CONSTRAINING THE SEVERITY OF PLIOCENE STORMS: EXPERIMENTAL AND THEORETICAL HYDRODYNAMIC ANALYSIS OF MERCENARIA IN FLORIDA'S PINECREST BEDS

In Phase 8 of the SMR Aggregates Quarry, west-central Sarasota County, exposures in the Upper Pliocene Pinecrest beds display distinct lags at the base of depositional units of variable thickness. These lags are composed largely of whole, disarticulated Mercenaria mercenaria valves that are consistently the largest and heaviest invertebrate skeletal remains and also represent one of the largest sedimentary particles in nearly all Plio-Pleistocene shell beds in Florida. The specimens within these lags are often found in unstable concave-up positions, indicative of preferential size settling from suspension and characteristic of rapid-burial storm deposits. The largest specimens are approximately 14 cm in length and 4 cm in width (single valve), with calculated pre-weathering weights of up to 500 g, suggesting that strong flow would be necessary for entrainment and transport. A model for calculating bivalve hydrodynamics was provided by Olivera and Wood (1997), and, based on their equation, average transport velocities of 0.76 m/sec are required to entrain valves of the sizes measured here, using lift coefficients adapted from similar bivalve types. To further constrain the values, empirical hydrodynamic analyses were done using a 61 cm wide flume, where entrainment velocities were determined through incremental increase in flow velocity to the point of movement. Entrainment conditions were varied for shell orientation in flow and bottom substrate. Near flow velocity field and variance in shell drag coefficient as related to shell size and orientation were calculated using 2-dimensional Acoustic Doppler Velocimetry (ADV). With the use of independent-origin depth calculations, minimum wave size and energy is estimated for the entrainment of these shells. This allows the reconstruction of storm intensity in the formation of these Pliocene shell beds in Florida, as well as a means to compare past and modern storm severity.