ABSTRACT
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 interpreted to have been deposited under anoxia on a steepened outer ramp (“slope”) environment, based on its stratigraphic position above the laminated litho-facies, presence of storm deposits, and internally laminated beds. Complete bioturbation, scattered coated grains, and stratigraphic position below an ooid grain-stone indicate that the upper burrowed lithofacies was deposited in a mid-ramp environment.
Anoxia in the basin and slope resulted from stratification of the water column into an anoxic, hypersaline bottom layer separated by a pycnocline from an oxygenated surface layer. Because bottom waters were always sufficiently anoxic to preserve organic matter, the high concentration of organic matter found only in siliciclastic-rich layers indicates that organic matter accumulation was related to periods of high productivity (algal blooms) triggered by warm and wet climatic conditions and increased nutrient supply from the ancestral Mississippi River. Cool temperatures and high salinities ended the productivity cycles and resulted in mass killings of algae and deposition of layers (a few millimeters thick) with unusually high organic carbon contents near the tops of organic-rich beds.
Cycles in the basal laminated lithofacies contain an average of 12 or 24 varves, corresponding to sunspot and Hale solar cycles, respectively. Most cycles in the thin-bedded lithofacies are 5-15 cm thick, corresponding to a cycle duration of about 80-240 yr (accumulation rate of 60 cm/k.y.). Cycles in the laminated and thin-bedded mudstones formed by fluctuating riverine-derived siliciclastic influx and organic productivity, both controlled by solar forcing of flood-drought climatic variations.
