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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 354

Last Page: 361

Title: Paleoenvironments of Mowry Shale (Lower Cretaceous), Western and Central Wyoming: GEOLOGIC NOTES

Author(s): Charles W. Byers (2), David W. Larson (3)

Abstract:

The Lower Cretaceous Mowry Shale of western and central Wyoming contains three distinct facies, each of which is characterized by a unique suite of physical and biogenic sedimentary structures: (1) laminated mudstone, (2) bioturbated mudstone, and (3) bioturbated sandstone. We interpret the facies to represent the following marine biotopes (Schafer's terminology): (1) lethal isostrate--quiet, anaerobic, >150 m deep; (2) vital isostrate--quiet, marginally aerobic, 15 to 150 m deep; and (3) vital heterostrate--agitated, fully aerobic, <15 m deep. Bentonite datum planes show that the boundaries between facies are time transgressive, and indicate a west-to-east progradation of the shoreline and lateral filling of the Mowry sea. An east-dipping paleoslope inclined at 0 76;13^prime has been estimated. The depositional topography of the Mowry basin as reconstructed from the facies pattern shows that the thickest accumulation of sediment occurred along the western margin where water was shallowest, whereas in the deeper water basin center a much thinner section was deposited. Thus, isopach patterns of the Mowry in no way reflect the original depositional topography.

The significance of this study for petroleum geology is threefold: (1) a facies sequence is used to reconstruct the depositional basin configuration of a known source rock; (2) the facies correlate with previously published regional variations in organic carbon concentration in the Mowry; and (3) the facies also correspond approximately to facies based on electrical resistivity.

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