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An Early Permian Coastal Flora from the Central Basin Platform of Gaines Couniy, West Texas

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An Early Permian Coastal Flora from the Central Basin Platform of Gaines Couniy, West Texas 524 An Early Permian Coastal Flora from the Central Basin Platform of Gaines Couniy, West Texas WILLIAM A. DiMICHELE and DAN S. CHANEY Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560 WILLIAM H. DIXON 345 Fritz Zwicke Road, Seguin, TX 78155 W.JOHN NELSON Illinois State Geological Survey, Champaign, IL 61820 ROBERT W. HOOK Texas Memorial Museum, The University of Texas at Austin, Austin, TX 78758 PALMOS, 2000, V. 15, p. 524-534 INTRODUCTION The record of Early Permian plant fossils is based large- Fossil plants are abundant in dolomite of the Clear Fork ly upon collections gathered from alluvial to coastal plain Group (Leonardian; late Early Permian) in the North Rob- deposits. On occasion, subsurface samples from the oil and ertson Unit of the Robertson Clear Fork oil field of Gaines gas industry become available and provide data that en- County, West Texas. The unit is a portion of the larger Rob- rich and extend our understanding of the surface record. ertson field. Thirty-seven separate collections were made In this paper, such an example is reported based upon from drill cores taken in three wells, spanning an interval cores recovered from the deep subsurface of the Central 300 m thick from a depth below the surface of > 2000 m. Basin Platform in Gaines County, West Texas (Fig. 1). The flora is depauperate, comprised of Comia sp. (most The Early Permian was a time of transition from the abundant), Taeniopteris sp., Delnortea abbottiae, callipter- stereotypical Carboniferous flora of everwet climates to id-like foliage of uncertain affinity, catamite stems, and un- seasonally dry floras, which contained many of the ele- identified foliage of cycad-like character. Plants occur in ments that would characterize Late Permian and early discrete intervals that are a few cm to 3 m thick and have Mesozoic vegetation (Frederiksen, 1972; Broutin et al., limited lateral continuity (less then 0.5 km). Plant-bearing 1990; DiMichele and Aronson, 1992). The everwet flora dolomites contain interspersed fossil roots and burrows, was dominated in its later phases by marattialean tree weakly developed soil horizons, thin coaly accumulations of ferns, pteridospermous seed plants, and lycopsids. The organic material, and pedogenic and collapse breccias. Fu- seasonally dry flora was dominated by seed plants, includ- sain is common throughout. Between the plant-bearing lay- ing callipterids, conifers, and a variety of other forms, ers are heavily bioturbated dolomites with anhydrite-filled some not yet linked clearly to established clades. Although burrows, interpreted as lagoonal to shallow subtidal in or- the place of origin of the seasonally dry flora is uncertain, igin, up to 50 m thick. Contacts between marine units and it is clear that it did not arise within, and then gradually plant-bearing beds are sharp. displace the everwet flora. The rise of the seasonally dry The North Robertson Unit and the field is believed to be flora in the tropics appears to have been a replacement internal to the Central Basin Platform and bordered by a driven by changing environmental conditions, especially a shallow seaway to the east. The Central Basin Platform change from year-round rainfall to seasonal rainfall, probably represented a broad peninsula between the Mid- spreading eastward across the tropics (Knoll, 1984; Phil- land Basin to the east and the Delaware Basin to the west. lips and Peppers, 1984). The site probably consisted of innumerable little islands The flora described here is clearly allied with others of separated by a network of waterways. Plants occur in "is- seasonally dry aspect from late Leonardian of the south- land complexes" consisting of deposits formed in ponded western United States. It seems not to be the remnants of water, beach fronts, and lagoons. Vegetation must have been an ancestral flora. Rather, it appears to be a coastal biofa- tolerant of salt or salt spray, seasonal drought, wind, and cies, an ecological variant of the broader, more diverse re- storms; all species were robust in construction, with either gional Early Permian biota. thick leaves or heavy cuticles. This flora, although clearly allied with other Early Permian floras from western North America, is sufficiently peculiar to indicate that habitat dif- GEOLOGY ferentiation was well developed within the tropical low- lands during the Late Paleozoic. Study Area The North Robertson unit (NRU) of the Robertson Clear Fork field is located in Gaines County, West Texas, near the border with New Mexico (Fig. 1). Oil is produced from Copyright © 2000, SEPM (Society for Sedimentary Geoiogy) 0883-1351 /00/0015-0000/$3.00 EARLY PERMIAN COASTAL FLORA 525 ->•<— 0.45 km- NRU #3533 NRU #207 1 & Mitchell County yM Glasscock County Tom Greer County FIGURE 1—Location of plant collections discussed in this paper. Dots = core collections; squares = surface collections. (1) North Robertson Unit cores, Gaines County (details of NRU core locations are shown FIGURE 2—Comparative stratigraphy and relative location of North in FIGURE 2). (2) Specimen reported in core by Adams (1933), Glas- Robertson Unit cores examined in this study. Depths are given in feet scock County. (3) Flora reported by Mamay et al. (1988), Brewster below the surface. County. (4) Flora collected in fall of 1999 by the authors, Tom Green County. (5) SE Westbrook field. low subtidal to intertidal platform, somewhat like the multiple pay zones within the Clear Fork Group at depths modern Bahama Banks (Pregger and Dixon, 1998). Mean- of 1,780 to 2,255 m below the surface. Oil is found within while, the Delaware and Midland Basins, intracratonic virtually the entire thickness (424 m) of the Clear Fork, basins hundreds of meters deep, progressively were filled yet the reservoir is highly "compartmentalized," meaning by carbonate and fine-grained siliciclastic sediments. that zones from which oil can be produced are thin and discontinuous. In an effort to discover how oil production Lithostratigraphy from this and similar fields might be enhanced, a multi- year study of NRU was conducted jointly by oil companies The Clear Fork Group originally was defined on the ba- and universities, with major funding from the U.S. De- sis of outcrops along the Clear Fork of the Brazos River partment of Energy. Part of this study entailed drilling north of Abilene, Texas, on the Eastern Shelf of the Mid- many continuous cores of the reservoir interval, each ap- land Basin (Cummins, 1890, 1891). In the North-Central proximately 10 cm in diameter and as long as 417 m Texas type area (Fig. 1), the Clear Fork is composed dom- (Montgomery, 1998; Montgomery and Dixon, 1998; Atch- inantly of red mudstone, with minor amounts of gray mud- ley et al., 1999; Pregger and Dixon, in press). The coring stone, deposited on coastal plains and mudflats, sand- program at NRU presented a rare opportunity to collect stones of fluvial-channel origin, and thin but laterally ex- fossil-plant specimens from the subsurface. tensive beds of intertidal to shallow-subtidal dolomite The NRU is situated near the eastern margin of the (Nelson et al., in press). Although correlative rocks on the Central Basin Platform, a broad tectonic uplift that sepa- Central Basin Platform are totally different lithologically, rates the Midland Basin, on the east, from the Delaware the name Clear Fork Group is traditionally applied in the Basin, on the west (Fig. 1). Platform and basins developed subsurface of West Texas. during mid- to late Carboniferous time as a consequence of The Clear Fork is more than 400 m thick at NRU (Fig. the Ancestral Rockies orogeny, which affected a vast area 2), and consists dominantly of dolomite that contains com- of the western and south-central United States. A complex mon nodules of anhydrite and occasional interbeds l-to-60 set of steeply dipping, basement-seated fault zones sepa- cm thick of gray-to-black dolomite. These dark gray to rate the Central Basin Platform from the basins. By Early black fissile rocks resemble shale in hand sample but thin Permian time, the region generally became tectonically sections indicate a dolomitic composition. Such organic in- stable, but the Central Basin Platform remained as a shal- tervals yield high, or "hot", readings on gamma-ray logs. 526 DiMICHELE ET AL and are characteristic of the plant-bearing facies. An in- al lower contact and sharp upper contact. The Tubb is re- terval of siltstone to very fine sandstone, 3-to-lO m thick gionally widespread around the margins of the Midland and a little over 300 m below the top of the Clear Fork, is Basin. This unit presumably reflects a progradational correlated with the widespread Tubb Sandstone Member. event, but little information has been published. The old- The Clear Fork is overlain by the Glorieta Formation, est plant fossil at NRU, a leaf of Cor(iaites sp., was recov- which averages 35 m thick, and consists of interbedded ered from the Tubb. siltstone, silty dolomite, and anhydrite. The Glorieta, in The remainder of the Clear Fork is close to 300 m thick, turn, is overlain by dolomite of the San Andres Formation. and consists of several varieties of dolomite that contain Carbonate rocks of the Abo or Wichita-Albany Group pre- scattered thin (1 to 50 cm) carbonaceous layers. Among sumably underlie the Clear Fork but no wells at NRU the of dolomite lithofacies are: reach this unit. (1) Lagoonal facies: heavily mottled and burrowed, drab gray-brown dolomite to wackestone containing abun- Age dant anhydrite nodules. This facies occupies the largest volume and forms the thickest intervals (commonly 3-to- Well-preserved fusulinids are abundant in limestone 12 m thick). near the base of the cored interval, 70-to-80 m below the (2) Reef facies: micritic dolomite having a framework of oldest fossil plants (Fig.
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