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AAPG Bulletin


Depositional Environments, Organic Carbon Accumulation, and Solar-Forcing Cyclicity in Smackover Formation Lime Mudstones, Northern Gulf Coast1

Ezat Heydari,2 William J. Wade,3 and Laurie C. Anderson4


Upper Jurassic Smackover lime mudstones-a major hydrocarbon source rock in the U.S. Gulf Coast-show three distinct lithofacies. The basal laminated lithofacies consists of organic- and 
siliciclastic-rich lamina-sets that alternate with organic-poor, clean carbonate lamina-sets. Both lamina-sets are composed of lamina couplets interpreted to be varves. This lithofacies lacks current-induced structures, allochems, and rip-up clasts. Benthic and planktonic fossils are absent. One organic-rich lamina-set and one organic-poor lamina-set comprise one cycle (0.5-3 cm thick). The middle thin-bedded lithofacies is composed of organic- and siliciclastic-rich carbonate beds that alternate with organic-poor, clean carbonate beds. Both bed types are internally laminated. One organic-rich bed and one organic-poor bed comprise one cycle 
(2-60 cm thick). The upper burrowed lithofacies is extensively bioturbated, massive, shows no evidence of cyclicity, and contains little organic carbon.

Sedimentologic characteristics suggest that the basal laminated lithofacies was deposited under anoxic conditions, below storm wave base in a basinal environment. The middle thin-bedded lithofacies is 

©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.

1Manuscript received March 5, 1996; revised manuscript received August 2, 1996; final acceptance December 10, 1996.

2Basin Research Institute, Louisiana State University, Baton Rouge, Louisiana 70803.

3LSS International, 1440 Lake Front Circle, Suite 180, The Woodlands, Texas 77380. Formerly with Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803.

4Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803.

The study was supported by grants from the Center for Energy Studies and the Basin Research Institute of Louisiana State University. We thank 
H. D. Klemme and G. T. Moore for providing unpublished data. P. Meyers and L. M. Pratt reviewed an earlier version of the manuscript and offered valuable comments. J. Wrenn kindly provided characteristics of organic matter. We thank J. Kropog, W. S. LeBlanc, K. Lyle, P. O'Neill, and 
B. Simpson for technical assistance. We appreciate the helpful comments of R. L. Folk, B. Kirkland, and C. H. Moore. Critical comments by K. T. Biddle, N. B. Harris, B. A. Prather, and P. D. Wagner significantly improved the quality of concepts presented.

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