The settling pattern of brachiopod shells: stratigraphic and taphonomic implications to shell bed formation and paleoecology

Abstract

In order to understand the effects of shell morphology on the hydrodynamic behavior and potential for sorting by hydraulic processes, the settling velocities and falling patterns of Bouchardia rosea (Mawe) shells (Brachiopoda, Rhynchonelliformea) from the northern coast of Sao Paulo State, Brazil, were studied under laboratory and field conditions. B. rosea is a small, epifaunal, free-lying brachiopod with shells fabric similar to some forms of Paleozoic brachiopods. Bouchardiid-rich shell-beds are also common in many Tertiary (Miocene) rocks of South America. Twenty valves (10 dorsal and 10 ventral) and three closed articulated shells, encompassing the typical size range of B. rosea were selected to the experiment. Shells were dried and weighed in air to the nearest 0.01g. The diameters of the three major axes of shells were measured to the nearest 1mm. Hydraulic settling velocities were recorded by releasing shells in a 35 cm wide and 56 cm deep column filled with natural salt water. Settling times were measured to the nearest 0.01 second, but falling durations were rounded to the nearest 0.1 second for calculation of settling velocities. Each shell was released just beneath the water surface for ten consecutive runs (230 releases total). The average settling velocities is 15.35 cm/s for ventral valves and 16.65 cm/s for dorsal ones. Articulated shells have average settling velocities of 26.48 cm/s. The lower average settling velocities of ventral shells indicate that they can be transported to greater distances than the less convex dorsal valves. This may be explained, in part, by the fact that ventral valves have greater cross-sectional areas than dorsal ones of the same weight. Hence, differential fragmentation of valves is not the only process likely to produce such bias, observed in some modern beach deposits, and many fossil concentrations. Finally, disarticulated shells settle in a concave-up orientation, regardless the size or degree of concavity of shells. Hence, biofabrics with shells in a preferred concave-up orientation will be common in fossil concentrations generated by turbulent flows with suspension settling of particles.