Thermopolis Shale


The Thermopolis Shale is a geologic formation which formed in west-central North America in the Albian age of the Late Cretaceous period. Surface outcroppings occur in central Canada, and the U.S. states of Montana and Wyoming. The rock formation was laid down over about 7 million years by sediment flowing into the Western Interior Seaway. The formation's boundaries and members are not well-defined by geologists, which has led to different definitions of the formation. Some geologists conclude the formation should not have a designation independent of the formations above and below it. A range of invertebrate and small and large vertebrate fossils and coprolites are found in the formation.

Geological history

The Western Interior Seaway was an inland sea that existed from the Late Jurassic to the end of the Paleogene. It existed in the middle of North America, extending from the Arctic Ocean to the Gulf of Mexico. It was roughly long and wide. The seaway was relatively shallow, with a maximum depth estimated at.
A foreland basin existed just to the east of the Sevier orogenic belt, which was inundated by the Western Interior Seaway. A forearc on the western side of the basin made this deeper than the eastern side, encouraging the build-up of sediment and, in time, sedimentary rock. Erosion of the Western Cordillera also contributed to the build-up of sedimentary rock on the western edge of the basin, while the more low-lying area to the east provided much less. Changes in the amount, type, rate, and other aspects of the sedimentation were caused by uplift, subsidence, sea level changes, and other factors. The water in the basin made at least two major advances and one major retreat during the Cretaceous, adding complexity to the rock and permitting the creation of riverine, marsh, and estuarine rock in addition to the principal shallow and deep marine rock.
Dating of bentonite and palynological evidence indicate that the Lower Thermopolis Member was deposited between 100.3 and 98.5 Ma. A study of Inoceramidae bivalves confirmed a Late Albian age. Deposition of the upper three members of the Thermopolis Shale occurred over approximately 7 million years.

Identification

The Thermopolis Shale was first identified in 1914 by geologist Ferdinand F. Hintze, Jr. He called it the "Lower Benton Shale", and included the Mowry Shale in the same formation. Hintze described three members: The basal "rusty beds", a lower shale, a thick "Muddy Sand", and an upper shale member.
The Thermopolis Shale was first named by geologist Charles T. Lupton in 1916. Lupton described the rocks as a formation lying conformably atop the Cloverly Formation, and conformably underlying the Mowry Shale. The Thermopolis Shale was the basal of four formations making up the Colorado Group. He described the Thermopolis Shale as Late Cretaceous in age, generally dark in color, from thick, and with sandstone lenses common. At least one member of the Thermopolis Shale was also noted, a "muddy sand" layer about thick. No type locality was identified, but the formation was named for the town of Thermopolis in Hot Springs County, Wyoming—where, nearby, outcroppings of the shale were well exposed. Lupton's division of the Thermopolis Shale was adopted by the United States Geological Survey and used for the next 50 years.
The stratigraphic history of the Thermopolis Shale was first outlined by geologist Don L. Eicher in 1962.

About the formation

The Thermopolis Shale belongs to both the Colorado Group and Dakota Formation. Both historically and currently, the stratigraphic units in these groups, and in the Thermopolis Shale, have been unclear, and the nomenclature used by geologists is not standardized. The identification of beds, members, and formations and their names have changed over time as well.
The Thermopolis Shale is said by Eicher to overlie the Cloverly Formation, although Rice, Porter et al., and Lash that in Montana and Wyoming the Kootenai Formation is geologically equivalent to the Cloverly Formation and thus conclude that the Thermopolis Shale overlies the Kootenai Formation. There is disagreement as to the stratigraphic definition of the basal member of the Thermopolis Shale, however. Eicher has argued that the "rusty beds" division is clearly distinguishable in many ways from the Cloverly Formation, and thus belongs to the Thermopolis Shale. Seeland and Brauch assigned the "rusty beds" to the Cloverly Formation in 1975, an assessment concurred with by Finn in 2010. Porter et al., however, classified the "rusty beds" as part of the Fall River Sandstone in 1997.
What constitutes the upper boundary of the Thermopolis Shale is disputed, making it difficult to identify what overlays the Thermopolis Shale. In 1922, Collier identified the beds below the Mowry Shale as the Nefsy shale member of the Graneros Shale. This left the Thermopolis Shale underlying the Graneros Shale. But Rubey assigned these rocks to the Mowry Shale in 1931, so that now the Thermopolis Shale underlay the Mowry Shale. Eicher redefined these beds in 1960 as the Shell Creek Shale, separating them from the Mowry Shale. This effectively put the Thermopolis Shale below the Muddy Sandstone Formation. In 1998, Porter et al. identified the Shell Creek Shale as the upper member of the Thermopolis Shale, a position with which Lash agreed in 2011.
Depending on the definition of the shale, and the location, the Thermopolis Shale varies widely in thickness. Chester N. Darton estimated the size of the formation at in 1906. In 1914, Hintze described the formation as deep. Hewett and Lupton reported in 1917 that the shale to be thick in the Bighorn Basin, while Finn (not including the "rusty beds" reported a thickness in the same area of. On the Wind River Indian Reservation of Wyoming, using the inclusive definition, it was reported to be a more robust thick. It is only thick in the Shirley Basin of southwest central Wyoming.
Generally speaking, the Thermopolis Shale consists of a dark gray to black shale, with thin layers of bentonite, sandy claystone, and siltstone interspersed throughout the shale. Depending on the stratigraphic definition of the formation, a gray, thinly-bedded sandstone member exists between the upper and lower members.

Members

For the purposes of this article, the definition of the Thermopolis Shale used by Porter et al. and Lash will be used, recognizing that there is scientific disagreement about this issue. Using this definition, there are four members of the Thermopolis Shale:
Surface outcroppings of the Thermopolis Shale occur in central Canada, and the U.S. states of Montana and Wyoming. Marine-deposited rock thins toward the west, while nonmarine-deposited rock thins toward the east. The marine-deposited rock is primarily shale, with some limestone, sandstone, and siltstone. The nonmarine rock is primarily sandstone, with some coal, shale, "black" or carbonaceous shale, and siltstone.

Fossil record

The Thermopolis Shale is unusually rich in marine vertebrate fossils, consisting primarily of skeletal material, teeth, and coprolites.
A particularly rich marine vertebrate fossil zone exists in the lower beds of the Upper Thermopolis Member. Marine crocodile, plesiosaur, ray, sawfish, and turtle remains, as well as whole coprolites, are abundant. Hybodont shark, ganoid-scaled and teleost fish, and invertebrate fossil ammonoids are also found.