Abstract: Major Volcanic Ash Units in the Late Eocene of East Texas

YANCEY, THOMAS, and RENALD GUILLEMETTE
Texas A&M University, College Station, TX

Volcanic ash layers produced by voluminous silicic pyroclastic volcanism occur at many sites in the Late Eocene of Texas, containing hydrated, but otherwise unaltered and noncorroded, bubble-wall glass shards. In the Brazos River Valley in east Texas, major and minor elemental composition as determined by electron microprobe can distinguish between ash units. FeO content for an ash unit in the lower Manning Fm. is 0.6% compared to 0.7% for an ash unit in the upper Manning Formation. and 0.9% for an ash at the top of the Eocene. Variability in composition was examined in the lower Manning ash unit (1.5 m of thickness and 3 km of outcrop) by analyzing 700 shards from 12 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 upper Manning ash unit. For most elements, shard populations from relatively inplace deposits have smaller scatter of values than those from more reworked ash.

The Brazos River Valley section contains two major volcanic ash units: a lower Manning layer dated at 34.93 Ma and an upper Manning layer dated at 34.4 Ma. The Late Eocene section of central Texas in Fayette and Gonzalez counties also contains two major silicic volcanic ash layers, suggesting the presence of two major, regionally extensive ash sheets in the Late Eocene, with thicknesses of 1-2 m. Major and minor element composition of a 1.2 m thick volcanic ash layer in Jasper County, easternmost Texas, compares most closely to the 34.93 Ma ash layer.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas