Smackover and lower Buckner formations, South Texas : depositional systems on a Jurassic carbonate ramp

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Date

1981

Authors

Budd, David A.

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Publisher

University of Texas at Austin. Bureau of Economic Geology

Abstract

The Smackover and lower part of the Buckner Formations (Oxfordian) comprise a thick regressive sedimentary sequence deposited on a Jurassic carbonate ramp. Four major depositional systems are recognized: (1) basinal, (2) low-energy open shelf, (3) high-energy shoal, and (4) sabkha. Lithofacies boundaries within each system and between systems follow paleobathymetrical contours. High-energy grainstone facies were concentrated landward; muddy low-energy facies were deposited seaward.

Basinal facies are dominated by laminated carbonate mudstones, deposited from suspension, and irregularly laminated carbonate mudstones, the product of sediment reworking by oscillatory bottom currents. The outer-shelf facies is characterized by burrowed carbonate mudstones containing crustacean pellets and a pelagic fauna. The inner-shelf facies is composed of burrowed wackestones containing a benthic fauna. Burrowed oncolite and pellet packstones characterize the outer-shoal system, and crossbedded mixed-allochem, oolite-intraclast, and oolite grainstones compose the high-energy inner shoal system. Increased sorting and decreased grain size within these facies suggest increasing energy levels landward even within the shoal system.

The sabkha system consists of cyclic subtidal to supratidal facies. Subtidal units are represented by burrowed gastropod and pellet wackestones and oolite wackestones to grainstones, whereas the intertidal zone is characterized by cross-laminated sandstones and algal-laminated dolomite mudstones. The supratidal facies consists of anhydrite nodules intercalated with carbonate and terrigenous mud, and siliciclastic sand and silt.

Depositional systems of the Yucatan Shelf of the Gulf of Mexico may be an approximate analogue for those of the Smackover Formation. Smackover moderate-energy packstones may have originated in stabilized grainflats, whereas Smackover grainstones were deposited in mobile sand waves, shoals, spits, spillover lobes, beaches, and eolian dunes. Depositional environments of the Trucial Coast of the Persian Gulf are in part analogous to the Buckner sabkha system. Subtidal units of the lower part of the Buckner Formation were deposited in coastal lagoons, tidal-channel deltas, spits, and beaches. Intertidal facies include sediment deposited in sand flats and algal mat zones. Supratidal facies are characterized by anhydrite nodules.

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