ABSTRACT: Controls of Platform Development in the Leonard Series (Middle Permian) of West Texas: Significance of Multifrequency Cyclicity and Paleotopography

RUPPEL, STEPHEN C., Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

The Leonard Series of West Texas comprises a thick (~800 m) sequence of cyclic, shallow-water carbonates that accumulated on a broad (~9000 sq. kilometers), flat to gently sloping platform. Detailed local and subregional study of this sequence illustrates that depositional facies architecture and patterns of diagenesis are the result of (1) frequent changes in relative sea level and (2) variations in depositional topography.

Vertical facies stacking patterns in the Leonard define three orders of cyclicity. Thicknesses average 1 to 2 m for high-frequency (fifth-order) cycles, 15 to 20 m for intermediate (fourth-order) cycles, and 200 m for long-duration (third-order) cycles. Facies stacking patterns within cycle families vary as a function of longer duration (lower order) cycles. Fifth-order cycles comprise (1) asymmetrical, progradational, upward-shallowing facies stacks in the upper parts of fourth-order cycles, (2) symmetrical, aggradational, upward-deepening stacks in the middle, and (3) asymmetrical, retrogradational, upward-shallowing stacks in the lower parts of fourth-order cycles. Fourth-order cycles exhibit relationships similar to third-order cycles. Stacking patterns are also modified by topogr phy; retrogradational and aggradational stacks are more poorly developed over paleo-highs, whereas paleo-lows may exhibit poor development of progradational stacks.

Patterns of diagenesis in the Leonard are also controlled by relative sea level rise/fall cycles. Leaching profiles, which cross-cut facies, are developed below progradational and retrogradational cycle tops by exposure and meteoric diagenesis. Leaching is most pronounced at the top of fourth-order cycles deposited, respectively, on lower and upper legs of third-order cycles. Diagenesis is most pervasive along paleotopographic highs where exposure duration and head were greatest.

Recognition of the interplay of cyclic fluctuations in relative sea level and paleotopography and their impact on patterns of depositional facies and diagenesis is critical to developing accurate models of platform evolution and also provides key insights into controls of reservoir heterogeneity in the Leonard Series, a major hydrocarbon-bearing sequence in West Texas.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)