FISC - St. Petersburg
Mosquito Bank, White Bank, Coral-Rock Ridges, and French Reef: Aerial photographs taken in 1975 were cropped and joined to form photomosaics of the shelf-wide seabed surface from the keys to the shelf edge. Mosaics of the area in Tile 1 reveal various sand-body forms and geologic structures typical of the outer shelf and margin (Fig. 33A, 33B). Features at the margin in the Tile 1 sector consist primarily of a generally continuous section of the Pleistocene shelf-margin reef capped by skeletal coral reefs and numerous sand chutes of the present reef ecosystem (see Benthic Ecosystems for Tile 1).
Mosquito Bank overlies a concave low-elevation indentation or reentrant in the bedrock depression beneath Hawk Channel (Figs. 22B, 33A, 33B). Sediments in Mosquito Bank are mostly muddy sands with patchy areas of lime mud that surround scattered patch reefs. White Bank is a backreef marine sand belt that has accumulated on the higher bedrock seaward of Mosquito Bank (Ball, 1967; Halley et al., 1983). Composed of coral, algal, and molluscan sand flats and sand waves, the sand belt is 1 to 2 km wide and 40 km long (Enos, 1977). Active (rippled) and stable bottoms (covered by sea grasses) both occur at water depths of 1 to 3 m, indicating that local hydrologic conditions control formation of the large scattered sand waves. The sand belt contains dipping layers called cross-bedding. A slope steeper at the landward than seaward edge (Enos, 1977) indicates landward sand transport. Ball et al. (1967) documented landward accretion after the 1960 passage of Hurricane Donna. Perkins (1977) referred to White Bank as a sand-shoal/patch-reef complex and considered it a modern analogue to similar underlying bedrock constituents.
Shelf-wide, the outer shelf seaward of Hawk Channel is lined with numerous rock ridges (Fig. 33B). Where cored behind Marker G in the lower Keys (Fig. 34A), two ridges have been shown to consist of narrow lines of massive Pleistocene and Holocene coral species separated by a sediment-filled swale (Shinn et al., 1977a). Coring of coral-rock ridges seaward of Bal Harbor, located north of Miami (Fig. 22B), has shown they developed on cemented sand dunes (Shinn et al., 1977a). It is believed that all coral ridges shelf-wide likely overlie cemented Pleistocene beach dunes (Shinn et al., 1991; Lidz et al., 2003; Lidz, 2004). The depositional environment in Florida during the Pleistocene is considered to have been similar to that in the Caribbean and Bahamas, where emergent Pleistocene sand dunes occur widely (e.g., Ball, 1967; McKee and Ward, 1983). Beach dunes forming along the arcuate Florida shelf edge would have created longitudinal or transverse ridges (Fig. 34B). The discontinuous coral ridges are mostly linear off the lower and middle Keys but can be quite sinuous seaward of Mosquito Bank. Hundreds of patch reefs line the ridges (Fig. 33A, 33B).
Other types of accumulations visible in the aerial mosaics include elongate or ovate zones of coral rubble behind reefs and numerous sand chutes at the shelf edge (Fig. 33A, 33B; see Benthic Ecosystems for Tile 1). Coral-rubble zones result from landward transport of coarse debris by storms and are found close to abundant quantities of loose rubble, usually behind major Holocene shelf-margin reefs. Low-elevation areas in the underlying Pleistocene reef form chutes through which sands near the shelf edge spill seaward. Also evident in the mosaics, reefs of the present ecosystem have numerous structural spurs of skeletal coral separated by sand-filled grooves (Figs. 15, 33A). Spurs and grooves form perpendicular to incoming waves and act as baffles to wave energy (Shinn, 1963). Present along most areas of the shelf-edge Holocene reefs, spurs and grooves are most prominent on the largest reefs.