Abstract: Global and Regional Controls on Origin and Burial of Organic Matter in Devonian-Mississippian Black Shales of North America

Frank R. Ettensohn

Devonian-Mississippian black shales on North America attain typical thicknesses of 10 to 730m, covered more than 2.5 million sq km, contain organic matter in amounts varying from 0.5 to more than 23 weight percent and accumulated continuously in places for 35 million years. No single, simple explanation can account for the origin and preservation of so much organic matter. Instead, nature, age, and distribution of the shales suggest a complex interplay between timing, tectonics, paleoclimate, and paleogeography.

Like most major black-shale depositional events, these deposits formed during a time of global transgressive and greenhouse states that enhanced organic productivity and preservation. At this time, three areas of partly

coeval black-shale deposition were initiated in foreland basins related to Acadian, Ouachita, and Antler orogenies, suggesting the importance of foreland-basin subsidence in generating the initial, deeper water environments conducive to sediment starvation and water stratification. These environments migrated cratonward in time largely by flexural subsidence and yoking with the intracratonic Michigan, Illinois and Williston basins. Several other mechanisms for generating abundant organic matter, preventing clastic dilution and establishing water stratification can be linked to unique sets of paleoclimatic and paleogeographic factors.

Presence of so many mechanisms, functioning together or in reserve, reflects a unique coincidence of paleoclimatic, paleogeographic, and tectonic factors during deposition of Upper Devonian-Lower Mississippian black shales in North America. Hence, depositional models are necessarily complex, and modern uniformitarian models may be incapable of explaining such extensive, epicontinental accumulations of organic matter.

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