The effect of sea-level and climate change on the development of a mixed siliciclastic-carbonate, deltaic coastline: Suwannee River, Florida, USA

Florida's mixed siliciclastic-carbonate, northwest-central Gulf of Mexico coastline is characterized by a 300-km-long, low-energy, sediment-starved, open-marine marsh system fronted by a broad, shallow limestone shelf. The only significant point source of sediment to this system is the Suwannee River, which forms a small (20 km(2)) delta at the center of this coastline. Late Pleistocene eolian dune formation along the Suwannee River coastline reflects slightly drier conditions and local sediment sources influencing the early geologic development of this region. As sea level rose in the early to middle Holocene, dunes on the low-gradient shelf were transgressed and provided the core for the modern offshore sandy shoals. With decelerating sea-level rise in the middle to late Holocene and probable increased riverine sediment input, coastal sedimentation switched from transgressive to aggradational. Paleo-distributary channels seaward of the delta coastline at approximately 4,000 cal yr BP indicate initial deltaic formation. At this same time, oyster reefs from the delta southward were able to keep pace with sea-level rise and began to form large (< 20 km(2)) offshore oyster bioherms. Away from the river mouth, the coastline continued to transgress until 2,350 cal yr BP to 1350 call yr BP, when the shoreline stabilized and the modern marsh system, south and north of the river mouth respectively, began to aggrade. The geologic development of this coastline and the relict eolian dunes preserved within the modern marsh system indicates that middle to late Holocene sea level did not exceed current elevations along this portion of the Gulf of Mexico coastline. This finding agrees with other studies from the low-gradient west-central Florida coast but contrasts with evidence for middle to late Holocene highstands along other Gulf of Mexico shorelines.