Abstract

The Balcones Igneous Province (BIP) occurs along a 250 mile length of the south-central Texas Balcones Fault Zone and is characterized by ultra-mafic volcanics extruded onto a mature, passive continental margin. The igneous rocks, with both outcrop and subsurface occurrences, are interlayered with Upper Cretaceous sedimentary strata and commonly are reported to straddle the Austin Group–Taylor Group stratigraphic boundary.

Recent geochronology studies using U–Pb and ⁴⁰Ar/³⁹Ar isotopic dating methods report samples collected at BIP exposures in Uvalde, Kinney, and Travis counties yield Santonian to Early Campanian age ranges of 81.5–84.1 Ma for ultramafic rocks and 76.2–78.8 Ma for phonolites. A limited sample population of mineral separates was analyzed to achieve this narrow age range determination, which was applied to the entire BIP igneous event. No data from subsurface occurrences, which numerically dominate the BIP deposits, was utilized in the application of this age range to the BIP. Current geoscience publications on mantle-aesthenosphere research and crustal plate movements have incorporated this reported isotopic age as a confining date for emplacement of the entire BIP.

A recently published stratigraphic study by this author of subsurface BIP volcanics which employed detailed well log correlation indicates that while most BIP deposits do straddle the Austin-Taylor age boundary, some subsurface igneous deposits within the eastern part of the BIP occur at older stratigraphic intervals throughout the Austin Chalk Group and down to the Lower Eagle Ford (late Cenomanian). Presented here are examples of well log stratigraphic cross sections from Zavala County in the western BIP of subsurface volcanic mounds that occur at a stratigraphic interval younger than the dates determined by the isotopic dating of the crystalline surface exposures. The volcanic mounds at Spinach Field and at Bear Salt Field occur in the middle of the San Miguel Formation, while at TRM and De Joria gas fields the volcanics are age equivalent with strata in the upper Olmos Formation. A late Olmos equivalency designation places the volcanics stratigraphically immediately below the Taylor-Navarro contact as defined by the Bigfoot Unconformity, which has been dated as middle Maastrichtian.

Combining findings presented here with those from the recent similar stratigraphic study of the eastern BIP deposits, it can be documented that subsurface BIP volcanics occur over the interval Late Cenomanian to Early Maastrichtian, which equates to an absolute age for the BIP of 94 Ma up to 70 Ma. This revised age range for the BIP significantly expands the time frame for BIP emplacement and will have a direct implication on geologic interpretation of Late Cretaceous tectonic events in the Gulf Coast.