Facies and fauna of the Pennsylvanian Buckhorn Asphalt Quarry deposit: a review and new data on an important Palaeozoic fossil Lagerstatte with aragonite preservation

The Pennsylvanian Buckhorn Asphalt Quarry contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed siliciclastic-carbonate rocks (mudstones, pack to grainstones, shell beds, and conglomerates) with hydrocarbons prevented aragonite destruction (Impregnation Fossil Lagerstatte). The exceptional preservation comprises shell microstructures, microornaments and early ontogenetic shells. Most gastropods had planktotrophic larval development indicating a high primary production although the remains of phytoplankton are very rare in this and other Late Palaeozoic deposits. Deposition occurred close to a shallow-water coastal area. Mass flow processes (density currents) triggered by storms were involved in the transport mechanisms of some units. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating at least partly euphotic conditions. The invertebrate fauna comprises about 160 species and is dominated by molluscs, which is unusual for a Palaeozoic deposit, suggesting that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera in the Buckhorn deposit are also known from other Pennsylvanian occurrences as recrystallised shells. This shows that preservation bias via preferential aragonite dissolution may be overestimated.