Abs. No. 50322
BENNINGTON, J Bret, FORSBERG, Nancy, SCHWAGER, Kathy, and VERDI, Elisheva, Department of Geology, 114 Hofstra University, Hempstead, NY 11549
Analyses of the sediments and fossils found in the Upper Cretaceous Navesink Formation at Big Brook and Poricy Brook in central New Jersey provide insights into the sequence of depositional environments developed during Navesink transgression and raise interesting questions concerning the paleoecology of Cretaceous oysters. At Big Brook, the Navesink is an approximately 8 meter sequence of quartz sand overlain by glauconite sand. A lower 2 meter interval of bioturbated, pebbly quartz sand is extensively burrowed, with a dense network of tunnels bearing the in-situ remains of decapod crustaceans (Protocallianassa mortoni). The presence of abundant callianassid burrows suggests a lower shoreface environment of deposition. The rare preservation of crustaceans within the burrows argues for episodes of catastrophic burial by storm events redepositing sediment below fair-weather wave base. The glauconite sands of the upper interval of the Navesink contain abundant benthic and planktic foramanifera and are inferred to have been deposited in a relatively deep water, outer shelf environment. Two macrofossil concentrations are found within the glauconite sands at the study localities, a lower horizon dominated by an assemblage consisting of pectens and the oyster Exogyra and an upper horizon dominated by the oysters Pycnodonte, Agerostrea, and the brachiopod Choristothyris. Fossiliferous horizons within generally unfossiliferous strata are usually attributable to storm redeposition or to colonization during periods of reduced detrital input. However, the morphology of the glauconite grains and abundance of planktic foraminifera in the upper Navesink implies slow, non-episodic sediment accumulation throughout the interval. Also, the regional extent of the fossil horizons and the mud-floating, non-cementing life-habit of exogyrid and pycnodontid oysters suggests that these are not `oyster banks' generated by substrate stabilization through taphonomic feedback. Instead, it is suggested that episodic changes in the benthic environment, unrelated to sedimentation, produced conditions favorable to colonization by oysters over a large region of the ocean floor. One effect that would allow colonization is an increase in oxygen, perhaps due to episodes of improved circulation and vertical mixing in the waters overlying the Cretaceous continental shelf.
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