Relationship Between Depositional Mechanism and Type of Organic Matter in Middle Cretaceous Mowry Shale, Wyoming

Hugh R. Davis, Lisa M. Pratt, Charles W. Byers

The Mowry Shale was deposited in the southern part of a marine embayment that extended from the Boreal ocean southward to central Colorado. In western Wyoming along the margin of the seaway, the Mowry is partly bioturbated and is composed primarily of repetitive centimeter-scale event strata. A typical event stratum in the Mowry fines upward and consists, in ascending order, of (1) scoured base, (2) silt layer, (3) alternating silt and mud laminae, and (4) homogeneous mud. These strata are interpreted to result from waning depositional currents, either turbidity or storm flows. In central Wyoming, the central basin facies of the Mowry is hemipelagic, and contains less silt and bioturbation and fewer event strata.

Shale from the marginal facies contains 1-2 wt. % organic carbon and is characterized by oxygen-rich (oxygen index = 50-200 mg/g) and hydrogen-poor (hydrogen index < 50 mg/g) organic matter primarily of terrestrial origin. Shales from the central basin facies contain 2-4 wt. % organic carbon and are characterized by oxygen-poor (oxygen index < 50 mg/g) and moderately hydrogen-rich (hydrogen index = 150-400 mg/g) organic matter primarily of marine origin.

Basin margins were periodically swept by currents that deposited fine clastics and terrestrial organic matter. Sediment-transporting currents rarely reached the central basin, where hemipelagic deposition in poorly oxygenated environments favored preservation of marine organic matter with a good potential for petroleum generation.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.