ABSTRACT

Volcanic ash layers produced by voluminous silicic pyroclastic volcanism occur at many sites in the Late Eocene of Texas, containing hydrated, but non-corroded, bubble-wall glass shards. In the Brazos River Valley in east Texas, major and minor elemental composition as determined by electron microprobe can help distinguish between ash units. FeO content for major ash units in the lower Manning Formation (Claypits unit) and upper Manning Formation (Somerville unit) differ slightly, but there is overlap in values. An ash unit at the top of the Eocene contains the highest FeO values (0.9%). Variability in composition was examined in the Claypits ash unit (2 m of thickness and 1.5 km of lateral outcrop) by analyzing 700 shards from 10 locations. As expected in a larger sample set, the range of values of FeO content is greater than previously documented and shows some overlap with the Somerville ash unit. Glass shards with average analytical totals less than 92% provide unreliable elemental data and comparisons between samples are based on shards with totals of greater than 93%.

The Brazos River Valley Late Eocene section contains two major volcanic ash units: the Claypits ash unit dated at 34.93 Ma and the Somerville ash unit dated at 34.4 Ma. The Late Eocene section of central Texas in Fayette and Gonzales counties also contains two major silicic volcanic ash layers, indicating the presence of two major, regionally-extensive ash sheets in the Late Eocene, with average thicknesses of 1 to 2 m. Major and minor element composition volcanic ash layer in Fayette County (central Texas) and Jasper County (easternmost Texas) are presented for comparison with ash units in the Brazos Valley.