--> Abstract: Composition and Provenance of Volcanic Glass in Late Eocene Manning Formation, East-Central Texas, by R. N. Guillemette and T. E. Yancey; #90950 (1996).

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Abstract: Composition and Provenance of Volcanic Glass in Late Eocene Manning Formation, East-Central Texas

Renald N. Guillemette, Thomas E. Yancey

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 from the upper part and top of the Manning Formation was determined by broad-beam electron microprobe analysis. Glasses from three ash beds analyzed to date are very similar in major and minor element composition. The mean and standard deviation in weight % oxides for 215 shards are: 73.0% SiO2 (sd 0.7), 11.7% Al2O3 (0.2), 5.7% K2O (0.4), 2.0% Na2O (0.3), 0.8% FeO (0.2), 0.4% CaO (0.1), and 6.2% H2O (by difference). Na and possibly K are 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) differs from the others in having 0.9% FeO compar d to 0.6-0.7% FeO. A possible source for these glasses would be the Sierra Madre Occidental in west Mexico. Age (34.4 Ma for the 2 older ash beds) and composition is dissimilar to most rhyolites in the Davis Mountains of the Trans-Pecos.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas