Cyclicity in the Middle Permian San Andres Formation, Palo Duro Basin, Texas Panhandle

The San Andres Formation in the Palo Duro Basin is a middle Permian carbonate-evaporite sequence situated between two red-bed units, the underlying Glorieta and the overlying undifferentiated Queen-Grayburg sequences. The San Andres Formation, deposited during relative structural quiescence in the region, is composed of cyclic sequences of dark anhydritic mudstone, skeletal limestone, dolomite, nodular anhydrite, bedded anhydrite, and halite. These cyclic vertical lithofacies sequences reflect sediment deposition by regressive, upward-increasing salinity cycles in a shallow platform setting. Thin interbeds of red siliciclastic mudstones and siltstones are present throughout the San Andres but become more abundant and thicker near the top of the formation. Textural evidence provided by core shows that the entire suite of cyclic facies reflects predominantly shallow-water deposition. Evidence of subaerial exposure is sparse within carbonate rocks, absent in anhydrite, but intermittently abundant in halite.Cycles can be traced across the entire Palo Duro Basin study area using geophysical log data. Much thinner sequences, on the order of tens of centimeters thick, also can be recognized using core data and correlated over distances of 118 km between cored wells. The extraordinary regional continuity of thin cyclic units, combined with textural evidence of their subaqueous origin, suggests that sedimentation occurred on an extremely broad, featureless shallow-water shelf characterized by nearly uniform depositional environments over large areas. The great width of the shelf combined with a shallow water depth restricted circulation, which caused development of a lateral salinity gradient soon after the initial, near-normal-marine transgressive phase of each cycle. This salinity gradient is reflected by the lateral equivalence of halite, anhydrite, and carbonate facies along a north-south trend. The dominant source of normal-marine water appears to have been the Midland Basin to the south, although a western normal-marine influence is indicated in some cycles. Vertical and lateral patterns of sedimentation in the San Andres Formation reflect changes in water circulation and resulting brine salinity, which are functions of the dynamic interplay between rates of eustatic sea-level change, regional basin subsidence, and sediment aggradation on a broad, low-slope, low-relief platform.Systematic vertical changes in the thickness and completeness of cycles allow division of the San Andres Formation in the Palo Duro Basin into three informal genetic sequences. Cycles of the lower San Andres sequence exhibit the complete ideal vertical facies sequence and have a relatively thick carbonate member. Cycles of the middle San Andres sequence contain relatively thin carbonate members and are incomplete because they lack halite members. Cycles of the upper San Andres sequence also have relatively thin carbonate members, but halite is present. The fundamental mode of cyclicity is the same in each of the genetic units, but the cyclic tempo is different. The temporal evolution of cyclic style reflects changes in regional basin subsidence rate or frequency of eustatic sea-level change or both.
Michael A. Fracasso
Susan D. Hovorka

Fracasso, M. A., and Hovorka, S. D., 1986, Cyclicity in the Middle Permian San Andres Formation, Palo Duro Basin, Texas Panhandle: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 156, 48 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation