Bivalve shell concentrations in Early Jurassic Lake Sichuan (Southwest China): expressions of the Toarcian super-greenhouse

During the Early Jurassic Period, the Sichuan Basin in southwest China was covered by a giant lake, nearly 250,000 km² in size, which deposited sediments now assigned to four members of the Ziliujing Formation. The youngest of these, the Da’anzhai Member, comprises widespread, unusually thick bivalve shell concentrations intercalated with dark organic-rich mudstones, which, based on radiometric dating and palynostratigraphy, are late Pliensbachian? to Toarcian in age. We logged and sampled nine outcrop sections of the Da’anzhai Member in the Eastern Sichuan Fold-and-Thrust Belt, which expose almost the entire stratigraphy of the unit. Collectively, these rocks preserve a rich but surprisingly low-diverse fossil assemblage. Life in Lake Sichuan was dominated by small pseudocardiniid bivalves, reaching no more than 30 mm in length, which, we assume, belong to a single, morphologically variable species in the genus Kija. Mudstones commonly contain Kija specimens in situ together with abundant darwinulid ostracods. Periodic storm events reworked dead and living bivalves into extensive shell concentrations, which locally reach amalgamated thicknesses greater than 10 m, containing an estimated 0.5 to 1 million bivalve individuals per cubic metre. The tempestites were home to several species of larger, unionid bivalves as well as an unknown trace maker creating networks of up to 80 mm wide horizontal burrows. Disarticulated bones and coprolites are evidence that fish were abundant. Additional taxa include rare gastropods, clam shrimps and charophytes. These fossil biota are evidence of a well-oxygenated freshwater lake. Palynomorph assemblages indicate a warm, generally arid climate, where seasonal (monsoonal?) rainfall coincided with storms accumulating the shells. Co-occurrence of myriads of bivalves and highly organic-rich mudstones in a generally shallow but extensive lake is striking. Both phenomena are testimony to an extraordinarily high primary productivity during the times of the Toarcian super-greenhouse.