Phosphatic intraclasts in shallow-water hemipelagic strata: a source of palaeoecological, taphonomic and biostratigraphic data (Upper Turonian, Bohemian Cretaceous Basin)

A prominent phosphatic lag exposed in the Upper Turonian hemipelagic strata near Býčkovice, Bohemian Cretaceous Basin, yields an enormous accumulation of diversified, phosphatized and unphosphatized fauna. Taphonomic, palaeoecological and sedimentological data suggest that the phosphatic lag records a complex history of burial, mineralization and exhumation of sediment and fossils. Three main stages can be distinguished in the formation of the lag: (1) deposition of a hemipelagic marlstone or limestone inhabited by a diversified, softground fauna, then (2) intense phosphatization, winnowing of unconsolidated sediment and exhumation of the phosphates due to a combined effect of elevated bottom shear stress and bioturbation, and (3) resumption of hemipelagic deposition and establishment of a new assemblage dominated by sponges attaching to the exhumed phosphatic intraclasts. The occurrence of sponges inhabiting phosphatic intraclasts is unique in both fossil and recent ecosystems. One of these sponges, a poorly known hexactinellid Laocoetis cretacea (Rauff) is therefore described in detail. Biostratigraphic data suggest that the lag represents the uppermost Subprionocyclus neptuni Zone and lowermost Mytiloides scupini Zone. Well-log correlation with coeval, orbitally tuned strata further suggests that the absolute duration approximated 350 kyr.