ABSTRACT: Processes Controlling the Deposition of the Sharon Springs Member of the Pierre Shale During the Early Campanian

T. Loutit, A. E. Bence, F. B. Zelt

Total organic carbon concentrations (up to 13 wt.%) for the Sharon Springs Member of the Pierre Shale are among the highest for source rocks deposited in the Cretaceous Interior Seaway. The organic characteristics and distribution of the Sharon Springs are controlled primarily by interactions of eustasy and subsidence and to lesser extents by climate and oceanography. First-order control on organic-rich rock deposition was provided by the formation of a semirestricted epeiric sea resulting from the development of a foreland basin toward the end of the Early Cretaceous. Second-order controls are related to the interaction of second- and third-order eustatic rises with major subsidence events produced by compressional tectonics to the west. Each of the Cretaceous organic-rich rock units in the Interior Seaway was deposited during a period of increasing rate of relative sea level rise.

An increase in subsidence rates in the early Campanian coincided with a eustatic rise during the ZC4.1 cycle (80 Ma). The resulting Claggett transgression significantly increased water depths across the basin, reduced its supply of terrigenous material and produced a widespread condensed section. Water depths in the deepest parts of the basin reached at least 180 m, which was sufficient to allow stratification and hence preservation of organic matter type. Increased supply of nutrients during the transgression and expansion of warm subtropical waters to the north also controlled the distribution and characteristics of the Sharon Springs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990