--> Abstract: Climate-Controlled Dilution Cycles in the Demopolis Chalk (Campanian, Alabama): Evidence from Relationships among Carbonate and Organic Parameters, by J. D. Warren and C. E. Savrda; #90937 (1998).

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Abstract: Climate-Controlled Dilution Cycles in the Demopolis Chalk (Campanian, Alabama): Evidence from Relationships among Carbonate and Organic Parameters

WARREN, JEFFREY D., Phillips Petroleum Co.; and CHARLES E. SAVRDA, Auburn University

Summary

Dilution cycles and productivity cycles, which are typically driven by orbitally induced climate fluctuations, are often difficult to discriminate in shelf-sea pelagic carbonate rhythmite sequences. Analyses of the Upper Cretaceous (Campanian) Demopolis Chalk of the eastern U.S. Gulf coastal plain demonstrate that cycle discrimination can be facilitated using relationships between carbonate content and organic parameters. Organic carbon contents of whole-rock samples (Ocw) vary inversely with carbonate. However, organic carbon contents of decalcified samples (Ocd) vary proportionally with carbonate up to a threshold of 80-90% CaCO3. Above this threshold, Ocd varies inversely with carbonate. Similar, albeit more crude, relationships exist between carbonate and hydrogen indices for decalcified samples (HId). The range in dHI generally increases with increasing carbonate content but then drops significantly at a comparable carbonate threshold. Qualitative models that predict the effects of productivity and dilution cycles on overall sedimentation rates and on accumulation and preservation of organic matter can explain the positive correlations between CaCO3 and both Ocd and HId. However, only the dilution model can adequately explain the reversal in relationships observed above carbonate threshold levels. Moreover, the dilution model best explains relationships between carbonate content and calculated organic accumulation rates. These results: (1) indicate that Demopolis carbonate rhythms were caused by climate-mediated variations in runoff and sediment delivery to the Gulf margin; (2) demonstrate the importance of analyses of organic parameters on decalcified samples as well as whole-rock samples; and (3) raise questions regarding the efficacy of oversimplistic “three component system” models for distinguishing productivity and dilution signals.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah