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Holocene Geology

Introduction
Setting
Pleistocene Geology
Holocene Geology
Hydrogeology
Water Resources
Case Study
Concluding Remarks
References

The many studies of carbonate depositional environments in the general vicinity of the Florida Keys are clearly beyond the scope of this chapter. Three aspects of the Holocene geology, however, form classic elements of the geologic story of the islands themselves: (1) Holocene sea-level history; (2) formation of modern dolomite at Sugarloaf Key; and (3) how the presence of the Keys has affected the Holocene buildup of reefs. The first two are discussed below and the third is the subject of the Case Study.

Florida Keys Sea-Level Curve

The Florida Keys are in the geographic region where curves of relative sea level during the late Holocene can be expected to show a history of continual submergence up to the present day (Zone III of Clark et al., 1978, and Peltier et al., 1978). The observed curve is that of Robbin (1984), and it is in general agreement with both the modeling by Clark et al. (1978) and the well-known curve of South Florida derived from the mangrove coast of southwestern Florida by Scholl and Stuiver (1968). Robbin's (1984) curve was based on 14C dates from soilstones and mangrove peat obtained by underwater drilling at six localities in the Upper Keys. The peat samples were obtained mostly by horizontal "push coring" into "walls of peat" exposed along the edges of channels cut through mangrove islands. The resultant sea-level curve shows a rise of 0.12 cm y-1 from about 7.0 m at 7 ka to about 0.75 m at 2 ka, followed by a rise of 0.03 cm y-1 from 2 ka to the present. The curve plots slightly below the curve of Scholl and Stuiver (1968), in which sea level was at about 1.6 m at 3.5 ka and 0.5 m at 1.7 ka. In neither case is there any indication of an emergence during the Holocene history.

Wanless (1982) called attention to the fact that tidal records from 1932 indicate that relative sea level has been rising at Key West at a rate of 0.23 cm y-1. Recently the Key West record has been examined in detail by Maul and Martin (1993). With newly discovered data going back to 1846, Maul and Martin (1993) report a 30-cm rise in the nearly 150-year period. For the period 1851 to 1987, the linear trend was 0.22 ± 0.05 cm y-1. Breaking this submergence into its geodetic and oceanographic components, Maul and Martin (1993) used the model of Peltier (1986) to infer that about 1/3 of the rise (0.08 cm y-1) was due to global isostatic adjustment to deglaciation. The remainder, they concluded, could be explained by a trend during the same period of dynamic height anomaly of the upper 1,000 m of the adjacent water column.

Lidz and Shinn (1991) reconstructed the paleogeography of the general vicinity of the Keys to illustrate how the platform was flooded, reef growth was displaced shoreward, and the region of the Keys was progressively drowned and separated. According to those authors, if sea level continued to rise at its present rate (0.38 cm y-1), most of the Keys would be flooded in 260 years (+1 m), and all but a few islands would disappear in 520 years (+2 m).

Holocene Dolomite

One particularly interesting aspect of diagenesis in the Florida Keys was the early identification of Holocene dolomite (Shinn, 1964). This mineral, common in ancient carbonate rocks, was known to be forming in only one other Holocene setting at the time of its identification in South Florida. Dolomite from Holocene mud in Florida Bay was initially thought to be authigenic (Taft, 1961), but the absence of 14C activity and other characteristics demonstrated its detrital origin (Deffeyes and Martin, 1962). The dolomite identified by Shinn (1964) is lithologically distinct and demonstrably Holocene by 14C dating. It occurs disseminated in cemented crusts of the supratidal zone of Sugarloaf Key, and its discovery became a key to recognizing analogous supratidal environments in ancient carbonate rocks. Carballo et al. (1987) have emphasized the importance of tidal pumping of sea water through these sediments to produce the dolomite.




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