18: Isotope Chemostratigraphy of the Lower Mississippian St. Joe Group in Northeastern Oklahoma and Southwestern Missouri
Published:January 01, 2019
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Julia P. S. Sessions, Christina L. Opfer, Robert W. Scott, 2019. "Isotope Chemostratigraphy of the Lower Mississippian St. Joe Group in Northeastern Oklahoma and Southwestern Missouri", Mississippian Reservoirs of the Midcontinent, G. Michael Grammer, Jay M. Gregg, James Puckette, Priyank Jaiswal, S. J. Mazzullo, Matthew J. Pranter, Robert H. Goldstein
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The St. Joe group (Lower Mississippian, Tournaisian) is petrographically and isotopically analyzed using δ13C and δ18O bulk sample stable isotopes in central, northeastern Oklahoma, and southwestern Missouri. Determined to be conformable in Oklahoma, this group represents deposition in the mid- to outer-ramp setting during one long-term depositional cycle and can be used as a reference section for geochemical chronostratigraphy. Minor diagenetic alteration did not overprint the initial isotope signal, and the resulting curve is similar to those from previous studies and is integrated with published conodont biostratigraphy. The resulting correlation indicates that the St. Joe group was deposited in the upper Tournaisian Stage.
The intraformational stratigraphy and depositional environments of the St. Joe group are debated among scholars. Most of the published research on the St. Joe group is on outcrop sections in Missouri and Arkansas and subsurface analysis in central to western Oklahoma. The St. Joe group was deposited during the Early Mississippian. The Mississippian is recognized as having a shifting climate from predominately ice-free to ice-covered (Mii et al., 1999; Saltzman et al., 2004). The Lower Mississippian isotope curve has been determined through sampling locations around the world; the value ranges are determined by multiple factors including variation in local seawater composition (Mii et al., 1999; Saltzman, 2002; Saltzman et al., 2004; Koch et al., 2014). Stable-isotope analyses have not been published on outcrop sections of the St. Joe group. Carbon and oxygen isotope curves can identify diagenetic alteration, overprinting of the initial isotope signal, and conformability of a section. An unaltered stable-isotope curve, which represents the values at the time of deposition, can be used as a chronostratigraphic tool. Bulk sample δ13C and δ18O and petrographic analyses in the study are area from one core and four outcrops slightly oblique to paleodip of a shallow ramp depositional environment. The initial isotope signal of the studied sections was unaffected by meteoric waters and sustained a minor amount of diagenetic alteration (Figure 1).
Stable isotopes are a chemical fingerprint of sedimentary rocks and can be combined with biostratigraphy to determine stratigraphic correlation (Renard, 1985; Weissert et al., 2008). Brachiopods were common throughout the Paleozoic and are often tested to determine general isotopic signatures for specific time periods because of their original low-magnesium calcite composition (Saltzman, 2002). Brachiopods are considered to represent the oxygen isotope equilibrium at the time of precipitation of waters with normal saline conditions; their signal can, however, be isotopically fractionated and must be analyzed for diagenetic alteration (Grossman et al., 2008). Bulk rock sample results are similar to data from brachiopod studies and commonly have similar trends, especially for stable isotopes (Nelson and Smith, 1996; Saltzman, 2002; Saltzman et al., 2004; Fouke, 2005; Allègre, 2008; van der Kooij et al., 2009). The Lower Mississippian St. Joe group has previously been analyzed biostratigraphically but not chemostratigraphically (Boardman et al., 2013). This paper aims to provide new chemostratigraphic data of four midcontinent Lower Mississippian outcrop sections, combine this with existing biostratigraphy, and re-evaluate suggested depositional models for the area.
The St. Joe group is overlain by the Reeds Spring Formation or Boone Group and was deposited during the latter half of the Tournaisian Epoch and early Visean Epoch (Figure 2). Lower Mississippian carbonates on the Ozark uplift and Cherokee platform provinces represent a transgressive system tract across the southern shelf margin of the Burlington shelf (Lane and DeKeyser, 1980; Shelby, 1986). The onset of sea-level rise is represented by the basal member of the St. Joe group, the Bachelor shale. The overlying St. Joe group formations—the Compton Limestone, Northview Formation, and Pierson Limestone—represent succeeding cyclic carbonate and shale deposition as water continued to deepen and shallow (Shelby, 1986; Mazzullo et al., 2011; Mazzullo et al., 2013).
Northeastern Oklahoma, southeastern Kansas, and southwestern Missouri occupy the southwestern flank of the Ozark uplift, a broad, asymmetrical cratonic dome covering an area of approximately 104,000 km2 (40,000 mi2) in Missouri, Arkansas, Kansas, and Oklahoma (Huffman, 1959; Manger, 2014). In the western Ozarks, Mississippian rocks dip gently away from the Ozark uplift in an arcuate outcrop belt. Regional dip varies between 10 m/km (19 ft/mi) to the south and southwest to 24 m/km (50 ft/mi) and is associated with beds proximal to major faults (Huffman, 1959, 1960). The gentle dip in the western Ozarks is significantly interrupted by a series of northeast-trending folds and faults in Lower Mississippian strata; the faults are generally aligned parallel to the axis of the Ozark uplift (Manger, 2014).
The north-central Oklahoma Cherokee platform is situated west and southwest of the Ozark uplift. The platform is composed of gently dipping strata with low, east-facing escarpments and isolated buttes that formed in nonorogenic conditions with localized small sedimentary folds trending with underlying structures (Berryhill, 1961; Merriam, 1999). The Cherokee platform is bounded on the west by the Anadarko Basin and on the south by the Arbuckle uplift and Arkoma Basin, all of which are outside the field area discussed in this paper.
The four rock exposures examined in this study area are of the lower “Mississippian limestone” in Oklahoma and Missouri and representative of the upper Tournaisian stage. The basal unit is the Bachelor shale, which is only present to the northeast at the outcrop near Jane, Missouri, on U.S. Route 71 The Oklahoma outcrops are situated oblique to the strike of the region in Oklahoma along U.S. Route 412 in Delaware County and along Oklahoma State Highway 10 in Cherokee County . A core in Pawnee County, Oklahoma, west of the outcrops was studied petrographically. Lithologic continuity of the St. Joe group formations across the Cherokee platform enabled subsurface correlation between the outcrops and core in the study area.
REGIONAL STRATIGRAPHY AND BIOSTRATIGRAPHY
Sea level transgressed during the Early Mississippian and flooded the southwestern margin of Laurentia, allowing for the accommodation and deposition of strata assigned to the St. Joe group. The St. Joe group, first proposed by Hopkins (1893) and used by Girty (1915), was defined as the chert-free, basal member of the “Mississippian limestone” as used by Cline (1934). Girty designated the type location to be St. Joe, Arkansas (Cline, 1934). The St. Joe group stratigraphically is composed of the Bachelor shale, the Compton Limestone, the Northview Formation, and the uppermost unit is the Pierson Limestone. Where present, the Bachelor shale is green shale with phosphatic nodules (Cline, 1934). The overlying Compton Limestone is pyritic, gray, medium-bedded limestone, which thins in upper portions and weathers as thin nodular beds (Huffman, 1960; Anglin, 1966). The Northview Formation is a thin “olive green calcareous shale or marlstone” (Huffman, 1960, p. 93). The uppermost unit, the Pierson Limestone, is gray, finely crystalline, thick-bedded limestone, which “locally passes into a thick, crinoidal reef facies” (Huffman, 1960, p. 93). The thickness of the St. Joe ranges up to 30.5 m (100 ft), which varies depending on the presence of reefs (Harbaugh, 1957). St. Joe fauna has been well documented and includes crinoids, bryozoans, brachiopods, mollusks, ostracods, corals, and echinoderm spines; crinoids are the most prominent, followed by bryozoans (Anglin, 1966).
Crinoids, rugose corals, echinoderms, brachiopods, bryozoans, ostracods, and gastropods are found in the St. Joe group. Crinoids and bryozoans are the most common specimens (Hopkins, 1893; Girty, 1915; Cline, 1934; Laudon, 1939; Harbaugh, 1957; Huffman, 1959; Anglin, 1966; and others). Mississippian biostratigraphy in the region is based on conodonts. Thompson and Fellows (1970) recognized six Tournaisian conodont zones and subzones. The conodont zonation of Lane and DeKeyser (1980) and others based on high-resolution sampling (Boardman et al., 2013) has produced the currently accepted Lower Mississippian conodont zonation of the region. The Compton Limestone is in the Siphonodella cooperi–Gnathodus delicatus Biozone (Thompson and Fellows, 1970; Boardman et al., 2013). The overlying zone in the Northview Formation is defined by the appearance of Gnathodus punctatus and the abundance of Siphonodella cooperi hassi, which defines the Siphonodella cooperi hassi–lower Gnathodus punctatus Biozone (Thompson and Fellows, 1970; Boardman et al., 2013). The Pierson Limestone is marked by the first appearance of Polygnathus communis carinus and Gnathodus semiglaber and encompasses three zones: the Polygnathus communis carina–upper Gnathodus punctatus Zone, the lower Pseudopolygnathus multistriatus Biozone, and the upper Pseudopolygnathus multistriatus–Gnathodus cuneiformis Biozone (Boardman et al., 2013). Many of these zones have been identified at the Jane, Missouri, outcrop (Boardman et al., 2013).
One hundred and eighty-six powdered bulk samples for isotope analysis were directly collected from the outcrop face at 20 cm (7.9 in) intervals and above all formation contacts. Two samples were taken at each sampling horizon at least 20 cm (7.9 in) apart laterally. All samples were taken at least 3 m (9.8 ft) from visible fractures. All samples were drilled using a 1.3 cm (0.5 in) titanium carbide bit in a rotary hammer drill. Each sample was caught in a new sterile container and then secured in a sealed sample collection bag. The outcrop was first drilled at least 15 cm (6 in) deep and the bit retracted from the hole prior to catching the rock powder to minimize the amount of possible isotopic alteration and contamination at the outcrop face. The bit was cleaned using isopropyl alcohol between each sample collection.
The powdered samples were sent to the University of Arkansas Stable Isotope Laboratory (UASIL) in Fayetteville, Arkansas, for analysis. Samples of 0.03–0.05 mg were weighed into aluminum boats, transported to a test tube, flushed for all atmospheric gases, and reacted with H3PO4 acid at 25°C for 18 hours to ensure a complete reaction of the CaCO3. The carbon and oxygen gasses created by the reaction of the limestone with the acid were then analyzed using a Finnigan DeltaPlus XP isotope mass spectrometer equipped with Gasbench II continuous-flow sample analysis. Interspersed with the outcrop powder samples were NBS-18 and NBS-19 standard samples. The δ13Ccarb data were reported per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) isotopic standard. The δ18Ocarb data were reported per mil (‰) relative to the Vienna Standard Mean Ocean Water (VSMOW) standard and converted to VPDB using the equation (Friedman and O’Neill, 1977):
To ensure sample accuracy, all samples from the Oklahoma outcrops were analyzed. Once the sampling method was verified to provide data with a deviation from the standard samples of 5.4‰ for δ13CVPDB and 11.3‰ for δ18OVSMOW, only one set of samples from the Missouri outcrop was analyzed. The reported deviation from the standard samples for the Missouri set of samples is 3.9‰ for δ13CVPDB and 11.5‰ for δ18OVSMOW.
X-ray diffraction (XRD) analysis was completed at the University of Tulsa. Samples were powdered at the University of Tulsa and analyzed using a Rigaku Miniflex 1000. Quantitative modeling was carried out by Rietveld refinement RIQAS (MDI). Brindley microabsorption correction was applied to the quantitative XRD results for pyrite abundance, with the weight fraction sum normalized to 1.0. Modeled mineral concentrations were normalized to 100%.
Outcrop Thicknesses and Observations
The four outcrops in this study strike obliquely to paleoshore along a distally steepened ramp from the northeast–southwest (Figure 3; Lane and DeKeyser, 1980). The Compton and Pierson limestones generally thicken toward the paleoshore, whereas the Northview is uniformly thick in the study area. The overall fossil assemblage of the outcrops is similar to previous studies, with brachiopods, crinoids, and rugose corals being common throughout the formations. Postdepositional fractures striking northeast–southwest were visible at all outcrops. All isotopic sampling avoided proximity to fractures to minimize diagenetic alteration. One fracture at the 412-S outcrop was filled with calcite crystals.
The trace fossil, Zoophycus, is reported for the first time at the base of the Northview Formation in the Jane outcrop and at the middle of the Northview at the N-412 and S-412 outcrops in Oklahoma. Bedding planes at the base of the Northview are not visible in Oklahoma, so Zoophycus may be present throughout the section, but this is not visible in outcrop. Zoophycus is a bacterial farming trace fossil on Northview bedding planes (Figure 4; Löwemark et al., 2007; Butaois and Mangano, 2011; Zhang and Zhao, 2015). This trace fossil was not observed at the Oklahoma State Highway 10 outcrop because bedding planes of the Northview are not exposed. Zoophycus is traditionally recognized in shelf to basin, quiet water facies below storm wave base in muddy marine environments (Miller, 1991). However, at other localities of the Northview, evidence of tidal conditions is present (Mazzullo et al., 2013; Childress and Grammer, 2015) suggesting that bathymetry of the Northview varied across the area.
Regional Structural and Isopach Map
The St. Joe group crops out in northeastern Oklahoma and southwestern Missouri nearly 305 m (1000 ft) above sea level and dips westward into the subsurface. A persistent structural nose is located in eastern Kay County, and a deep structural terrace is in northern Osage County (Figure 5). Both features are located near faults shown on the recent Oklahoma Geologic Survey fault maps (Darold and Holland, 2015). Thickness variations observed in outcrop are similar to subsurface thickness changes. The St. Joe group thickens along an east-to-west ridge with thinning to the north and south. In outcrop, the Compton ranges in thickness from 65 cm to 14 m (2–46 ft), the Northview ranges from 0.7 to 1.5 m (2–5 ft), and the Pierson ranges from 0.8 to 3.5 m (3–11.5 ft). In the subsurface, the Compton Limestone ranges in thickness from 1.4 to 12.2 m (4–40 ft) with thicker pinnacles scattered across the study area. The Northview Shale varies in thickness between 0.6 and 8.5 m (2–28 ft) and is primarily controlled by positive remnant structures in the Compton Limestone so that the Northview thins where the Compton Limestone thickens. The Pierson Limestone caps the St. Joe group with thicknesses ranging from 1.2 to 12 m (4–39 ft), thickening to the west and south. In northern Osage County, the Pierson thins to less than 1.5 m (5 ft) on the east side of a fault-related structure. Remnant pinnacles are observed in Osage, Kay, and Pawnee counties as well as additional minor pinnacles across the middle of Osage County in a northwest-to-southeast trend (Figure 5). Smaller carbonate buildups are observable in multiple St. Joe outcrops (Laudon, 1939; Harbaugh, 1957; Anglin, 1966; Unrast, 2013; Morris et al., 2013). Additional structure and isopach maps of each formation of the St. Joe group have been previously published (Opfer, 2015).
The Compton Formation is a coarsening-upward wackestone–packstone with the primary fossils of echinoderms, crinoids, and brachiopods. Closer to shore at the Jane outcrop, sponge spicules and trilobites are more prevalent. The Northview Formation is a wackestone–grainstone with interbedded shale layers, with the more fossiliferous lithologies occurring closer toward paleoshore. Bioturbation, graded bedding, and lamination are present throughout the formation, particularly toward the upper half of the section. The primary fossils present in the thin sections are crinoids, echinoderms, brachiopods, and ostracods. The Northview Formation is the most fossiliferous of the St. Joe group. The Pierson Formation is a wackestone–packstone with echinoderms, crinoids, and brachiopods prevalent throughout the formation. The Pierson is more fossiliferous than the Compton Formation and less fossiliferous than the Northview Formation (Figure 6).
As seen in outcrop and thin section, Jane, Missouri, was the most fossiliferous outcrop and Oklahoma State Highway 10 (T-10) was the least. All outcrops in this study area were determined to have fragmented and unfragmented fossils, dolomite concentrated along fractures, minimal porosity, and compaction. Pyrite and dolomite are interpreted as having formed during burial diagenesis. Pyrite is common in the St. Joe group and occurs in multiple stages. Framboidal pyrite is typically observed in finer grained facies. Coarse cubic pyrite crystals and bioclastic replacement pyrite are present in fractures and bioclasts. Iron staining is associated with the pyritization. Multiple stages of dolomitization are inferred based on zoning and crosscutting relationships. Early dolomite rhombs, planar-euhedral and planar-subhedral, are present in the matrix and vary in size from fine to medium crystalline. Crosscutting stylolites indicate dolomite precipitation prior to burial diagenesis. Fine-crystalline, planar-subhedral dolomite rhombohedra occur in skeletal fragments, nonskeletal grains, cements, and fractures. By crosscutting relationships, this is inferred to be late dolomitization. Multistage fracturing and late-stage calcite dissolution account for the majority of the observed microporosity. Precipitation of calcite, pyrite, and dolomite during burial diagenesis limit observed porosity to <5% in petrographic samples.
A red clay layer at the top of the Northview Formation is only observed at the Jane outcrop. This clay was sampled for XRD analysis and the mineral assemblage indicates a mixed-marine and terrigenous source. The transition from green to red color represents a change from iron mineral reduction to oxidation. The present minerals include quartz (36.9%), illite 2M (26.3%), goethite (13.2%), hematite (6.5%), microcline (4.5%), smectite (3.9%), I/S-67 (3.8%), magnesium calcite (2.6%), chamosite (1.7%), pyrite (0.5%), and I/S-80 (0.1%).
The data spread for this study is (full dataset in Sessions, 2016). Diagenetic effects on carbon and oxygen isotopes can be assessed using the variation among values of cross-plotted isotope data (Allan and Matthews, 1982; Figure 7). The spread of data from this study is compared to the estimated Lower Mississippian marine calcite initial isotope values of and 3.5‰ to 4‰ , which is represented by the red box (Frank and Lohmann, 1995; Mii et al., 1999). The subsurface study on the Anadarko shelf in western Oklahoma determined the modern bulk rock sample range for the Mississippian to be (Koch et al., 2014). All data points in this new study are slightly more negative than the estimated initial of values of Mississippian seawater, indicating that some amount of diagenetic alteration and isotope replacement has occurred. The spread and configuration of the data curves, however, in this study are consistent with those of past studies, further indicating the presence of the depositional isotope signal (Mii et al., 1999; Saltzman, 2002; Saltzman et al., 2004; Koch et al., 2013). In our study, the mean r2 value of 186 isotope samples from three formations is 0.20 and the r2 value of 0.38 for the Compton Limestone, 0.21 for the Northview Formation, and 0.18 for the Pierson Limestone. These low r2 values measure the consistency of the data and the linear regression trend lines suggest that a minimal and uniform amount of diagenetic alteration affected the system. The isotopic values do not plot in an inverted J-curve shape, which would indicate extensive diagenetic alteration caused by rock–fluid interaction (Meyers and Lohmann, 1985; Lohmann, 1988; Bishop et al., 2014). The data are also statistically significant with calculated Pearson r values for a linear correlation at p < 0.05 being 0.61 for the Compton Limestone, 0.46 for the Northview Formation, and 0.43 for the Pierson Limestone. The r and r2 values of the data suggest that the original carbon and oxygen isotope values have not been changed greatly. Therefore, we suggest that the isotope curves can be used to correlate the St. Joe group with other Lower Mississippian sections.
The and δ18OVPDB data curves are correlated to the corresponding stratigraphic sections with a two-point moving average curve (Figure 8). The stratigraphic section at Jane, Missouri, has been correlated with the conodont biostratigraphy determined previously (Boardman et al., 2013). In the four sampled sections, the gradually increases from the base to the top of the stratigraphic section. A negative deflection is above the base of the Pierson Limestone. The values for the Oklahoma sections are similar to the Missouri sections, which are up to 1‰ more positive. This may result from paleohydrothermal fluid-flow through formation fractures that formed in the lead-zinc tristate mining district, differences in local depositional conditions, or modern weathering. The precise reason for the presence of lower values in Oklahoma than in Missouri was not a focus in this study. The isotope curves from this study can be reliably used as a correlation tool despite the value differences between the outcrops because of the similarly shaped curves, variances in the data within each outcrop and between formations, and the fact that bulk rock was sampled. Bulk rock sample data may be a different overall value, but the results are similar to the data presented in brachiopod studies and commonly have similar trends (Nelson and Smith, 1996; Saltzman, 2002; Saltzman et al., 2004; Fouke, 2005; Allègre, 2008; van der Kooij et al., 2009). Unlike the δ13CVPDB, the δ18OVPDB values for all four outcrops are similar with no one outcrop having heavier or lighter oxygen values than the others. The oxygen values are also relatively uniform for the entire St. Joe group in these sections.
Depositional Environment Interpretation
This study agrees with previous studies that the Compton and Pierson formations were deposited in mid- to outer-ramp conditions (Mazzullo et al., 2013). By projecting these models onto the Cherokee platform, it is inferred that outer-ramp conditions will be observed west of this study area. This depositional model has been recently challenged by authors who argue that additional unconformities are present in the St. Joe group, specifically at the Northview–Pierson contact, and that these represent periodic regressive cycles (Childress and Grammer, 2015). These authors state that the Northview Formation is a shallow to exposed tidal flat environment, as represented by ripples and tidal bundles observable in outcrop and the fossil assemblage. Debris flow blocks in the Compton are also described, and the depth of their deposition would be more consistent with a deeper Northview depositional interpretation (Childress and Grammer, 2015). The Northview is composed of interbedded shale and limestone beds that have observable hummocky cross-stratification at Jane, and have Zoophycus trace fossils (Sessions, 2016). The Zoophycus and hummocky cross-stratification both indicate a mid- to outer-ramp depositional environment between storm and normal wave base. The fairly uniform thickness and distribution as plotted on surface and subsurface isopach maps is consistent with a mid-ramp depositional environment (Opfer, 2015).
Lithofacies of each formation based on thin-section petrography range from mudstone to packstone, which here represent deposition in the mid- to outer-ramp zones ranging from quiet, subtidal marine conditions near storm wavebase to relatively shallow, normal marine conditions between fair wavebase and storm wavebase (Wilson, 1975; Read, 1980; Burchette and Wright, 1992; Franseen, 2006; Mazzullo et al., 2009, 2011). No petrographic or outcrop evidences of subaerial exposure, such as oxidized crusts, karsts, cracks, or precipitation of evaporates, were observed.
In outcrop, shoaling upward lithofacies and stratigraphic continuity observed at the four outcrops in this study area indicate that the Compton Limestone was deposited on a mid-ramp facies. At the N-412 outcrop, the base of the Compton transitions from bioclastic mudstone–wackestone representing outer-ramp depositional conditions into two thin beds of echinoderm packstone deposited as shallowing occurred in a higher-energy environment. Shoaling upward lithofacies at the S-412 outcrop transition from a bioclastic wackestone–packstone to a bioclastic packstone. Multiple shallow water lithofacies transitions are recorded for the remainder of Compton deposition evidencing minor sea-level fluctuations in mid-ramp conditions. In the core, the Compton is a bioclastic mudstone–wackestone representing a more distal environment than facies observed in outcrop.
The Northview Formation is a wackestone–grainstone with interbedded shale layers and is more fossiliferous toward paleoshore. Notably, Zoophycus is observed on bedding planes in three of the four outcrops. Zoophycus is evidence of a mid- to outer-ramp depositional environment below normal wavebase (Miller, 1991). At the N-412 outcrop shale has parallel to subparallel laminations with in-situ brachiopods. Climbing ripples are observed below the Northview–Pierson contact suggesting sediment transport in a lower flow regime. The Northview is interpreted to have been deposited in a mid- to outer-ramp setting between fair-weather wavebase and storm-weather wavebase in the Oklahoma sections, representing the deepest water conditions observed in the St. Joe group. The Northview represents the maximum flooding interval in the Compton–Pierson Formation sequence. This differs from previous interpretations of the Northview at other outcrops, where it is either silty or mud-lean, interbedded limestone with desiccation cracks that was deposited on a tidal flat (Mazzullo et al., 2013; Childress and Grammer, 2015). The Northview in the Jane section previously has been interpreted as being the shallowest depositional area and was subaerially exposed due a eustatic sea-level drop (Haq and Shutter, 2008; Kammer and Matchen, 2008; Sneddon and Liu, 2010). In the core, the Northview is represented by calcareous siltstone, and the quartz is interpreted to be authigenic.
The Northview–Pierson contact in our sections is a sharp change from marine clay to marine limestone, which records a change from deposition below local wavebase to within wave or current energy. There is no evidence of reworking, scouring, or other basal erosion and the contact is sharp at all four outcrops in the study area. This contact represents the deepest environment and the maximum flooding of the depositional cycle. At Jane, the clay layer may represent a submarine depositional hiatus, but does not represent a subaerial exposure surface. Such a change is consistent with the three units comprising one deepening-shallowing depositional cycle. The change from carbonate to siliciclastic and back is a response to increased terrestrial sediment influx, which suggests possible increased runoff. The environmental transition from deposition below normal wavebase to above wavebase and the absence of scouring and reworking at the contact support the conformable nature of the Northview–Pierson contact.
The basal interval of the Pierson Limestone is lithologically and depositionally similar to the Northview. As shoaling occurred, depositional facies coarsened upward in a marine sequence. The lower Pierson Limestone is a laminated wackestone–packstone representing mid- to outer-ramp depositional conditions. The upper Pierson lithofacies are characterized by diverse heterozoan and siliceous fossil assemblages, common bioturbation, and evidence of hydraulic reworking as mid-ramp conditions returned (Wilson, 1975; Burchette and Wright, 1992; Franseen, 2006). The Northview–Pierson contact is conformable in these Oklahoma sections.
The St. Joe group represents one complete long-term depositional cycle. The transgressive system is observed stratigraphically as a lithofacies transition from mid-ramp conditions in the Compton Limestone to outer-ramp conditions in the Northview Shale. Deepening is interrupted near the Northview–Pierson Formation contact by a drop in relative sea level that may have only exposed the Jane section in this study area, and subsequent shallowing produced the observed coarsening upward textures of the Pierson Limestone (Kammer and Matchen, 2008). Using biostratigraphy to correlate the onlap curve, three high-frequency cycles are represented within the conodont zones proposed by Boardman et al. (2013). However, the multiple high-frequency cycles are not observed within the study area.
Stratigraphic Succession and Chronostratigraphic Correlation
Similarity between the δ13CVPDB and the δ18OVPDB isotope curves from this study and previous studies of Lower Mississippian sections indicate that these isotopic systems can be used to correlate sections regionally and internationally (Saltzman, 2002; Koch et al., 2014; Figure 9). Both the conodont biostratigraphy at the Jane, Missouri, outcrop and the isotope curves correlate these sections with the upper Tournaisian Stage (Boardman et al., 2013; Figure 10). The correlation of the new data with previous studies estimates a depositional period of approximately 8 Myr from 357 Ma to 349 Ma (Figure 8A–D; Koch et al., 2014). Because units of the St. Joe group in the eastern Oklahoma sections are apparently conformable and have been integrated with conodont biostratigraphy, we propose that the St. Joe in these sections can be used as a chronostratigraphic reference section in the U.S. midcontinent. The Compton–Northview contact represents deepening from above the zone of normal wavebase to below. The absence of reworking supports the conformable nature of this contact. There is also no visible reworking or clast inclusions at the Northview–Pierson contact.
The absence of large negative carbon isotope deflections at the Compton–Northview and Northview–Pierson formation contacts and the gradational boundaries suggests that the St. Joe group is internally conformable and does not have subaerial exposure surfaces in the study area. The conformable nature of the contacts was observed in both outcrop and thin section, because features such as reworked clasts, scours, and dolomite are not present. These observations are consistent with the Kinderhook–Osage paleogeographic shoreline map that shows the study area was below sea level at this time (Kammer and Matchen, 2008). The negative carbon and oxygen isotope deflections in the Northview have been observed in previous regional and international studies of Lower Mississippian sections, further clarifying that the up to 5‰ negative shift in data is neither erroneous nor an indication of exposure (Mii et al., 1999; Saltzman, 2002; Saltzman et al., 2004; Grossman et al., 2008; Koch et al., 2014; Figure 10). These negative deflections may represent an increase in the burial and preservation of organic carbon and have been interpreted to signal polar glaciation (Grossman et al., 2008).
This study has determined that the St. Joe group is conformable in the study area and represents one long-term transgressive–regressive cycle. The depositional environment was mid- to outer ramp for the Compton, Northview, and Pierson formations, with the Northview Formation representing the deepest depositional environment between fair-weather and storm-weather wavebase. The St. Joe group has been slightly altered through diagenesis, but the bulk rock δ13C isotope curves are comparable to previous regional isotope studies. This isotopic correlation indicates that the St. Joe group was deposited in the Tournaisian Epoch from 357 Ma to 349 Ma, which is consistent with the broader range determined by previous conodont biostratigraphy. The intraconformable nature and minimal diagenesis allow for the St. Joe group to be used, when combined with conodont biostratigraphy, as a reference section for Tournasian geochemical chronostratigraphy.
We would like to thank Dr. Bethany Theiling, Dr. Kerry Sublette, Dr. Winton Cornell, Dr. Dennis Kerr, and Dr. Junran Li at The University of Tulsa and Dr. Erik Pollack and Lindsey Conway at UASIL for their contributions to this project. We would also like to thank The University of Tulsa for funding and Keith Summar (Petro-Quest) for partial funding and use of their well database.