ABSTRACT

Airfall volcanic ash beds in the Late Eocene Manning Formation, Brazos River Valley, Texas, contain large quantities of hydrated, but otherwise unaltered and non-corroded, rhyolitic glass shards, along with mica, sanidine, and quartz crystals. Enclosing deposits contain well preserved siliceous microfossils (diatoms, sponge spicules, phytoliths). Unaltered glass occurs only where volcanic ash was deposited in nonmarine, non-swamp environments: lacustrine, fluvial, and forested soil surface.

Composition of glass shards from the upper part and top of the Manning Formation was determined by broad-beam electron microprobe analysis. Glasses from four ash beds analyzed to date are very similar in major and minor element composition. The mean and standard deviation in weight% oxides for 398 shards are: 72.7% SiO₂ (Standard deviation 0.7%), 11.6% Al₂O₃ (0.2),5.7% K₂0 (0.5),2.4% Na₂O (0.3),0.7% FeO (0.2), 0.4% CaO (0.1), and 6.5% H₂O (by difference). Na and possibly K have likely been substantially leached from glass shards during hydration. MgO, MnO, TiO₂, P₂O₅, and SO₃ are all less than 0.2% (below detection limit). The youngest ash layer (top Manning) contains 0.9% FeO compared to 0.7% FeO for two ash layers in the middle Manning and 0.5% FeO for the oldest ash layer in the lower Manning. A possible source for these glasses would be the Sierra Madre Occidental in west Mexico. Age (34.9 and 34.4 Ma for two of the ash beds) and composition is dissimilar to most rhyolites in the Davis Mountains of west Texas.