--> ABSTRACT: Facies Distribution within Upper Albian Carbonate Ramp Sequences defined by a time-significant correlative horizon, West Texas, by Laura C. Zahm; #91019 (1996)

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Facies Distribution within Upper Albian Carbonate Ramp Sequences defined by a time-significant correlative horizon, West Texas

Laura C. Zahm

This study documents the sequence stratigraphy of the upper Albian-lower Cenomanian(?) (Lower Cretaceous) Segovia Fm. (360 ft thick) along an inner ramp (Comanche Platform) to intrashelf basin (Ft. Stockton Embayment) transect in Crockett and Pecos Counties, West Texas. Undeformed, flat-lying ramp strata of the Segovia Fm. were described in roadcuts extending 40 mi along Interstate-10, between Ozona and Iraan, Texas. Correlations were aided by a laterally extensive clay horizon interpreted as an event bed. The Segovia Fm. can be subdivided into two sequences (~500 ky duration) on the basis of facies distributions.

Sequences consist of a transgressive systems tract (TST) and an overlying highstand systems tract (HST) which together comprise an upward-shallowing succession from below fair weather base to above fair weather wave base. Toucasid mounds, fine peloidal-skeletal wackestones to packstones, isotropically burrowed-argillaceous wackestones, and oyster (Gryphaea sp.) wackestones, stack vertically to define meter-scale, outer ramp, retrogradational cycles within the TST. Cycles of the TST are interpreted to reflect higher accommodation and deeper water depths. The HST is defined by current-stratified, Caprinid grainstone that averages 30 ft thick and extends 40 mi laterally in a down dip direction. Grain size, sorting, and stratification types within the shoal complex define inner r mp, ramp crest, and outer ramp facies tracts. The ramp crest grainstone of the HST contains large, in situ caprinid that form mounds and represents the shallowest water depths. These facies reflect deposition within fair weather wave base which in turn reflects low accommodation and progradation of the ramp margin. The sequence stacking patterns of the Segovia Fm. allow a detailed understanding of ramp margin trend, facies distribution along a time-synchronous surface, and an understanding of surface and subsurface facies distributions in terms of facies continuity, geometry, and thickness.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California