Seismic and core investigation off Panama city, Florida, reveals sand ridge influence on formation of the shoreface ravinement

A high-resolution seismic reflection and vibracore survey was conducted on the inner shelf offshore of Panama City, Florida in April, 2011, to provide seabed characterization for Office of Naval Research acoustic reverberation experiments. The seafloor in this region is part of the MAFLA sand sheet: marine sands up to 5.5 m thick extending from Mississippi to the Florida panhandle. The survey area spanned a transition in sand ridge morphology: northwest of the St. Andrews Bay inlet channel, sand ridges are up to ~4 m tall, but die off southeast of the inlet channel. Analysis of reflection and core data documents a basal reflector beneath the ridges interpreted to be the shoreface ravinement separating estuarine and Pleistocene sediments below from the marine sands above scavenged from the eroded sediments. In the northwest, this surface truncates buried river channels filled with estuarine sediment. In the middle of the survey area, where sand ridges are ~2 m high, the basal reflector is stratigraphically above the channel tops, and the presence of woody fragments in the intervening sand implies that it is relict barrier/estuarine material that has not been reworked into marine sand. In the southeast, where no sand ridges are present, the basal reflector is not observed, but the shoreface ravinement is identified in cores just ~0.4 m below the seafloor; unreworked barrier/estuarine sands are up to ~4 m thick above the channel tops. The stratigraphic progression implies reduced depth of shoreface erosion to the southeast, correlating with the reduction in sand ridge height. It is hypothesized here that sand ridge migration provides an addition to wave erosion in the formation of the shoreface ravinement along the lower shoreface and inner shelf, eroding through net negative bedform climb, and transferring sand to the sand ridges comprising the modern sand sheet.