SHEAR VERSUS SETTLING VELOCITY OF RECENT AND FOSSIL LARGER FORAMINIFERA: NEW INSIGHTS ON NUMMULITE BANKS

The genesis of nummulite banks remains a poorly understood topic and the aim of this work is to shed light on it by observing the hydrodynamic behavior of selected larger foraminifera collected from both bank and non-bank deposits. Entrainment and settling velocity of both recent and fossil larger foraminifera were measured using a flume channel and settling tube. Both velocities give information about the reaction of foraminiferal shells to different hydrodynamic conditions affecting their capability to build bank-like sedimentary structures. To assess entrainment, experiments were performed on different substrates to simulate bed load transport on smooth surfaces, fine sand, coarse sand, and bioclastic substrates. Thirty-four recent and 49 fossil shells of foraminifera were used; recent taxa used are Operculina ammonoides, Heterostegina depressa, and Palaeonummulites venosus and fossil taxa used are Nummulites perforatus, N. fabianii, and N. tavertetensis (only A forms were used in this study). Our results seem to differentiate the hydrodynamic behavior of shells collected from banks from those collected from non-bank deposits. The latter possess settling velocities significantly lower than their entrainment velocities, while for taxa collected from nummulite banks, settling velocities are always close to entrainment velocities. Therefore, the relation between hydrodynamics and shape and size may explain why modern larger foraminifera, consistently of smaller size than fossil forms, cannot produce banks and that transport as bed load in moving water was possibly the main trigger for the production of nummulite banks.