Abstract

Recent development around Pilot Knob volcano in southeastern Austin included considerable excavations. Numerous studies and field trips since 1890 provided a better understanding of subsurface volcanic features commonly referred to as “serpentine plugs,” from which nearly 50 million barrels of oil have been produced. The excavated rocks have provided a rare and more extensive view of rocks typically in subsurface environments than was previously accessible, offering the opportunity to better understand the geology of these features. The discoveries were made while preparing for geology conference field trips in 2019 and 2021. Excavated rocks included basalts, breccias/peperites, agglomerates, conglomerates, tuffs, and contact-metamorphosed crystalline limestone. Subsequent research indicated that the diversity of rocks may be unique not only in Texas but worldwide and deserves further study. Research of some of the most diverse submarine-to-subaerial volcanic environments had very few photos, and rock varieties were more limited than at Pilot Knob. Previous studies of Pilot Knob classify it as a submarine to subaerial Surtseyan phreatomagmatic or hydroclastic eruptive volcano of the Upper Cretaceous Austin Chalk Group (∼80 Ma). The volcano began as submarine eruptions in a shallow sea of the ancestral Austin Chalk and is estimated to have been relatively near the ancient shoreline, eventually forming an island. These changing environments are hypothesized to explain the complex rock assemblage. The diversity of rocks indicates numerous micro-environments and mixing during explosive eruptions, some below and others above sea level. The photographic documentation and hypotheses of how these rocks were formed will hopefully provide a new level of understanding and comparison for these challenging geological environments, help in understanding rock variability and development of “serpentine plug” oil fields, and identify similar features elsewhere.