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5. Hardbottom Development and Significance
Hardbottoms are common in carbonate and siliciclastic shallow marine settings worldwide, but these features have been poorly documented. Along west-central Florida, at least 50% of the inner shelf sea floor seaward of 5 km consists of hardbottom and extensive systems of these features, locally termed ledges, constitute the only areas of natural relief (up to 4 m) providing important marine habitat. The lithology and stratigraphy of these ledges are all that remains of an incomplete geologic record of Tertiary continental shelf evolution. Extensive research of hardbottoms in Onslow Bay, North Carolina by Riggs et al. (1996; 1998) has pointed out the importance of hardbottoms as sequence boundaries and condensed sections commonly constituting the preserved sedimentary deposits of highstand and lowstand sea-level events. The hardbottoms of Onslow Bay occur on a high-energy continental shelf, and lithology plays a key role in controlling the morphology of these features (Riggs et al., 1998). In addition, these features have been shown to contribute significant amounts of sediment to the inner shelf system (Riggs et al., 1998). West-central Florida represents a low-energy shelf, and the hardbottoms differ from those of Onslow Bay in lithology, stratigraphy, and relief. Initial studies have shown that these hardbottoms do contribute sediments to the shelf, but the total volume is unknown (Berman, 1998).
Two types of ledges (dolomitic and calcitic) trend roughly shore-parallel, exhibit varying relief (<1-4 m), and tend to be rich in siliciclastic and phosphatic sediments. Each ledge type provides a substrate for diverse floral and faunal assemblages that encrust and bore the outcrop, especially on the lee side (shoreward) of the ledge, producing a shoreward-facing scarp and overhang. Intense bioerosion causes the overhanging blocks to periodically fail, producing a rubble ramp/field and serves as a mechanism for releasing sediment grains onto the inner shelf, at a rate of 0.04 kg/m2/yr of siliciclastic sediment. High-relief (3-4 m), Quaternary calcitic ledges that can be underlain by a layer of blue-gray clay are located in the southeast portion of the study area and have not been correlated to any known formational unit. Low-relief (<2 m) dolomitic ledges, located in the northwest portion of the study area, are extensions of the underlying, Miocene Arcadia Formation, the known extent of which has been increased at least 40 km offshore.
Hardbottoms on the coast probably formed as non-deposition hardground surfaces during periods of sediment starvation and low-energy conditions. The difference in lithology is a function of time and diagenesis. The ledge orientation may be controlled by a pre-existing fracture pattern in the rocks, while the prevalence of undercutting may be a function of lithology, although this process is still uncertain.
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