Geologic Controls on Oyster Bed Distribution

The stratigraphic evolution of Apalachicola Bay has heavily influenced the present distribution of oyster bars in the estuary. Oyster beds in the Apalachicola Bay part of the estuary rest mainly on Late Holocene delta deposits (fig. 14D and fig. 15). Small beds also occur on Late Holocene flood-tidal delta deposits and anthropogenic deposits of dredged material along the Intracoastal Waterway (fig. 10). The seismic profiles show that the oyster beds consistently sit on high-amplitude reflections and are absent in areas where only low-amplitude reflections are present. This relationship suggests that the surfaces colonized by the oysters were sandier than the surrounding sea floor. The bathymetry and present distribution of oyster beds, as defined by the sidescan-sonar imagery, suggest that some sections of the delta surfaces were colonized and grew vertically, while other sections were either never colonized, or were abandoned at a later time because of burial by prodelta muds (fig. 15B). Sediment cores will be required to determine if oyster beds in the Apalachicola Bay part of the estuary initially had a broader aerial extent that has receded with time.

In the St. George Sound part of the estuary, oysters have colonized abandoned flood tidal deltas that extend into the bay along the northern edge of St. George Island, linear ridges of uncertain origin that rest on Pleistocene sands, and mounds of dredged material (fig. 10). Sand waves cover some of the broad shoals in the eastern part of the sound, which is well removed from the muddy, prodelta deposits, suggesting that tidal and storm generated currents are strong enough to sculpt the sandy estuary floor (fig. 10, and fig. 15C). Through sand-wave migration, larger shoals are generated, and where salinity is appropriate, these shoals can be colonized by oysters. Green and Porter's Bars may be shoals that were formed by sand-wave migration then stabilized when oysters colonized the tops.

Click on figures for larger images.

Figure 10. Map showing the distribution of eleven sedimentary facies identified on the floor of Apalachicola Bay superimposed on the shaded-relief image of the bathymetry.

Figure 14. Schematic block diagram showing important steps in the evolution of the Apalachicola Bay region since the last lowstand of sea level.

Figure 15. Seismic profiles showing (A) oyster mounds that accumulated on an older, sandy-delta surface, and were subsequently buried by younger mud, (B) an oyster bar that accumulated on a sandy-delta surface and remains exposed at the sea floor, and (C) sand waves in the eastern part of St. George Sound.