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Abstract: Tonsteins and Clay-Rich Layers in Coal-Bearing Intervals of the Eocene Gibbons Creek Member, Manning Formation, East-Central Texas

Leslie F. Ruppert, Peter D. Warwick

Samples from five clay-rich layers collected from the coal-bearing interval in the upper part of the Eocene Gibbons Creek member of the Manning Formation were mounted in epoxy, polished, and analyzed by scanning electron microscopy and energy-dispersive x-ray fluorescence to determine their origin. Two layers were from surface-mine exposures of the 3500 bed near Bryan, Texas, and the other layers were from an exposure of a correlative interval at the Lake Somerville Spillway about 60 km southwest of Bryan. Preliminary data suggest that both a 2-cm-thick claystone from the upper part of the 3500 bed and the upper part of an 11-cm-thick mudstone from the floor of the lower coal bed at the spillway may be derived from volcanic ash falls.

Both possible tonsteins are composed of kaolinite and accessory quartz, euhedral to subhedral zircon, feldspars, and A1 phosphates (crandallite?). Only K-feldspars were observed in the parting from the 3500 bed, whereas both alkali and plagioclase feldspars, as well as high-Ti biotites, were observed in the sample from the spillway. These compositional differences

are suggestive of two separate volcanic sources. Because waterborne, recycled volcanic minerals could have been brought into the paleopeat swamps, we must examine the associated minerals from these sites to verify ash-fall origins.

The other layers contain textures that are more suggestive of detrital rather than ash-fall origin; rounded to subrounded zircons and feldspars are present as accessory minerals.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994