--> ABSTRACT: Depositional Sequences in the Leonardian Portion of the Transpecos Supercycle, Glass Mountains, West Texas, by Charles A. Ross, June R. P. Ross; #91020 (1995).

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Depositional Sequences in the Leonardian Portion of the Transpecos Supercycle, Glass Mountains, West Texas

Charles A. Ross, June R. P. Ross

The Permian System is the middle part of the first-order Absaroka Sequence (the Absaroka II subsequence of Sloss, 1963, 1979). For this group of distinctive depositional sequences Ross and Ross (1987a, 1987b) applied the name Transpecos supercycle or Transpecos second-order depositional sequence based on its excellent development and exposures in West Texas and southern New Mexico. The Permian Transpecos supercycle starts in the Wolfcampian with third-order cycles that suggest relatively rapid, major sea-level rises and falls that are not unlike those of the Upper Carboniferous. By the Leonardian, the duration of the third-order depositional sequences appears to become longer and/or rate of carbonate production increases and the magnitude of sea-level fluctuations appears to diminish. These changes are progressive and had a major sedimentary impact that progressively changed ramp-like shelves to nearly flat-topped platforms with abrupt shelf margins and steep slopes into deep basins.

The lower Leonardian Hessian Stage was a time of small, repeated transgressive sequences associated with stable carbonate ecosystems producing abundant lime mudstone and wackestone on nearly flat platforms. The steep-sided platform margins had a great variety of biohermal and small reefal facies which resulted in large amounts of unstable penecontemporaneous carbonate debris that was washed off the platforms on to the slopes. Four Hessian third-order depositional cycles correspond to the lower four carbonate sequences in the Artinskian on the eastern Russian Platform.

The Cathedralian sequences are thin, clastic-rich shelfal carbonates that prograde basinward into thick, clastic wedges of shelf margin sequences. The Cathedralian includes equivalents of the Kungurian evaporites. The lowest ledge of the Road Canyon Formation, a biohermal, near-shore facies limestone, caps the Cathedralian basinal sequences. It is discontinuous on the shelf margin and upper slope facies in the central Glass Mountains and seems entirely missing in the eastern Glass Mountains. It typically has relatively primitive species of the fusulinacean Parafusulina and species of the ammonoid Perrinites, typical of the Leonardian. Its discontinuous outcrop locally has karst and diagenetic features suggesting a significant time of lower sea levels, weathering, and erosion prior to the deposition of the succeeding middle and upper ledges of the Road Canyon Formation. This major unconformity is overlain by the basal transgression of the Guadalupian.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995