A Late Cretaceous, submarine, igneous extrusion and an associated biocalcarenite beach facies occur in the subsurface of the North Poth area, Wilson County, Texas. The volcano and reworked volcanic material built a framework upon which a well-formed carbonate beach developed in the Austin Formation.

The volcanic material was extruded on the shallow sea floor. Prevailing wave energy from the northeast reworked the volcanic materials that were deposited as a fringing platform off the flanks of the extrusion. West of the volcano and stratigraphically above the reworked volcanics is a 35-ft (11 m) bioclastic calcarenite. It was deposited as a barchanoid-shaped beach which lapped on the accumulated reworked volcanics.

The maximum thickness of the Austin Formation in the vicinity of the North Poth volcano is 300 ft (92 m). The Austin Formation thins rapidly toward the volcano; there it is less than 20-ft (6 m) thick near the crest. A considerable amount of draping around the volcano is indicated by the structure of the Austin Formation. Much of the draping is due to differential compaction.

The North Poth, Northwest Poth, East Poth, and the Helen K. Fields appear to be related to this igneous extrusion. Oil has been produced from the Buda and Austin Formations and from the biocalcarenite facies of the Austin Formation. Occurrence of oil appears to be related to porosity in the reworked volcanics and beach complex and to fractures in the Austin Formation. The fractures probably are associated with differential compaction of sediments over the volcano.

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