15 Structural Geology, Geophysics, Tectonics, and Volcanology (Posters)

Tuesday, 22 March 2016
8:00 AM - 5:30 PM
Riverview B

PRECISE 27 MA U/PB ZIRCON AGES OF THE MASSIVE LATE OLIGOCENE CATAHOULA ASH EXPOSED AT RIO GRANDE CITY, TX

Paper number: 15-9
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

The 20 m thick exposures of the Catahoula Ash adjacent to the Rio Grande floodplain in Starr County, Texas have been interpreted to represent a single, massive, rhyodacitic volcanic ash depositional event. The age of the Catahoula ash has previously been estimated to be ~25-30 million years based on its stratigraphic position; it overlies dark unconsolidated clay of the Frio Formation and is unconformably capped by a resistant, well-cemented fluvial conglomerate of the Pliocene Goliad Formation. Here we present the first radiometric ages from this unique ash from a sample collected 4-5 m above the contact with the underlying Frio Formation at the “Tampico” location. Sixteen euhedral to subhedral zircons averaging ~30x100 microns were analyzed at the UCLA ion microprobe dating lab. The main cluster of 12 concordant grains have a weighted U/Pb age of 27.2 Ma, ± 0.4 Ma (2 sigma). Three older, inherited(?) grains have U/Pb ages of 36, 38 and 336 Ma; these older grains have significantly greater U content than the 12 concordant grains. Depth profile analyses on 9 zircons by laser ablation mass spectrometry at University of Houston yielded a U/Pb zircon rim age of 26.7 Ma, ± 0.5 Ma (2 sigma). These independent ages overlap and are consistent with several felsic volcanic sources in the Sierra Madre of Mexico, the closest being ~400 km distant. Determining the age of this rapidly deposited volcanic unit is important as it represents an excellent stratigraphic marker in southeast Texas.

Geological Society of America Abstracts with Programs. Vol. 48, No. 1

doi: 10.1130/abs/2016SC-273768

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