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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. The sea advanced into the western interior from the north temporarily joining a northward-transgressing Gulf sea. A regressive period followed, and the southern margin of the sea retreated at least as far north as Wyoming. Studies of depositional environments in the central and eastern parts of the seaway indicate that a second Early Cretaceous transgression followed. However, evidence for the second transgression is not apparent in the study area (Madison and Gallatin Ranges of southwestern Montana). Eastward progradation of marginal marine environments continued on the western side of the seaway despite the sea's second advance. Fluvially dominated delta systems developed on the western side with only minor reworking by marine processes. The western side also received significant amounts of volcaniclastics producing additional lithologic and environmental differences across the seaway.
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. The following are some of the changes in the Thermopolis, Muddy, and Mowry formations which occur across the Early Cretaceous seaway as a result of the following differences. (1) The lower, informally designated "rusty beds member" of the Thermopolis Shale is more calcareous on the western side of the seaway. Paleozoic carbonates provided sediment from the west while siliciclastic sediment was shed in o the seaway from the east. (2) The Thermopolis Shale was subaerially exposed on the eastern side of the seaway is separated from overlying Muddy Sandstone by an unconformity. In contrast, the depositional sequence is continuous on the western side. (3) An unconformity separates lower, regressive Muddy Sandstone deposits from overlying transgressive Muddy Sandstone deposits in the central and eastern parts of the seaway. The Muddy Sandstone on the western side of the seaway is a continuous regressive deposit. (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 off hore marine environments in most of the Northern Rocky Mountains and Great Plains region. However, in most of southwestern Montana, the Mowry Shale and equivalents were deposited in nonmarine environments. (6) Siliceous claystone is the dominant lithology of the Mowry Shale deposited in marine environments on the western side of the seaway. Much of the sediment on the western side was derived from volcanic sources. The Mowry Shale is less siliceous to the east, probably because volcanic sediment decreases eastward. Organic carbon content is higher to the east, increasing the petroleum source potential of the Mowry Shale on the eastern side of the seaway.
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