EXPERIMENTAL MODELING OF GAS-ESCAPE VS. RAINDROP ORIGIN OF CIRCULAR PITS IN SHALES OF THE POTTSVILLE FORMATION, STEVEN C. MINKIN PALEOZOIC FOOTPRINT SITE (PENNSYLVANIAN, ALABAMA)

The Steven C. Minkin Paleozoic Footprint Site, north of Birmingham, AL has recently been recognized as a Pennsylvanian lagerstätten. Here, abundant shales of the upper Pottsville Formation, lying approximately 1 to 6 m below the Newcastle coal seam, have yielded numerous tetrapod and invertebrate traces, as well as diverse compressed plant fossils. In addition, many beds contain abundant, mm- to cm-scale circular impressions. Initially considered raindrop impressions, a variety of field observations suggest that these impressions are better interpreted as gas escape structures (Rindsberg, 2005). First, the craters' rims show no sign of spattering common to raindrop impressions. Second, the circular pits occur on the same surfaces as undertracks. Where cross-sections are visible, pits are observed as only part of larger vertical structures that may penetrate several laminae. Furthermore, the impressions found here do not overlap one another and are not associated with mud cracks. Finally, curvilinear arrays of gas escape structures have been observed to follow tetrapod trackways, suggesting a genetic relationship between the two structures.

The aim of this study was to compare morphology and distribution of experimentally-produced gas escape structures and raindrop impression in a mud-silt mixture designed to simulate the sediment preserved in the Pottsville shales. Gas escape structures were generated by adding a sugar-yeast-water mixture to a water-saturated silty mud and allowing gas bubbles to be produced by the yeast. The production and abundance of gas escape structures was strongly dependent on the ratio of mud to silt. Raindrop impressions were difficult to produce in water-saturated sediment, and are better preserved in sediment that is dry at the surface, supporting the idea that raindrop impressions should co-occur with mud cracks. Preliminary results suggest that the morphology, distribution, and depositional setting of circular pits in the Pottsville shales are consistent gas-escape structures and unlikely to be raindrop impressions.

Reference: Rindsberg, A.K., 2005, Alabama Paleontology Society Monograph 1 (Buta et al., eds.), pp. 177-183.