Experimental rain prints and gas escape structures as a framework for interpreting circular imprints in shales of the Pottsville Formation (Pennsylvanian, Alabama)

Abstract

AbstractCoal‐bearing, fine‐grained clastic rocks of the Pennsylvanian Pottsville Formation contain well‐preserved vertebrate and invertebrate trace fossils and disarticulated plant fossils. These fossil‐bearing rocks also contain small, circular imprints that resemble raindrop impressions but have also been interpreted as structures formed by gas bubble escape from wet sediment. Because rain prints are associated with arid, subaerially exposed depositional conditions and gas escape structures with water‐saturated, organic rich sediments, the interpretation of the circular structures profoundly impacts the palaeoenvironmental reconstruction and interpretation of the life habits of the trace‐makers. In this project, rain prints and gas escape structures were produced to compare these two hypotheses regarding mode of formation and to gain insight into interpretations of similar circular pits in the geological record. The results indicate that gas escape structures form most readily in fine‐grained sediment under water‐saturated conditions, in contrast to rain prints, which form most readily in unsaturated sand. A comparison with experimental structures suggests that the Pottsville shales preserve gas escape structures, not raindrop impressions, suggesting that the environment was rarely desiccated. Because rain prints and gas escape structures are superficially similar, genetic interpretations should not be based on these similarities, but rather on the differences observed between them. Reexamination of circular pits in the sedimentary rock record might require adjustment of environmental interpretations.