A Bathysiphon (Foraminifera) ‘shell bed’ from the Cretaceous of northern California, USA: Example of a parautochthonous macro-skeletal deposit in deep-ocean turbidites

Shell beds consisting of concentrations of minimally transported, slightly damaged skeletal remains of indigenous organisms—comparable to bedded shelly accumulations of certain shallow-marine environments—have rarely been reported from truly deep-ocean turbidites. The general expectation is that shelly accumulations, when they do occur, ought to be derived from upslope sources and many kilometers away from the site of deposition. A Cretaceous thin-bedded turbidite in the Franciscan Complex of northern California, however, hosts a concentration of large specimens of the giant foraminiferan, Bathysiphon aaltoi, reflecting localized transportation and deposition in the original life habitat. The tests were derived from a densely populated thicket of the bathysiphonid probably located only a few metres/10s-of-metres away, decimated by a turbidity current that either overflowed an active submarine fan channel or spread outward from a suprafan lobe. As such, this unusual bathysiphonid-rich deposit can be viewed as a kind of deep-ocean level bottom ‘shell bed’.