Interpretation of the Surficial Geology of Apalachicola Bay and St. George Sound, Florida (SURFICIALGEOLOGY)

This sea floor type is characterized by a moderate to high backscatter signature on the sidescan imagery. Sediment samples from these areas recovered a mix of sand and shell hash (NOAA, Coastal Services Center, 1999). The shelly sand facies is found in three settings. First, it is found fringing parts of many of the oyster bars. Specifically, this facies is found along the eastern sides of Porter's, Platform, and St. Vincent Bars, it is found along both sides of Hotel and Norman's Bars, and occurs in patches around and between many of the smaller oyster bars. Second, this facies covers some shoal areas where there is no evidence of oysters. The shoal off Cedar Point and two areas west of St. Vincent Bar are examples of such areas. The third setting is the floor of troughs where strong tidal currents actively rework the sea floor. One of these troughs extends eastward along the southern side of the bay from West Pass, and another extends west into Apalachicola Bay from the shallow gap between Cat and East Hole Bars. In these areas, the shelly sand probably represents a lag deposit associated with active reworking by strong tidal currents. The shelly sand facies covers 16.33 km2 of the study area.

This sea floor type was only identified in a small, relatively deep depression south of Cat Point. This facies has a high-backscatter return, and in this area bottom samples recovered shell hash and some gravel. This shell and gravel appears to be a lag deposit resulting from the strong tidal currents in this area. This facies covers 0.69 km2

Sections of the estuary floor have a moderate-to-low backscatter signature that is interrupted by small high-backscatter patches that are mostly 5-15 m in diameter. This mottled sea floor occurs in narrow bands along parts of the southern side of the survey area in St George Sound, and in some larger patches in the western part of Apalachicola Bay on Higgins Shoal and along both flanks of a second low-relief ridge west of St. Vincent Bar. Sediment samples form these mottled areas are sandy mud. The origin of the mottled pattern is unknown. This facies covers 2.60 km2.

This sea floor type refers to the flood tidal delta deposit shoreward of Government Cut, the manmade inlet that separates St. George Island from Little St. George Island. The surface of this feature is rippled, and inferred to be primarily of sand composition. West Pass does not have a flood tidal delta on its shoreward side. The flood tidal delta shoreward of Government Cut occupies 0.23 km2.

Two patches of sand waves were identified in the eastern part of St. George Sound where the sea floor is primarily sand. These sand waves have 50-150 m wavelengths, 0.5-0.8 m heights, and crests that trend northwest. Many of the sand waves are asymmetrical with their steep sides facing to the west. The asymmetry of these bedforms suggests a net westward migration. Sand waves cover 1.39 km2 of the survey area.

This sea floor type refers to clean sand containing no more than small amounts of mud and shell hash. It covers most of the floor of the eastern part of St. George Sound, a large part of the floor of Apalachicola Bay west of St. Vincent Bar, and discontinuous sections of the southern edge of Apalachicola Bay. This facies covers 52.21 km2 of the study area.

Areas inferred to be covered by oysters show as high backscatter on the sidescan-sonar imagery, coincide with shoal areas on the bathymetry, and bottom observations show living oysters on a shelly sand sea floor. Oyster beds cover 21.91 km2 of the study area

The mud facies covers the largest part of the study area. It appears on the sidescan-sonar imagery as low backscatter, and samples are comprised of at least 70% silt plus clay. The mud facies covers 134 km2 of the study area; an area that includes much of the floor of Apalachicola Bay and the western part of St. George Sound. It occurs mostly in sections of the study area where water depths are deeper than 2 m.

Oyster beds that have developed on top of dredge-spoil material.

These manmade deposits have the same origin as the dredge material/oyster areas however these deposits have a moderate backscatter surface and are inferred to not be covered with oysters. Areas classified as Dredge material cover 0.78 km2 and are found in the eastern part of St. George Sound, and along the western side of the section of the Intracoastal Waterway that trends northward to the mouth of the Apalachicola River.