.Interpretation: Domal stromatolites grew on eroded pinnacles of ravinement surfaces during expansion of the Belt lake. Calcitic Baicalia and Conophyton biostromes may indicate freshening of a balanced-fill lake during deposition of the Baicalia member of the Wallace Formation.

Contents

GeoRef

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