Near-surface geophysical mapping of an Upper Cretaceous submarine volcanic vent in Austin, Texas, USA

Abstract

Geophysical surveys were conducted at the Williamson Creek site south of Austin, Texas, to determine the structural relation of the Upper Cretaceous volcanic rocks (lava and tuff) with the associated Austin Chalk limestone. At this site, resistivity and magnetic methods were performed over the exposed volcanic and limestone rocks. Geophysical results indicate an excellent correlation between high-magnetic and low-resistivity anomalies. The pseudo-3D resistivity data show a steeply dipping funnel-shaped vent formation over the high-magnetic anomaly (up to 3000 nT). Magnetic anomalies are consistent with a uniformly magnetized body, like a volcanic vent, and the anomaly's dimensions are consistent with eroded volcanic vents in other distributed volcanic fields in the United States. Magnetic data has been integrated with resistivity data and geologic observations and subjected to 2.5D forward potential-field modeling. Modeling has revealed a perfect fit with three magnetic zones: (1) the central part corresponds to the main magma feeder (vent); (2) the surrounding zone corresponds to undifferentiated interbedded tuffs and lavas; and (3) the low-magnetization zone. Geophysical results show that additional resistivity surveys, in conjunction with magnetic surveys, could offer useful information on the shallow volcanic plugs (serpentines), which are potential oil and gas traps in Texas, and their adjacent sedimentary rocks. The procedures developed here may have applications in other areas with comparable geologic conditions.

Subsurface structure of St. Elmo submarine volcanic mound (Austin, Texas) and its volcanic conduits imaged using geophysical methods

“…In this study, RMS values of six inverted resistivity sections range between 3 and 8, and the seventh profile has a RMS value of 11. These are acceptable results, being of the same magnitude of the values that characterize inversion of data collected in volcanic environments [Bruno et al 2007;Loke 2000;Xia et al 2010;Di Giuseppe et al 2017;Saribudak and Caran 2015;Saribudak 2016].…”

“…More geophysical results (2D and 3D resistivity and magnetics) over the known outcrops of tuff and lava at the Williamson Creek site revealed a volcanic vent [Saribudak 2016]. Results of these geophysical studies [Saribudak and Caran 2015;Saribudak 2016], for the first time, revealed the presence of volcanic features that included vents, dikes and craters. This study is the extension of those previous studies, with particular emphasis on the structures of tuff mounds.…”

“…The electrical resistivity method inolves imaging the bulk electrical resistivity distribution of the subsurface in two, three, or four dimensions (the three dimensions of space plus time when monitoring is performed) from the meter to hundreds of meter scale depending on the electrode spacing [Hermans et al 2014]. Resistivity has been well documented in the geophysical literature [Loke 2000;Dahlin and Loke 1998;Griffiths and Barker 1993;Bruno et al 2007;Saribudak 2016]. Resistivity values (Ω m) measured by the method are highly affected by several variables, including the presence of water or moisture, the amount and distribution of pore space in the material, and temperature [Rucker and Glaser 2015].…”

“…Near-surface geophysical imaging is used in a wide range of volcanic settings to understand the internal structure of volcanic centers and the nature of volcanic deposits [e.g. Courtland et al 2012;Barde-Cabusson et al 2013;Di Giuseppe et al 2017;Saribudak and Caran 2015;Saribudak 2016;Blaikie et al 2014;Di Giuseppe et al 2015;George et al 2015;Marshall et al 2015]. In this study, electrical resistivity imaging is used to resolve near-vent structures.…”

“…Structure of St. Elmo submarine volcanic mound Saribudak, 2019 Figure 1: Maps showing the Balcones Magmatic Province and the Balcones Fault Zone, and the study area (modified after Saribudak [2016]). The Balcones Magmatic Province is defined by the surface extent of Cretaceous igneous rocks.…”