Saribudak reported in 2019 that 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.
The author concluded that three resistivity profiles were interpreted as subsurface evidence that the St. Elmo submarine mound has high resistivity limestone blocks scattered randomly within the resistivity sections as deep as 35 meters. The distribution of these chaotic limestone blocks is more evident on the eastern resistivity profiles than the western resistivity profiles. These limestone blocks are interpreted to be erratic blocks of Austin Chalk, which were probably torn from the walls of the volcanic vent and ejected. Resistivity data from eastern profiles indicate three conduit-like anomalies, and are probably part of the eruption center in the study area. Magnetic data along one profile correlates well with the location of contact between the mound and the limestone unit. Magnetic data along two other profiles show a high magnetic anomaly that correlate with the approximate location of an inferred conduit. These high magnetic anomalies correlate well with the resistivity anomalies, strengthening the interpretation of the resistivity data.
Results of this study and others [Saribudak and Caran 2015; Saribudak 2016] showcase the effectiveness of integrated resistivity and magnetic imaging for mapping and characterizing volcanic mounds in detail, as well as constraining the lateral and vertical boundaries of volcanic mounds, and in this case, the adjacent Austin Chalk formation. These results also indicate that the combination of resistivity and magnetic data may provide valuable information in terms of delineating volcanic vents and dikes and defining the geological contacts of volcanic rocks in the Austin area and in the state of Texas. Additional resistivity surveys, in conjunction with magnetic surveys, could also offer useful information on the structure of volcanic plugs, which are potential oil and gas traps in the state of Texas. The procedures developed here may have applications in other areas with comparable geological conditions.
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