Multibeam and CHIRP sonar imaging of sand ridge morphology and basal stratigraphy on the inner shelf offshore Panama City, Florida
Abstract
Reconnaissance surveys were conducted with a multibeam swath bathymetry sonar and a CHIRP subbottom profiler, and vibracores were collected on the inner shelf offshore Panama City, Florida in April, 2011, to provide seabed characterization for an upcoming ONR acoustic reverberation experiment. The seafloor in this region is part of the MAFLA sand sheet: Holocene shelf marine sands extending from Mississippi to the Florida panhandle, 0-5 m thick and dominated by oblique sand ridge morphology. Coring typically samples a thin shelly layer, associated with the shoreface ravinement, at the base of the sand sheet, followed by finer-grained and organic-rich estuarine sediments. Prior CHIRP data collected off Fort Walton Beach, NW of Panama City, revealed an intermittent reflector beneath the sand ridges that can be correlated to the base of the sand sheet identified in cores. The Panama City CHIRP data also display an intermittent reflector at the base of the sand ridges, often outcropping in the swales between the ridges. Estuarine layering can also be identified, contained within erosional channels beneath the sand ridges. Three spatially correlated morphologic/stratigraphic transitions occur across the survey area. To the NW, the shoreface is narrow and steep, sand ridges are larger, and the base of the sand ridges is coincident with the top of the channel-fill deposits and can therefore be identified as the base of the sand sheet. To the SE, the shoreface is broad and gradual, sand ridges are smaller, and the reflector at the base of the sand ridges is distinct from the top of the channel fill. A core through the reflector at the base of the sand ridges, in a location where it is distinct from the top of the channel fill, sampled a ~0.5 m-thick shell layer coincident with the reflector, with well-sorted sand above and poorly-sorted sand with woody fragments beneath. The reflector at the base of the sand ridges therefore appears to be the transgressive ravinement surface. In the SE part of the survey area, this surface did not excavate down to the estuarine sediments, instead separating winnowed marine sands above from presumably barrier sands below it. Furthermore, the core demonstrates that shells are likely responsible for this intermittent reflector, and thus that the shells are themselves probably very heterogeneous in concentration at the ravinement surface. The transitions across the survey area may be related to sand delivery to the coast by the Apalachicola River, SE of Panama City, and subsequent NW along-shore transport.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS24A..01G
- Keywords:
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- 3002 MARINE GEOLOGY AND GEOPHYSICS / Continental shelf and slope processes;
- 3022 MARINE GEOLOGY AND GEOPHYSICS / Marine sediments: processes and transport;
- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics