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-
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
1997. The American Association of Petroleum Geologists. All rights reserved.
received March 5, 1996; revised manuscript received August 2, 1996; final
acceptance December 10, 1996.
Research Institute, Louisiana State University, Baton Rouge, Louisiana
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.
of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana
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and G. T. Moore for providing unpublished data.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
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.