Sharon Springs Research Articles

A pteranodontid wing with traces of extensive invertebrate scavenging from the Pierre Shale (Sharon Springs Member, Campanian–Maastrichtian), east central Wyoming (Niobrara County)

Here we report the associated and semi-articulated partial wing of a mid-sized pterosaur from an exposure of the Pierre Shale, in east central Wyoming. All elements suffer from compaction but are identified as those of a pteranodontid based on the warped deltopectoral crest. Most probably, the wing is that of Pteranodon longiceps, although the length of the ulna and of the metacarpal IV relative to that of the humerus slightly exceeds that reported from any previously documented Pteranodon specimen. The surfaces of all wing elements are littered with circular indentations, ranging from 1.1 to 6.8 mm in diameter. These indentations formed postmortem and appear to be borings left by invertebrate scavengers.

Chemostratigraphy of unconventional shale reservoirs: A case study of the Niobrara Formation within the Denver-Julesburg basin

The Upper Cretaceous carbonate-rich Niobrara Formation is a prominent petroliferous unconventional reservoir that is generally characterized by fine-scale heterogeneities that complicate local and regional correlation of target zones, well placement and completion strategies. Within rhythmically interbedded reservoirs like the Niobrara Formation, the occurrence of thin marl and chalk beds within chalk- or marl-dominated intervals respectively, further impedes identification of marl or chalk benches. This study uses a combination of petrophysical and geochemical data from 6 oil and/or gas wells within the Niobrara interval of the DJ basin to identify and correlate eight (8) chemozones (C1, C2, C3, C4, M1, M2, M3 and M4) and four (4) chemostratigraphic packages (Sharon Springs, Smoky Hill, Fort Hays and Codell Sandstone) that are essential for constructing a robust and dynamic stratigraphic framework.The results showed synchronous variation in petrophysical trends, elemental, mineralogical and organic composition at contacts, ubiquitous cyclicity within, and across the Niobrara Formation in the northern DJ basin. The stratigraphic units are geochemically separated based on TOC, total clay, total carbonate, and quartz + feldspars, and Ca, Sr, Si, Al, Ti, Rb, Zr, Nb, K, and Cr, while the internal stratigraphic units within the Niobrara Formation are defined by integrating gamma-ray (GR), resistivity, carbonate proxy, marl proxy, carbonate/marl proxy and terrigenous input. The chalk and limestone units (C1–C4) have higher Ca, Sr, total carbonate concentrations and relative brittleness index relative to the marl units (M1-M4). This multiproxy approach reveals variable calcareous and terrigenous/detrital input, and paleoredox conditions as the main controlling factors behind the changing lithologies/cyclicity within the Niobrara Formation. Hence, an adequate characterization and correlation of inherent heterogeneities within marls and chalks of the Niobrara Formation may require incorporation of a multiproxy approach at various scales. By integrating color scale, geochemistry, TOC and wireline well log curves responses, higher resolution identification of chalk reservoirs and marl source beds can be achieved. Spatial distribution of chalks and marls consistently show a trend of total carbonate and Ca increasing southwards towards the Tethys Sea with highest average concentrations recorded in Fort Hays Limestone while quartz + feldspars, and total clay concentrations increasing northwards, suggesting a southern and northern provenance, respectively.

Codell carrier-bed play, Denver Basin

Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.

Geochemical and micropaleontological evidence of paleoenvironmental change in the Late Cretaceous Western Interior Seaway from the Niobrara to the Pierre Shale Formation

Deposition of the Upper Cretaceous Niobrara Formation and overlying Sharon Springs Member of the Pierre Shale occurred during transgressive phases within the Western Interior Cretaceous (WIC) Seaway, however their respective geochemical and biological differences are significant. The Niobrara Formation is an alternating calcareous chalk/marl system, biologically defined by fecal pellets and robust foraminifers including Heterohelix globulosa, Globigerinelloides ultramicrus, Hedbergella, Giimbelina and two Archaeoglobigerina species. However, the Sharon Springs is primarily argillaceous in composition with lesser amounts of biogenic calcite and silica. Larger foraminifers within the Sharon Springs are primarily arenaceous whereas foraminifers of the same species found in the Niobrara are dwarfed in the Sharon Springs. Dwarfism of foraminifers species and the presence of dispersed diatoms, which flourish in cold waters, indicate a shift in paleocurrents. Deposition of the Niobrara occurred during times when warm Gulfian currents allowed for carbonate production through the Early Campanian. Paleogeographic maps of the Middle Campanian indicate a southerly restriction of the WIC during the time of the southerly Claggett Transgression. The influx of cold, southerly waters resulted in an environment conducive to diatoms and environmentally stressful to foraminifers. Lithological and geochemical evidence from the Sharon Springs indicates that the cold-water Claggett Transgression resulted in a stratified water column in the middle of the WIC as well as increased organic matter production. Petrographic analysis of preserved organic matter show clumped floccules or flattened amalgamations of floccules in laminated facies. While dispersed amorphous organic matter is also present, flocculation had a major influence on organic matter preservation. Two possible depositional mechanisms, each dependent on water density, probably produced these water conditions. If density contrasts existed between the cold water influx and the warm waters present during the deposition of the Niobrara, the cold waters would have progressed along the bottom of the basin leading to upwelling along the basin margin and basin stratification. If no significant density contrast existed, caballing would occur at the mixing front, leading to downwelling, rapid transport of organic matter to the sea floor, better organic matter preservation and basin stratification.

Sequence stratigraphy of the lower Pierre Shale of the southern Powder River Basin: A ramp margin sequence that terminates Niobrara Formation carbonate deposition

The Upper Cretaceous section of the southern Powder River Basin of the Rocky Mountain region includes the Niobrara Formation, which is one of the most significant source rocks of the Western Interior Cretaceous Seaway, and it is overlain by the lower Pierre Shale interval. The lower Pierre Shale is divided into eight members, which are the progradational highstand deposits of the Gammon Ferruginous Member; lowstand prograding wedge deposits of the Shannon Sandstone, the Unnamed Member, and the Sussex Sandstone; transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members; and highstand deposits of the Mitten Black Shale and Red Bird Silty members. The Shannon and Sussex Sandstone members are known targets for oil production. Based on integrated research of 1490 well logs with two cores, this study demonstrates that the lower Pierre Shale interval is a Type 1 Ramp Margin Sequence. Although previous studies primarily focused on individual sandstone members or parts of the lower Pierre Shale section in relatively limited areas, this study provides an in-depth sequence stratigraphic analysis of the lower Pierre Shale interval. A depositional model was created, which demonstrates that the Shannon Sandstone, Unnamed, and Sussex Sandstone Members were deposited as an encased lowstand prograding wedge between the progradational Gammon Ferruginous Member and the transgressional Ardmore Pedro Bentonite Beds and Sharon Springs members hundreds of miles basinward from the stratigraphically equal Mesaverde lowstand sandstones of the Bighorn Basin. The shift of facies across long distances is explained with a forced regression that has taken place during the deposition of the uppermost Gammon Ferruginous Member and the following deposition of the Shannon and Sussex sandstone members. Rapid eastward progradation of the clastic sediments of the lower Pierre Shale and the following rapid forced regression effectively terminate the deposition of the Niobrara Formation carbonates in the Western Interior Seaway.

Interpretation of wrench faulting and faultrelated pressure compartmentalization, Wattenberg Field, Denver Basin Colorado

A structural interpretation of a 128 km2 3D seismic data set from Wattenberg Field, Colorado indicates a northeast-trending step-over fault system in the Niobrara Formation. These step-over faults are the product of a fault block rotation that is caused by two large-scale dextral wrench faults: the Longmont and the Lafayette wrench fault zones. An investigation into different fault orientations between the Niobrara and the overlying Pierre Shale indicates that a rotation of the maximum horizontal stress has occurred during the Laramide Orogeny. The step-over faults and stress rotation result in reservoir compartmentalization in the Niobrara. Seismic inversion shows the different stress field compartmentalization. Low impedance occurs in a lower effective stress environment caused by high pore pressure. These compartments may constitute ‘sweet spots’ in exploration and production in the Niobrara Formation. The Niobrara Formation is a major hydrocarbon reservoir deposited during the Late Cretaceous (Figure 1a). This formation consists of the Smoky Hill Member and Fort Hays Limestone. The Smoky Hill member consists of A, B, and C chalks, and A, B, and C marls. Bratton (unpublished work, 2015) indicates that the overpressure regime extends into the overlying Sharon Springs Member of the Lower Pierre Formation. Figure 1B shows the geologic events of the Late Cretaceous Petroleum System (Higley and Cox, 2007). The Laramide Orogeny occurred during the Late Cretaceous and Early Tertiary (between 70-50 ma.). This event is thought to be responsible for the structural features in the study area (Higley and Cox, 2007). The products of this movement include several wrench fault zones located in the area mapped by Weimer (1996).

Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A.

A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2–4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1–2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1–2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarse-grained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.

Geographic and stratigraphic distribution of the Late Cretaceous suspension-feeding bony fish Bonnerichthys gladius (Teleostei, Pachycormiformes)

ABSTRACT The stratigraphic and paleogeographic distribution of the suspension-feeding pachycormiform fish Bonnerichthys is reviewed. Fossils attributable to this genus are known from the Western Interior Seaway (Niobrara Formation of Kansas, Sharon Springs Formation of Kansas, Nebraska, and North Dakota, DeGrey Formation of South Dakota, and possibly Mobridge Formation of Nebraska), the Eastern (Eutaw Formation of Mississippi and Mooreville and Demopolis formations of Alabama and Mississippi) and Western (Ozan Formation of Texas and Marlbrook Marl Formation of Arkansas) Gulf Coastal Plain, the Atlantic Coastal Plain (Wenonah Formation of New Jersey and possibly Marshalltown Formation of Delaware), and Pacific coast (Moreno Formation of California). Chronologically, occurrences of Bonnerichthys range in age from Coniacian to Maastrichtian. Fossils of Bonnerichthys from the Atlantic Coastal Plain, Eastern and Western Gulf Coastal Plain, and Pacific coast are definitively identified and figured here for the first time. Candidate deposits outside the United States that might yield Bonnerichthys are reviewed.

A partial skeleton of the Late Cretaceous lamniform shark, Archaeolamna kopingensis, from the Pierre Shale of western Kansas, U.S.A.

ABSTRACT All previous records of the lamniform shark, Archaeolamna kopingensis, are based on isolated teeth. Here we describe a partial skeleton from the Sharon Springs Formation of the Pierre Shale Group of western Kansas, U.S.A. The specimen includes portions of the upper and lower jaws with articulated teeth. The dentition consists of two files of upper and lower anterior teeth that, together with a single file of intrabullar intermediate teeth, are housed in a dental bulla, as well as multiple files of lateral teeth, along with at least two files of lower symphysial teeth and a single file of upper symphysial teeth. The intrabullar intermediate tooth is slightly shorter than the other anterior teeth and has a median cusp with distinctive distal curvature. The dental sequence of A. kopingensis is unique among both extinct and extant lamniforms. Associated with the jaws are fragments of the neurocranium and multiple vertebral centra. A sagittal section through a centrum shows that this shark deposited 18 annual marker bands after its birth and adult size was attained by the 10th band. The robust but penetrating tooth morphology and large jaw circumference suggest that A. kopingensis likely fed upon large prey items.

Stratigraphy of the Lower Pierre Shale (Campanian): Implications for the Tectonic and Eustatic Controls on Facies Distributions

The lower Pierre Shale represents a time of significant changes in the Cretaceous Western Interior Seaway, resulting from complex interactions of tectonism and eustatic sea level changes. The recognition and redefinition of the units of the lower Pierre Shale has facilitated understanding of the dynamics of the basin. The Burning Brule Member of the Sharon Springs Formation is restricted to the northern part of the basin and represents tectonically influenced sequences. These sequences are a response to rapid subsidence of the axial basin and the Williston Basin corresponding to tectonic activity along the Absoroka Thrust in Wyoming. Unconformities associated with the Burning Brule Member record a migrating peripheral bulge in the Black Hills region corresponding to a single tectonic pulse on the Absoroka Thrust. Migration of deposition and unconformities supports an elastic model for the formation and migration of the peripheral bulge and its interaction with the Williston Basin.

Soot and palynologic analysis of Manson impact-related strata (Upper Cretaceous) of Iowa and South Dakota, USA

The Campanian Manson impact structure of Iowa represents the best-preserved, large-diameter complex crater within the continental U.S. To assess the timing and potential mode of crater infilling and the possible presence of an impact event horizon, we analyzed samples from both within and distal to the impact structure for their elemental carbon, soot and palynomorphs. Within the impact structure, identifiable soot occurred in fragmented impact breccia and suevite but not in lower impact-melt breccia. Although most of this soot likely represents reworking of material from older Cretaceous marine shales, one high soot concentration occurs with melt material in a Keweenawan Shale–Phanerozoic clast breccia mix. This represents the first association of soot and impact-generated materials within an impact structure and the best sample candidate for Manson impact-generated soot. No palynomorphs occurred in the impact melt breccia. Overlying suevite (Keweenawan Shale clast breccia) of the central peak yielded sparse and thermally altered palynomorphs, indicating deposition prior to full cooling of the crater debris. Presence of easily degraded soot also argues for rapid backfilling of the crater. Distal samples from South Dakota represent the Sharon Springs and Crow Creek members of the Pierre Shale 230 km northwest of the Manson impact structure. Although containing shocked grains, the Crow Creek preserves no soot. In contrast, the Sharon Springs, generally considered as predating the Manson impact, has significant soot quantities. Palynomorphs differ markedly across the unconformity separating the two members with the Crow Creek containing more terrestrial forms, normapolles, and older reworked palynomorphs, consistent with a terrestrial impact to the east. Origin of the Sharon Springs soot remains unclear. Given soot occurrence within four of the five Cretaceous marine units sampled, the relatively shallow, anoxic bottom conditions of the Western Interior Cretaceous Seaway may have simply favored soot preservation. Until a better understanding of the broader occurrence and preservation of soot is achieved, some soot-impact associations will remain ambiguous.

Paleoenvironmental interpretations of rare earth element signatures in mosasaurs (reptilia) from the upper Cretaceous Pierre Shale, central South Dakota, USA

Rare earth elements were analyzed from fossil marine reptile (Mosasauridae) bones collected from five superposed members (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the upper Cretaceous Pierre Shale at localities along the Missouri River in Brule, Buffalo, Hughes, and Hyde counties in central South Dakota. Fossil bones from each member of the Pierre Shale have different rare earth element (REE) signatures that may be distinctive over wide geographic areas. Fossils from the Sharon Springs Member have distinctive REE signatures that may be further subdivided into three superposed groups that correspond with the upper, middle, and lower Sharon Springs Member. REE signatures are distinctive from each stratigraphic unit; therefore, fossils eroded from their stratigraphic context may be assigned to their proper depositional unit based on REE signature comparisons. Differences in REE compositions of bones among members appear to result from differential mixing of oxygenated and anoxic seawaters. If differences in mixing are interpreted as depth differences, the lower Sharon Springs Member was deposited in deep, anoxic water; water depths decrease in the middle and upper Sharon Springs, and the overlying Gregory and Crow Creek Members were deposited in even more shallow water. Finally, according to this interpretation, the overlying DeGrey and Verendrye members were deposited in progressively deeper marine waters, but not as deep as the lower Sharon Springs. These interpretations are generally consistent with those based on faunal diversity and eustatic sea level curves.

Paleoenvironmental interpretations of rare earth element signatures in mosasaurs (reptilia) from the upper Cretaceous Pierre Shale, central South Dakota, USA

Abstract Rare earth elements were analyzed from fossil marine reptile (Mosasauridae) bones collected from five superposed members (Sharon Springs, Gregory, Crow Creek, DeGrey, and Verendrye) of the upper Cretaceous Pierre Shale at localities along the Missouri River in Brule, Buffalo, Hughes, and Hyde counties in central South Dakota. Fossil bones from each member of the Pierre Shale have different rare earth element (REE) signatures that may be distinctive over wide geographic areas. Fossils from the Sharon Springs Member have distinctive REE signatures that may be further subdivided into three superposed groups that correspond with the upper, middle, and lower Sharon Springs Member. REE signatures are distinctive from each stratigraphic unit; therefore, fossils eroded from their stratigraphic context may be assigned to their proper depositional unit based on REE signature comparisons. Differences in REE compositions of bones among members appear to result from differential mixing of oxygenated and anoxic seawaters. If differences in mixing are interpreted as depth differences, the lower Sharon Springs Member was deposited in deep, anoxic water; water depths decrease in the middle and upper Sharon Springs, and the overlying Gregory and Crow Creek Members were deposited in even more shallow water. Finally, according to this interpretation, the overlying DeGrey and Verendrye members were deposited in progressively deeper marine waters, but not as deep as the lower Sharon Springs. These interpretations are generally consistent with those based on faunal diversity and eustatic sea level curves.

An Elasmosaur with Stomach Contents and Gastroliths from the Pierre Shale (Late Cretaceous) of Kansas

A nearly complete skeleton of an elasmosaurid plesiosaur (NJSM 15435) from the Sharon Springs Member (Middle Campanian) of the Pierre Shale, Logan County, Kansas, is associated intimately with fragmentary fish remains and numerous gastroliths. The fish bones and gastroliths were located just behind the pectoral girdle in the abdominal region. Identifiable prey includes Enchodus and other small clupeomorph fishes. An isolated tooth of the anacoracid shark Squalicorax cf. S. pristodontus also was recovered in this area. Ninety-five gastroliths (6.8 kg) were present, with the largest stone measuring 15.1 × 8.5 × 5.7 cm (5.0 × 3.3 × 2.2 in.) and weighing 1.06 kg (2.3 lb.). Many of the gastroliths are composed of pink or gray Sioux Quartzite, which suggests that the source of these stones was about 600 km (475 mi) to the northeast of where the elasmosaur remains were discovered. The association of fragmentary fish remains and gastroliths within the abdomen of NJSM 15435 supports the contention that the stones aided in the breakdown of food in plesiosaurs.

Gastroliths Associated with Plesiosaur Remains in the Sharon Springs Member of the Pierre Shale (Late Cretaceous), Western Kansas

Field work conducted in 1991 and 1998 recovered 47 gastroliths in association with the incomplete and disarticulated remains of a large plesiosaur (KUVP 129744). The specimen was discovered in the upper Sharon Springs Member of the Pierre Shale (Late Cretaceous), Logan County, Kansas. The gastroliths are unusually large in size when compared to those documented from other plesiosaur remains, and larger than those associated with the giant sauropod, Seismosaurus. The shapes of the gastroliths are consistent with similar sized stones that occur in river gravel. This specimen provides new data in regard to the range of sizes and the occurrence of gastroliths in these extinct marine reptiles.

The 21st Century Alliance: A Dry Edible Bean Cooperative

AbstractProducers of the 21st Century Alliance in western Kansas had an opportunity to vertically integrate in the dry edible bean industry in the winter of 1998. Demand for dry edible beans, especially pinto beans, has been increasing in recent years. The major decision in this case was to determine whether to (a) build new facilities, (b) build a dual‐purpose grain elevator for beans and other crops, (c) not buy, or (d) purchase an existing facility in Burlington, Colorado, or Sharon Springs, Kansas.

The Sharon Springs Roundup and Prairie Rattlesnake Demography

In the present century rattlesnake roundups have become popular in various parts of the United States as a form of recreation and as a means of generating income for the sponsoring agencies. Various species of rattlesnakes including Crotalus atrox, C. adamanteus, C. horridus, and C. viridis are hunted. The only Kansas roundup was initiated in 1992 in the town of Sharon Springs and C. viridis is the species hunted. Attributes of the Kansas prairie rattlesnake as a game animal are its fertility (mean litter 10.7) and early maturity (third year). About 80% of females produce young each year, unlike more northern populations of the species that reproduce biennially or triennially. In the Kansas prairie, rattlesnakes natural attrition amounts to from one-fourth to one-third annually. Relatively few of the snakes survive for as long as ten years. In order to assure maintenance of a supply of snakes sufficient to provide the recreation and income expected from the roundup, conservation measures must be adopted. Size limits and bag limits should be enforced. About one-fourth of the snakes captured are gravid females, and these should be released unharmed to ensure a continuing supply.

Abstract: High-Resolution Analysis of the Subsurface Stratigraphy and Organic-Matter Content of the Niobrara Formation and Sharon Springs Member of the Pierre Shale, Cretaceous, Western Interior Seaway

Abstract: High-Resolution Analysis of the Subsurface Stratigraphy and Organic-Matter Content of the Niobrara Formation and Sharon Springs Member of the Pierre Shale, Cretaceous, Western Interior Seaway

Paleobiogeography of the Cretaceous Western Interior Seaway of North America: the vertebrate evidence

Distribution patterns of marine vertebrates of the Cretaceous Western Interior Seaway of North America are analysed in a biogeographical context. The analysis is based on fossil vertebrate assemblages from the following Lower Campanian localities: (1) Anderson River fauna, N.W.T. (2) Pembina fauna, southern Manitoba (3) Sharon Springs fauna, South Dakota-Wyoming (4) Niobrara fauna, Kansas (5) Mooreville fauna, Alabama. Due to the cosmopolitan nature of many marine vertebrates, relative abundance is used in preference to presence-absence data in identifying faunal distribution patterns. The analysis demonstrates the presence of two faunal subprovinces in the seaway. The Northern Interior Subprovince is characterized by a low diversity in all groups and is dominated by plesiosaurs, hesperornithiforms and the mosasaur Platecarpus. The Southern Interior Subprovince is characterized by a high diversity in all groups and is dominated by sharks, turtles and the mosasaur Clidastes. These biogeographical subdivisions support the hypothesis of warm temperature and cool temperate zones in the seaway that have been proposed on the basis of invertebrate faunas.

Anatomy of an Organic-Rich Condensed Section: The Campanian Sharon Springs Member of the Pierre Shale, Powder River Basin: ABSTRACT

The Sharon Springs Member of the Pierre Shale is an early Campanian organic-rich rock deposited over a wide regiin of the Cretaceous Interior Seaway. Widespread exposure and detailed subsurface cross sections provide an ideal setting to reconstruct details of its depositional system. The Sharon springs, whose organic matter component is dominated by marine algae, has concentrations of total organic carbon that average about 5 wt. % in the eastern Powder River basin to about 13 wt. % in eastern South Dakota. Thicknesses range from approximately 140 ft in the west, near Redbird, Wyoming, to about 10 ft near Chamberlain, South Dakota. The Sharon springs section near Redbird can be divided into two parts, a lower shale having smectite-dominated clays, total organic carbon (TOC) = 2-5 wt. %, TOC/S = 0.1-0.2, and Ni(Ni+V) = 0.2, and an upper shale with more illite-rich clay, TOC = 5-8 wt. %, TOC/S = 0.2-0.6, and Ni/(Ni+V) = 0.05-0.1. Progressive changes in La/Sm from the lower unit (La/Sm = 2-6) to the upper unit (La/Sm = 7-10) and in K{sub 2}O, AI{sub 2}O{sub 3}, and TiO{sub 2} reflect changes in the provenance of the clastic component. The two units can be correlated over a largemore » area of the Interior Seaway using inorganic and organic chemical markers and physical stratigraphy. The bulk inorganic and organic properties of the Sharon Springs, as well as the molecular characteristics of the soluble organic fraction, suggest that it was deposited at relatively low sedimentation rates under anoxic bottom-water conditions in a stratified water column.« less