Comparison of western facies of Thermopolis, Muddy, and Mowry formations with other areas of the early Cretaceous Seaway, northern Rocky Mountains and Great Plains region
The Thermopolis, Muddy, and Mowry formations were deposited in environments associated with an Early Cretaceous sea in the area of the modern Northern Rocky Mountains and Great Plains. Facies of the Thermopolis, Muddy, and Mowry formations on the western side of the seaway are compared with other parts of the seaway through compilation of regional paleogeographic maps for five Early Cretaceous episodes. These comparisons show that significant differences in sediment source, amount of sediment input, and tectonic setting existed from one side of the seaway to the other. (1) The lower, informally designated rusty beds member of the Thermopolis Shale is more calcareous on the western side of the seaway. (2) The Thermopolis Shale was subaerially exposed on the eastern side of the seaway and is separated from overlying Muddy Sandstone by an unconformity. (3) An unconformity separates lower, regressive Muddy Sandstone deposits from overlying transgressive Muddy Sandstone deposits in the central and eastern parts of the seaway. (4) The transgressing sea reworked lower Muddy Sandstone into extensive winnowed bar deposits in the eastern and central parts of the seaway. These excellent oil and gas reservoirs apparently are not present on the western side of the seaway. (5) The Mowry Shale was deposited in offshore marine environments in most of the Northern Rocky Mountains and Great Plains region. Siliceous claystone is the dominant lithology of the Mowry Shale deposited in marine environments on the western side of the seaway. Organic carbon content is higher to the east, increasing the petroleum source potential of the Mowry Shale on the eastern side of the seaway.
- Research Organization:
- Montana Bureau of Mine Geology, Butte
- OSTI ID:
- 6432741
- Report Number(s):
- CONF-8309274-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Vol. 67:8; ISSN AAPGB
- Country of Publication:
- United States
- Language:
- English
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Geology
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COMPARATIVE EVALUATIONS
CRETACEOUS PERIOD
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
GREAT PLAINS
LITHOLOGY
MESOZOIC ERA
MINERAL RESOURCES
NORTH AMERICA
PETROLEUM DEPOSITS
RESOURCE POTENTIAL
RESOURCES
SEDIMENTATION
SEDIMENTS
USA