Inner structure of monogenetic Pilot Knob submarine volcano (Austin, Texas) revealed by electrical resistivity tomography and magnetic surveys

Abstract

Pilot Knob is the eroded and exposed core of an extinct subaqueous to emergent volcano that formed in a shallow Late Cretaceous epicontinental sea in the ancient Gulf of Mexico during the deposition of Austin Chalk Limestone. With the aim of understanding the inner structure of this volcano, we have conducted geophysical surveys (resistivity and magnetometry) to provide an insight into the past eruptive history. High-resolution electrical resistivity and magnetic data reveal, for the first time, a buried tuff mound, an explosion crater, several dikes and vents, basaltic flows, and limestone blocks within the volcano. A combination of geologic and geophysical data suggests that Pilot Knob volcano started with an explosion as a maar-type volcano beneath a shallow sea floor. Then, it likely has evolved into a tuff mound-type volcano with “Surtseyan” eruptions, before finally evolving into an emergent volcanic center. Major resistivity and magnetic anomalies align approximately in the east–west and north–south directions, respectively. Such alignments suggest that Pilot Knob is situated on east–west and north–south fractures, along which its most significant volcanic activity occurred. Results also provide valuable information in delineating tuff mounds and their structural characteristics.