ABSTRACT

Mollusks and benthic foraminifera were examined to evaluate Pensacola Bay and Perdido Bay estuarine systems as potential sources for, or modern analogs of, Holocene facies of the northeastern Gulf of Mexico shelf. Grab samples were collected at 34 stations, most located in or near seagrass beds. Stations on the sandy backbarrier platform of Perdido Key in Big Lagoon contained a depauperate fauna of typical grassbed mollusks including Crepidula spp., Cerithium muscarum, and Anomalocardia auberiana. Most stations on the backbarrier platform of western Santa Rosa Island in Pensacola Bay showed low molluscan diversity and low bioclast abundance. Anomalocardia auberiana was most common. One station in this area has a much higher species diversity and bioclast abundance, and this concentration probably formed via physical processes rather than in situ productivity. Pensacola Pass sands contain molluscan assemblages with low species diversity and moderate bioclast abundance. These samples are dominated by Ervilia nitens and small lucinids. Modern Pensacola and Perdido Bay assemblages contain species that also occur in Holocene estuarine and marine facies of the shelf, although common species typically differ. Bioclasts of bay assemblages are generally well preserved, and preservation is less variable than in offshore subsurface facies, indicating the bay assemblages are less time-averaged. Surficial deposits of Pensacola and Perdido Bay estuarine systems are not a source of mollusk bioclasts to the offshore and are not modern analogs of subsurface shell beds in offshore facies.

Foraminiferal assemblages of backbarrier and bay stations are dominated by Ammonia and Elphidium, similar to Mississippi Sound, Mobile Bay, and St. Andrew Bay. Backbarrier samples have a higher proportion of miliolid species, indicating a greater marine influence, possibly due to washover events, tidal transport, or higher salinities through evaporation. Bay foraminiferal assemblages are comparable to subsurface estuarine facies on the shelf, but offshore marine death assemblages can include large well-preserved soritid foraminifera, including Archaias angulatus and Peneroplis proteus. The source of these soritids is not grassbeds of the Perdido and Pensacola Bay estuarine systems.