Tonsteins and clay-rich layers in coal-bearing intervals of the Eocene Manning formation, east-central Texas

Six samples from clay-rich intervals in the coal-bearing upper part of the Eocene Manning Formation were analyzed by scanning-electron microscopy and energy-dispersive X-ray fluorescence to determine the origin of minerals in the samples. Two samples were from surface-mine exposures of the 3500 coal bed near Bryan, Texas, and the remaining samples were from an exposure of a correlative interval at the Lake Somerville spillway about 60 km (37 mi) southwest of Bryan. Preliminary data suggest that both a 2-cm-thick (0.75-in) claystone from the upper part of the 3500 bed and the upper part of an 11-cm-thick (4.25-in) mudstone from the floor of the lower coal bed at the spillway were derived from volcanic ash falls.

Both clay layers identified as possible tonsteins are composed of kaolinite and accessory quartz, euhedral to subhedral zircon, feldspars, and Ca-Al phosphates (crandallite?). Both alkali and plagioclase feldspars are observed in the two samples, but K-feldspar predominates in the upper clay layer of the 3500 bed, and plagioclase, with accessory Ti-bearing biotite, predominates in the sample from the floor of the spillway. These compositional differences suggest two separate volcanic ash falls.

The other sampled clay layers contain rounded to subrounded zircons and feldspars in a mixed-layer clay groundmass, which suggests detrital rather than ash-fall origins.