Geophysical and Isotopic Investigations of the Uvalde Intrusive Field, South Central Texas

Charles D. Blome, Daniel P. Miggins, and David V. Smith
U.S. Geological Survey, Denver Federal Center, Denver, Colorado

The volcanics and subvolcanic intrusives of south central Texas are centered in Uvalde County. They extend eastward to Bexar County. Ultramafic and hypabyssal rocks occur primarily as plugs and shallow intrusions. Dominant rock types are alkali basalt, melilite-olivine nephelinite, olivine nephelinite, nepheline basanite, and phonolite.

Preliminary ⁴⁰Ar/³⁹ Ar isotopic ages of a high level of confidence have been obtained for various mineral and groundmass separates from Uvalde County rock exposures. Fractions of K-feldspar were obtained from Interstate Hwy 90 exposures east of Uvalde and at Fort Inge. Plagioclase and groundmass concentrates were obtained from an intrusive exposure north of Cline. Resulting age spectra indicate two distinct phases of magmatic activity for the Uvalde intrusive field. The first was emplacement approximately 82-80 million years ago and a younger emplacement, phonolites, was approximately 74-72 million years ago. The ⁴⁰Ar/³⁹ Ar ages are more precise than previous K/Ar ages.

Three years of geophysics have successfully provided definition of the structures within the intrusive field. A high-resolution aeromagnetic survey revealed dozens of unmapped volcanic pipes and associated flows. In part, the data were interpreted using new semi-automated potential field software that generated 3 dimensional visualizations. The size and distribution of the igneous intrusive rocks in Uvalde probably reduces hydraulic transmissivity on a regional scale and influences ground-water flow and quality. The vesicles of the intrusive rocks are dominated with secondary mineralization, thereby decreasing the porosity and permeability in and around the intrusive field. Surface surveys employing gravity, magnetometry, and DC resistivity also delineate the structure of a prominent pipe west of Sabinal.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004