Abstract: Controls on Accumulation of Organic Matter in Western Interior Sea: Evidence from Late Cenomanian Hartland Shale Member

B. B. Sageman, M. A. Arthur

Although depositional models for Phanerozoic epicontinental black shales around the world vary in the roles attributed to primary production vs. anoxia in the process of organic carbon accumulation, a common interpretation for most is stable density stratification of the water column. Recent findings have suggested the need to re-evaluate this assumption. In many modern marine environments organic matter accumulation occurs due to high production and concomitant oxygen deficiency, regardless of stable stratification. Paleoecologic studies have shown increasing evidence of repeated, if not frequent colonization of ancient organic carbon-rich muddy substrates, suggesting events of water column mixing and benthic oxygenation. And recently, preliminary results from a 3-D circulation model of the Cenomanian-Turonian Western Interior sea, an interval of significant global organic matter burial, have suggested that the basin may have been readily mixed by storms from the surface to the bottom.

One possible explanation for the accumulation of large volumes of organic matter in shallow epicontinental basins like the Western Interior is the idea of "dynamic stratification." In this model, water column mixing is repetitive and geologically frequent, but the re-establishment of oxygen-depleted (but not anoxic) bottom waters is rapid and widespread. The model depends on the balance of surface water primary production vs. mixing frequency of bottom waters, on sedimentation rates vs. organic matter flux to the sediment, and on the temperature and salinity characteristics of different water masses. The average state of bottom waters is dysoxic, and that of substrates is anoxic. Data from the Hartland Shale Member, Western Interior basin, will be used to argue the dynamic stratificat on hypothesis.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994