--> ABSTRACT: Depositional Environments and Sedimentary Tectonics of Subsurface Cotton Valley Group (Upper Jurassic), West-Central Mississippi, by Ben D. Sydboten Jr. and Richard L. Bowen; #91042 (2010)

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Depositional Environments and Sedimentary Tectonics of Subsurface Cotton Valley Group (Upper Jurassic), West-Central Mississippi

Ben D. Sydboten Jr., Richard L. Bowen

Study of data from 65 selected wells in a 6-county area (about 60 by 60 mi) north and west of Jackson, Mississippi, discloses that Cotton Valley strata, now within the axial trough of the Mississippi embayment, display thickness variations which demonstrate that Late Jurassic sedimentation was strongly controlled by maximum subsidence along the same trough axis. Examination of well logs, other records, and cutting sets from 38 wells has resulted in preparation of dip and strike cross sections that permit informal definition of lower, middle, and upper parts of the Cotton Valley Group throughout the area evaluated. Within these lithostratigraphic diversions, lithofacies are discriminable that represent alluvial, upper delta plain, lower delta plain, and prodeltaic environm nts. These facies display a general variation from coarse, commonly red, oxidized sediments on the north and east, to mudrocks, locally calcareous and carbonaceous, on the southwest. Within the Cotton Valley Group examined, two persistent clastic lobes demonstrate relative environmental stability while deposits ranging in thickness from 1,500 ft (northwestern corner of study area) to 4,500 ft (axial depocenter on the south) accumulated.

During Cotton Valley deposition, west-central Mississippi was the site of a two-toed birdfoot delta within which lignites were deposited. Major sediment supply was from the east and north; a minor source was to the northwest (Ouachita-Ozarks). Irregularities in both rates of supply of clastics and of shelf subsidence permitted intermittent shallow, clear-water, marine incursions from the south during which thin carbonate beds were deposited, interfingering with the clastics. Thus, potential source and host rocks for hydrocarbon traps are closely associated, for thick, organic-rich, interlobate mudrocks pass laterally and vertically into fluvial sands of the delta lobes.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.