TAPHONOMIC ANALYSIS OF A LATE CRETACEOUS OYSTER BED AT THE TOP OF THE MOUNT LAUREL FORMATION IN MONMOUTH COUNTY NEW JERSEY: LONG TERM HIATAL CONCENTRATION, STORM DEPOSIT OR TSUNAMI?

Fossil concentrations of large bivalves, such as Exogyra sp. and Pycnodonte sp., are not uncommon features of the Cretaceous Atlantic Coastal Plain. Most show evidence of heavy bioturbation, which leaves a more random orientation to the fossils. This study focuses on a more unusual fossil concentration occurring at the top of the Mt Laurel Fm in the Crosswicks basin of New Jersey. The shell bed is approximately 20cm thick, bioclast supported and stacked concordantly. The matrix is composed of medium to coarse glauconitic quartz sand. It is dominated by large oysters (Exogyra cancellata, Exogyra costata and Pycnodonte convexa), as well as belemnites (Belemnitella americana). There is a wide range of preservation, from articulated well-preserved shells to bored and broken ones. Eighty-five percent of the large oysters were found with bottom valves concave down, suggesting transport. Initial findings show little to no apparent preferred orientation to the belemnites possibly due to influence of the large oysters on the sea bottom during deposition. Three possible processes were considered to explain the concentration of fossils and the preferential orientation of the large oysters: storms (event or composite concentration), tsunami (event concentration) or long-term lag due to sediment bypassing (hiatal concentration). The lack of sedimentary features in the strata associated with storm or tsunami deposits seems to rule these out as likely processes. A more likely explanation is long term lagging possibly due to sediment bypassing on the continental shelf. The fragmentary nature of the deposit, as well as apparent time averaging of the oysters and belemnites, suggests this as the most likely scenario. The uppermost occurrence of E. cancellata and the lowermost occurrence of E. costata are mixed within the shell bed. There is also a change in the belemnite fauna across the bed. Within the bed, B. americana and its variants were found together with B. subfusiformis, whereas above the shell bed, only B. americana and its variants seem to occur. The relatively high concentration of glauconite also suggests slowing depositional rates. We argue the shell bed represents a hiatal concentration related to eustatic sea level change towards the end of the Campanian.