Oolite-Bar Progradation, San Andres Formation, Midland Basin, Texas

Abstract

The Upper Permian San Andres Formation is a widespread carbonate unit which is a prolific oil producer throughout much of the Permian basin of West Texas and New Mexico. In most places the San Andres is a progradational sequence consisting, from the base upward, of (1) shallow basinal limestones and sandstones transitionally overlain by (2) shallow-shelf skeletal micrites, (3) oolite or coated-grain calcarenite-bar deposits, (4) lagoonal dolomitized pelletal lime mudstones and interbedded sandstones, and finally (5) intertidal algal-flat and supratidal salt-flat (sabkha) sedimentary rocks. This sequence is present in the central part of the Midland basin and represents the close of normal-marine sedimentation in the basin. The San Andres prograded across the Midland basin as a series of oolite bars and associated intertidal and supratidal facies. Paleogeographic maps utilizing a regionally extensive bentonite as a datum show how the oolite bar migrated across the basin and demonstrate the diachronous nature of the facies. Facies patterns produced by the rapid progradation in the comparatively shallow San Andres marine environment reflect deep structure and can provide a useful exploration tool. It is suggested that the controversy of the Leonardian versus Guadalupian age of the San Andres might be resolved by recognizing its progradational sedimentary history. Thus, the oldest San Andres may be Leonardian in age at its type locality in central New Mexico and in correlative outcropping beds on the Eastern shelf, but by the time the San Andres had prograded into the Midland basin it had transgressed sufficient geologic time to be of Guadalupian age.