DEDMON, RUSSELL G., and STEVEN L. DOROBEK, Department of Geology, Texas A&M University, College Station, TX; and TAD M. SMITH, and RICHARD E. McARTHUR, Amoco Production Company, Houston, TX

ABSTRACT: Sequence Stratigraphic Relationships of the Grayburg Formation, Midland Farms and Foster Fields, Central Basin Platform, West Texas

The Grayburg Formation in Midland Farms and Foster fields along the eastern margin of the Central Basin Platform is located stratigraphically between restricted peritidal deposits of the upper San Andres Formation and red siltstones and evaporites of the overlying Queen Formation. Two sequences can be identified within the Grayburg Formation; these consist of bundled, higher frequency parasequences and parasequence sets. Transgressive parasequences (5-7 m thick) are characterized by bioturbated peloid-fusulinid dolowackestone/packstone with a thin basal dolomudstone layer. Highstand parasequences (3-7 m thick) contain peloid/ooid/intraclast dolopackstone/grainstone that grade upward into peritidal facies.

Uppermost San Andres beds at Foster field are brecciated and have well-developed vadose diagenetic fabrics. The top of the San Andres Formation marks the sequence boundary beneath the Grayburg Formation. Another, more subtle sequence boundary occurs in the middle of the Grayburg Formation and can be picked by parasequence set stacking patterns. The lower Grayburg sequence is capped by peritidal parasequences at Midland Farms field and coarse-grained subtidal-dominated parasequences at Foster field. The base of the upper Grayburg sequence at Midland Farms field is marked by thick (6-10 m) ooid-peloidal dolograinstone/packstone. At Foster field, the base of the upper Grayburg sequence consists of muddy subtidal parasequences.

The upper Grayburg Formation also contains vadose diagenetic fabrics and peritidal facies that indicate proximity to the upper sequence boundary. However, the Grayburg-Queen transition is marked by several shallowing upward, subtidal to supratidal parasequences that can not be correlated between adjacent cored wells. This suggests parasequences formed thin, diachronous, shingled depositional packages that filled regional paleotopography on the Grayburg platform or low relief paleotopography formed by underlying parasequences. The upper Grayburg-lower Queen platform probably was very near base level during the transition from latest highstand to lowstand deposition. It may be more appropriate to describe a "sequence boundary interval" rather than pick a single surface as the sequence b undary between these units. This concept may be important for other fully aggraded platforms during the turn-around from late highstand to lowstand deposition, if the top of the platform stays near base level.

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