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The AAPG/Datapages Combined Publications Database
CSPG Special Publications
Abstract
Regional Synthesis and Concepts
Depositional Environments of the Early Cretaceous Western Interior Seaway in Southwestern Montana and the Northern United States
Abstract
The Thermopolis, Muddy and Mowry Formations were deposited in association with a shallow sea which entered the northern part of the Western Interior during the Early Cretaceous Albian stage. These units were studied in the southern Madison and Gallatin Ranges of southwestern Montana, situated on the western side of the Early Cretaceous seaway. Depositional environments in this area are compared with those in other parts of the Early Cretaceous seaway.
The basal sandstone of the Thermopolis Formation was deposited in shoreface environments as the sea first advanced into the area. As the sea deepened, the overlying offshore black shale of the Thermopolis Formation was deposited. A fluvially dominated, shallow-water delta system developed in the area as the sea regressed, depositing sediments of the upper Thermopolis and Muddy Formations. Prodelta, delta-front, distributary and fluvial environments are represented. The Mowry Formation and the upper Muddy Sandstone in the northern part of the study area were deposited in coastal plain environments which received thick deposits of volcanic ash.
Two marine transgressions into the northern United States have been interpreted from studies of depositional environments in the central and eastern parts of the Early Cretaceous seaway. However, the second transgression is not evident in the sedimentary record in the area of this study in southwestern Montana where eastward progradation of marginal marine environments apparently occurred in spite of the second advance of the sea. As a result, although extensive deposits of marine-winnowed sandstones exist in the central and eastern parts of the seaway in the northern United States, deposits of this type are not present in the area of this study in the southern Madison and Gallatin Ranges on the western side of the seaway. In addition, the western side of the seaway received much thicker deposits of volcanic sediment than the eastern side, producing additional lithologic changes across the seaway.
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