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

Abstract

Geophysical survey techniques including electrical resistivity imaging and magnetics were utilized to study the late-Cretaceous submarine volcanic mound of St. Elmo railroad cut located in south Austin, TX. The St. Elmo site cut exposes a sequence of late Cretaceous volcaniclastic rocks in contact with carbonate rocks. Resistivity imaging results provided subsurface evidence that the St. Elmo submarine mound has high resistivity limestone blocks scattered randomly within the resistivity sections. These limestone blocks are interpreted to be erratic blocks of Austin Chalk, which were probably torn from the walls of the volcanic conduits (vents) and ejected. Magnetic anomalies correlate well with the resistivity anomalies, strengthening the interpretation of the resistivity data. Resistivity results from the residential site, not far from the St. Elmo volcanic mound, are interpreted as a conduit which is similar to the conduit anomalies observed on the St. Elmo railroad cut site.