--> Abstract: Outcrop Observation and Preliminary Correlation of Transgression-Regression Cycles of the Cisco Group (Virgilian and Wolfcampian), Colorado River and Brazos River Valley areas, North-Central Texas, by W. Yang, M. A. Kominz, and W. Dharmasamadhi; #90987 (1993).

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YANG, WAN, MICHELLE A. KOMINZ, and WIDYA DHARMASAMADHI, Department of Geological Sciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Outcrop Observation and Preliminary Correlation of Transgression-Regression Cycles of the Cisco Group (Virgilian and Wolfcampian), Colorado River and Brazos River Valley areas, North-Central Texas

A hierarchical pattern of cyclic deposition has been delineated by outcrop study of Cisco strata. The outcrop is approx. 350 m thick, composed of fluvial, deltaic, shelf, and slope siliciclastics and carbonates deposited on the Eastern Shelf of the Midland basin. Nearly 100 sections were measured along three W-E traverses in a 150x20 sq mi area. About 50 transgression-regression cycles were delineated based on lithology, fossil content, depositional environment, and stratigraphic position. Cycles are 3-14 m thick, and 7-8 m on average. Typically, transgression is represented by thin, marly mudstone andlimestone; maximum transgression by thin black shale; and regression by thick limestone and marine/nonmarine siliciclastics, capped by variegated paleosol and erosional surfaces. Complet cycles are rare. Often, black shale was not present; transgressive and regressive limestones coalesce, or are absent. Cycles composed entirely of siliciclastics or marine deposits were observed.

Preliminary cycle correlation between traverses indicates that major limestone markers are correlatable, so are many minor cycles. Cycle correlation has differentiated regional transgression-regression events from local ones, suggesting that cyclic deposition was controlled by autocyclic processes, such as supply change and local subsidence, as well as allocyclic processes. such as global sea-level change and shelf-wide subsidence. The average cycle periods are 195 to 120 ky according to the estimated time spans of the Cisco Group. They comprise a higher frequency component than depositional sequences of lowstand, transgressive, and highstand system tracts established by Brown et al (1990). The outcrop transgression-regression cycles will serve as a model for subsurface cycle delineat on and correlation of the Cisco strata on the Eastern Shelf.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.