UNIQUE OLIGOCENE-MIOCENE CATAHOULA VOLCANIC ASH EXPOSURE AT RIO GRANDE CITY, TEXAS

A unique, 10-15 m thick, volcanic ash unit crops out at the eastern edge of Rio Grande City, Texas, 4 km north of the Rio Grande River in Starr County. The exceptional outcrop covers an area of about 3 km by 5 km. Steep slopes characterize the clean, white, ash, which is unconformably capped by a resistant, well-cemented coarse fluvial conglomerate of the Goliad Formation (Pliocene-Pleistocene age). Where the cap has been breached, the ash is eroded to its base. An outcrop of the conglomerate 3 km to the east, contains occasional round clasts of the ash up to 8 cm in size, 1-3 m above the base of the conglomerate.

Internally the ash unit is massive, it displays no sedimentary stratification or other structures. This leads to an interpretation of it being deposited as a single, large, air-fall deposition event. The ash rapidly thins to the north, is mixed with clastic sediment and recognized for more than 400 km to the north and northeast (Galloway, 1977).

The mineralogy is dominated by clay of the montmorillonite group which gives the ash the property of making indurated clots when wet. Some of the quartz component is probably volcanic and rare sanidine crystals have been reported (W. Crook, pers. comm.); occasional grains of magnetite can be extracted with a magnet, but mafics and other “heavy minerals” are very rare. Secondary minerals include large amounts of calcite, and quartz in three forms: fine grained clear or white; brown, well crystallized, coarse aggregates filling vertical “fumarole” holes from 0.2-3 cm diameter; white filling widely spaced long joints which seem to be more frequent in the top of the ash unit. Much of the secondary quartz likely formed from silica released by the devitrification of the original volcanic glass fragments.

The age of this ash is poorly constrained in literature references. As a rapidly deposited volcanic unit, an accurate age would provide an excellent time-stratigraphic marker in southeast Texas stratigraphy.