Taphonomic and Paleoenvironmental Evidence of Holocene Shell-Bed Genesis and History on the Northeastern Gulf of Mexico Shelf

Abstract

Subsurface shell beds that are common on the eastern Alabama/western Florida Panhandle shelf provide paleoenvironmental and taphonomic data that demonstrate a dynamic depositional history with reactivation and amalgamation to the base of the Holocene marine transgressive package. In most of the ten shell beds we examined, shallow-marine mollusks, large soritid foraminifera, and cupularid bryozoans are common. Bioclasts occur in all preservation states, but a majority are pristine or only slightly altered. Normal grading, and concave-up, stacked, and random fabrics are common. In three of these shell beds, a relict estuarine component characterized by poorly-preserved Chione cancellata can be detected. Three other shell beds contain 1 of 2 estuarine molluscan assemblages. One assemblage is characteristic of fine-grained and the other of coarser-grained substrates. In all estuarine shell beds, bioclast preservation ranges from excellent to mixed, and bioclasts have random fabrics and locally are normally graded. Most molluscan remains are indigenous and assemblages range from within-habitat time-averaged to environmentally condensed. Shell beds are thick (up to about 75 cm) and overlie bay or shoreface ravinement surfaces, indicating that shell beds first accumulated as coarse transgressive deposits, and are composite concentrations modified by multiple events. Shell beds, however, are amalgamated. Only fabrics reflecting the final modifying events are preserved, and evidence of previous accumulation processes are obliterated. For marine shell beds especially, these final agents were, and possibly continue to be, highenergy events such as storms (winter cold fronts, hurricanes) or currents associated with Loop Current eddies. Episodically high sedimentation rates associated with these events may have acted as a buffer, preventing a long history of reworking and exposure (thus reducing shell alteration) typical of transgressive lags. Our results indicate that: 1) within-habitat time-averaged and environmentally condensed assemblages can be distinguished by combining taphonomic and environmental data, even when the ranges of environmentally disparate species overlap, 2) transgressive-lag deposits can be composed of well-preserved bioclasts, and 3) transgressive lags can be reworked and amalgamated but still be recognizable on the basis of their stratigraphic context.