FISC - St. Petersburg
Overview MapReef and Sediment Thickness: Dates on coral reefs and sediments of the present ecosystem indicate they have been accumulating since the sea began to cover the Florida shelf approximately 7 to 6 ka. As in the case of the bedrock components and fossil reefs, thickness and distribution of the modern reefs and sediments contain records of environmental quality and physical/biological processes. Coral reefs flourished between ~7 and 2 ka (e.g., Shinn et al., 1989a) and began to decline as Florida and Biscayne Bays filled with sea water between 4 and 2 ka (Lidz et al., 1991). Flooded tidal passes between the keys allowed bay water, unsuitable for coral growth, to access the offshore reefs. The modern reef system is Holocene (Fig. 7B).
The thickest (>40 m) sediment buildup visible on the map occurs on the upper slope southwest of the Marquesas Keys in 80 to 190 m of water. Thicker sediments (> 55 m) are known to fill an outer-shelf sinkhole located off north Key Largo (Fig. 6C; Shinn et al., 1996).
Although less extensive in distribution and dimensions than the older bedrock reefs, reefs of the modern ecosystem are comparable. Thickness of their skeletons indicates they grew vigorously, but their ages indicate growth occurred only over ~5 ka (e.g., Shinn et al., 1977a, 1989b). The thickest Holocene reef drilled (~17 m) is at Southeast Reef in the Dry Tortugas (Fig. 6A, 7B; Shinn et al., 1977a) and indicates that environmental quality upon initial flooding of the shelf (at ~7 to 6 ka) was very favorable for coral growth. The least healthy modern reefs are off the middle Keys, where wide tidal passes allow diurnal exchange of turbid water from Florida Bay (e.g., Hallock and Schlager, 1986) and cold winter water from the Gulf of Mexico onto the reef tract (e.g., Roberts et al., 1982).
When viewing the sediment thickness map, envision the physical and sedimentary processes responsible for the distribution of reefs and sediments. Reefs grow on bedrock highs where sediments do not collect. Sediment accumulates in enclosed bedrock lows, such as those north of New Ground Shoal and the sinkhole off Key Largo, and piles up behind rock barriers, such as east of Halfmoon Shoal. Sediment is being transported off the shelf, forming a thick sediment wedge southwest of the Marquesas Keys, and strong currents are preventing accretion in Boca Grande Channel (Shinn et al., 1990). Deep water on three sides of the Marquesas-Quicksands ridge and current-swept, sediment-free Boca Grande Channel indicate that ridge sediments are being formed in place. In the case of The Quicksands where the sediments consist primarily of Halimeda grains (Hudson, 1985), sediments can be considered biologic (algae) and geologic (in-situ source) indicators of physical processes.
Considered together, the bedrock and sediment-thickness datasets show that bedrock elevation is several meters lower in the southwest than in the northeast (e.g., Perkins and Enos, 1977), yet sediment cover is about 3 to 4 m thick shelf-wide (Lidz et al., 2003). In other words, except for localized enclosed bedrock depressions, sediments are not thicker in extensive open areas where bedrock is deeper, which reveals differences in physical processes in the southwest and northeast. Prevailing winds and waves impinging on the shelf margin generally arrive from the southeast, i.e., onshore in the northeast areas, but tangential to shore in the southwest. Sediments in the southwest region are being removed from the shelf due to the generally westward winds and waves, a more open exposure to strong oceanic currents, and other conditions associated with the deeper-water setting and location at the end of the island chain.