KAUFFMAN, ERLE G., and THOMAS VILLAMIL, Department of Geological Sciences, University of Colorado, Boulder, CO
ABSTRACT: New Depositional Models for Cretaceous Source Rocks
The Cretaceous marks one of the greatest periods of source rock development in geologic history, especially in coastal and epicontinental marine basins where the number, duration, and geographic extent of Corg-rich intervals exceeds that of oceanic basins. Large-scale factors regulating Cretaceous source rocks include sealevel, sedimentation rate/type, paleoclimate and marine thermal gradients, paleoceanography (circulation, stratification, chemistry, upwelling, nutrient supply), and surface water productivity. Marine depositional settings favored as models for Corg concentration include silled and tectonically depressed basins, intersection of OMZ's with shallow continental seas, coastal upwelling, highly stratified shallow seas, and oceanic anoxic events (OAE's). All of these settin s are thought to be characterized by stagnant, anoxic/highly dysoxic water masses above the sediment-water interface, and highly stressed benthic environments. This is seemingly supported by fine lamination, sparse bioturbation, high pyrite and Corg content of most source rocks. But high-resolution (cm-scale) sedimentologic, paleobiologic, and geochemical analyses of Jurassic-Cretaceous source rocks reveal instead dynamic benthic environments with active currents, episodically crowded with diverse life in event communities, and persistently characterized by longer term, low diversity resident benthic communities. These characteristics indicate rapidly fluctuating, predominantly dysoxic to oxic waters at and above the sediment-water interface for most Corg-rich black shales. A new model f r source rock generation is proposed which retains the redox boundary at or near the sediment-water interface over large areas of seafloor, in part aided by extensive development of benthic microbial mats which may contribute up to 30% of the Corg to marine source rocks.
AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.