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Abstract

DOI: 10.1306/05111817310

Dip-related changes in stratigraphic architecture and associated sedimentological and geochemical variability in the Upper Cretaceous Eagle Ford Group in south Texas

Ahmed Alnahwi,1 Robert G. Loucks,2 Stephen C. Ruppel,3 Robert W. Scott,4 and Nicolas Tribovillard5

1John A. and Catherine G. Jackson School of Geosciences, The University of Texas at Austin, 23 San Jacinto Boulevard & E 23rd Street, Austin, Texas 78712; present address: Saudi Aramco, Emerging Unconventional Assets Department, Southern Unconventional Resource and Exploration and Appraisal Division, 31311 Dharan, Saudi Arabia; [email protected]; [email protected]
2Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, 10100 Bureau Road, Building 130, Austin, Texas 78713-8924; [email protected]
3Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, 10100 Bureau Road, Building 130, Austin, Texas 78713-8924; [email protected]
4Precision Stratigraphy Associates, 149 West Ridge Road, Cleveland, Oklahoma 74020; [email protected]
5Laboratoire Géosystèmes, The French National Center for Scientific Research 3298, Université Lille 1, Bâtiment SN5, 59655 Villeneuve d’Ascq cedex, France; [email protected]

ABSTRACT

A detailed, rock-based investigation of three Upper Cretaceous Eagle Ford Group cores situated behind, at, and downdip of the Lower Cretaceous Stuart City paleoreef-shelf margin in south Texas was conducted to understand stratigraphic, sedimentological, and geochemical relationships across this buried shelf margin. An understanding of how the Eagle Ford Group lithofacies vary across the paleoreef-shelf margin is currently lacking. We therefore examined a dip section of three cores across the antecedent shelf margin and delineated seven Eagle Ford lithofacies: (1) massive argillaceous mudstone, (2) massive to laminated foraminiferal lime wackestone, (3) radiolarian and foraminiferal dolomitic to lime packstone, (4) massive to bioturbated skeletal lime wackestone, (5) laminated foraminiferal lime packstone, (6) laminated inoceramid and foraminiferal lime grainstone, and (7) massive to bioturbated claystone. A basinward decrease in calcite from 60% to 48% is accompanied by an increase in clay minerals from 12% to 20%. The low-relief raised rim of the older, buried Stuart City paleoshelf margin may have acted as a barrier, dividing the Eagle Ford Group into two sedimentological systems: (1) a restricted drowned shelf to the north, and (2) an open-marine basinal setting to the south. The lower to upper Cenomanian Eagle Ford strata on the drowned shelf are cyclic and enriched in molybdenum, suggesting anoxic to euxinic water masses. The anoxic, open-marine, basinward strata are less cyclical and have a lower molybdenum (compared with the drowned shelf) content. Ash beds and gravity-flow deposits are rare south of the margin. A depositional model was constructed of the lower and upper Eagle Ford formations.

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