SHELL BED FORMATION, BIOTURBATION, AND SEA LEVEL RISE: DEVELOPMENT OF A REEF-LAGOON SEDIMENT PACKAGE, ST. CROIX, USVI

Many paleoecological studies seek to understand shell bed formation in the fossil record by looking for a relationship between modern life and death assemblages in surface and near-surface sediments. They then attempt to extrapolate their results to ancient fossil shell accumulations. An analysis of the sediment package in Tague Bay lagoon (St. Croix, US Virgin Islands) reveals that the mollusc content of the surface and near-surface sediments bears little relationship, either taphonomically or taxonomically, to the faunas preserved throughout the underlying lagoon sediments. This assertion is based on twelve sediment cores across 1.3km (parallel to the reef), including detailed stratigraphy and constituent content within each core. To augment the sedimentological history, thirty additional probe locations establish depth to the basal surface aross the width of the lagoon (0.5km).

In Tague Bay, the sediments have been extensively bioturbated by callianassid shrimp that sequester the coarser shell content into lag deposits that sit atop the hard Pleistocene subsurface. The lag deposits can be as deep as 4m below the sediment surface and approach 1m in thickness. Above the lag molluscs are scarce, but where they occur they belong to infaunal taxa and are in good taphonomic condition. Even the shell-rich lag is dominated by infaunal species, which contrasts with surface death assemblages that are rich in epifaunal gastropods and large, shallow-infaunal clams. This result urges caution for those who would relate results from surface fauna in modern environments to their potential fossil assemblages in the subsurface, at least in environments affected by deep bioturbation.

Probing the sediment revealed sediment thicknesses of 1m to 4m above the Pleistocene hard surface. Four meters of sediment (the thickest measured) extrapolated over 7000 years since sea level flooded the lagoon (data based on Caribbean-wide sea level curves) indicate a maximum sedimentation rate of 0.5m/1000yrs. These data, along with the molluscan content, constituent analysis, and evidence for deep bioturbation of the sediments allow a reconstruction of the history of sea level rise and sediment accumulation in this lagoon over the past 7000 years.