Abstract

Fossil plants are abundant in dolomite of the Clear Fork Group (Leonardian; late Early Permian) in the North Robertson Unit of the Robertson Clear Fork oil field of Gaines County, West Texas. The unit is a portion of the larger Robertson field. Thirty-seven separate collections were made from drill cores taken in three wells, spanning an interval 300 m thick from a depth below the surface of > 2000 m. The flora is depauperate, comprised ofComiasp. (most abundant),Taeniopterissp.,Delnortea abbottiae, callipterid-like foliage of uncertain affinity, calamite stems, and unidentified foliage of cycad-like character. Plants occur in discrete intervals that are a few cm to 3 m thick and have limited lateral continuity (less then 0.5 km). Plant-bearing dolomites contain interspersed fossil roots and burrows, weakly developed soil horizons, thin coaly accumulations of organic material, and pedogenic and collapse breccias. Fusain is common throughout. Between the plant-bearing layers are heavily bioturbated dolomites with anhydrite-filled burrows, interpreted as lagoonal to shallow subtidal in origin, up to 50 m thick. Contacts between marine units and plant-bearing beds are sharp.

The North Robertson Unit and the field is believed to be internal to the Central Basin Platform and bordered by a shallow seaway to the east. The Central Basin Platform probably represented a broad peninsula between the Midland Basin to the east and the Delaware Basin to the west. The site probably consisted of innumerable little islands separated by a network of waterways. Plants occur in “island complexes” consisting of deposits formed in ponded water, beach fronts, and lagoons. Vegetation must have been tolerant of salt or salt spray, seasonal drought, wind, and storms; all species were robust in construction, with either 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 differentiation was well developed within the tropical lowlands during the Late Paleozoic.

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