The Buckhorn Asphalt Deposit - An Unique Fossil Lagerstätte from the Pennsylvanian of Oklahoma

The Pennsylvanian Buckhorn Asphalt in Southern Oklahoma contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed carbonatic-siliciclastic rocks (mudstones, pack- to grainstones, shell beds, and conglomerates,) with hydrocarbons prevented aragonite destruction (“Impregnation Fossil Lagerstätte”). 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 Late Palaeozoic deposits. The deposition of the Buckhorn sediments occurred close to a coastal area partly involving mass flow processes (density currents) triggered by storms. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating euphotic conditions prior to final deposition. The invertebrate fauna comprises about 155 species and at least 120 genera. The assemblage is strongly dominated by molluscs which is unusual for a Palaeozoic deposit and corroborates that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera are also known from other Pennsylvanian occurrences (replaced shells). This shows that preservation bias via preferential aragonite dissolution may be overestimated.