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Abstract: Recognition, Correlation, and Hierarchical Stacking Patterns of Cycles in the Ferry Lake-Upper Glen Rose, East lexas Basin: Implications for Grainstone Reservoir Distribution
W. M. Fitchen (1), D. G. Bebout (1), C. L. Hoffman (2)
Core descriptions and regional log correlation/interpretation of Ferry Lake-Upper Glen Rose strata in the East Texas Basin exhibit the uniformity of cyclicity in these shelf units. The cyclicity is defined by an upward decrease in shale content within each cycle accompanied by an upward increase in anhydrite (Ferry Lake) or carbonate (Upper Glen Rose). Core-to-log calibration of facies indicates that formation resistivity is inversely proportional to shale content and thus is a potential proxy for facies identification beyond core control. Cycles (delineated by resistivity log patterns) were correlated for 90 mi across the shelf; they show little change in log signature despite significant updip thinning due to the regional subsidence gradient.
The Ferry Lake-Upper Glen Rose interval is interpreted as a composite sequence composed of 13 high-frequency sequences (4 in the Ferry Lake and 9 in the Upper Glen Rose). High-frequency sequences contain approximately 20 (±5) cycles; in the Upper Glen Rose, successive cycles exhibit decreasing proportions of shale and increasing proportions of grain-rich carbonate. High-frequency sequences were terminated by terrigenous inundation, possibly preceded by subaerial exposure. Cycle and high-frequency sequence composition is interpreted to reflect composite, periodic(?) fluctuations in terrigenous dilution from nearby source areas. Grainstones typically occur (stratigraphically) within the upper cycles of high-frequency sequences, where terrigenous dilution and turbidity were least and potential for carbonate production and shoaling was greatest.
Published mid-Cretaceous geographic reconstructions and climate models suggest that precipitation and runoff in the area were controlled by the seasonal amplitude in solar insolation. In this model, orbital variations, combined with subsidence, hydrography, and bathymetry, were the primary controls on Ferry Lake-Upper Glen Rose facies architecture and stratigraphic development.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78713
(2) Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712
Copyright © 1999 by The Gulf Coast Association of Geological Societies