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geology and hydrogeology of the florida keys >
Climatic and oceanographic setting
The Florida Keys enjoy a subtropical climate with mean January temperatures of 21oC (69oF) and mean July temperatures of 28oC (83oF). Winter freezes occur rarely in the Upper Keys and have never been known to extend to the Lower Keys. A well-developed gradient in rainfall extends across the Keys, with the northern Keys receiving an average of 140 cm (55 in.) and Key West averaging slightly less than 100 cm (40 in.). Rainfall is also highly seasonal. Almost two-thirds of the annual rainfall occurs as wet-season thunderstorms between May and October. The predominant wind direction is from the east and characterizes the summer months; however, this wind is a gentle summer breeze that rarely exceeds 10 km h-1 except during summer thunderstorms. The relative calm of the summer is occasionally punctuated by tropical storms and hurricanes that pass through the Keys with a recurrence interval of about 2 years. During the winter dry season, rain is associated with periodic, almost weekly, cold fronts sweeping through the Keys as they cross the eastern North American continent. These storms bring strong westerly and northerly winds between calmer periods of easterly and southeasterly breezes.
On the Atlantic Ocean side of the Keys, the Florida Current flows northward, parallel to the Keys and east of the 5-8-km wide shelf that supports the growth of modern coral reefs along its outer margin (Fig. 5-1). Pleistocene reefs form relict ridges along the margin a few hundred meters seaward of the modern reef tract (Fig. 5-2). The Florida Current varies from 22oC (71oF) in winter to 28oC (83oF) in summer and is a moderating influence on the climate of the Keys. Landward of the reef tract is White Bank, a shallow sand shoal studded with patch reefs, and Hawk Channel, an inner shelf lagoon 6-8 m deep. Just seaward of the Keys, bare limestone equivalent to that of the Keys is exposed for as much as a kilometer offshore (Fig. 5-2).
Florida Bay [q.v., Chap. 6] lies west of the Upper Keys (Fig. 5-1) and is a shallow lagoon (average depth, 1.3 m) characterized by mangrove-covered Holocene mud and peat islands, mudbanks, and shallow marine basins locally called "lakes." This shallow water typically remains close to atmospheric temperature and may be as warm as 35oC (95oF) in the summer and reach extremes of 13oC (55oF) during winter cold episodes. Lakes, mudbanks, and islands are underlain by Pleistocene limestone that is a facies variant of limestones forming the Florida Keys (Fig. 5-3).
Before the arrival of Europeans, vegetation of the Keys was of three kinds: (1) mangroves; (2) tropical and subtropical hardwoods; and (3) pines and palmettos. Typically the small and narrow islands support hardwoods. Larger, broad islands support pine-palmetto vegetation. The pine-palmetto ecotope, characteristic of much of South Florida, dominates where periodic fires sweep the larger Keys. All islands are fringed with mangroves; the lowest islands are typically entirely mangrove. Many of the islands were cleared for agriculture during the last century and early part of this century (Craighead, 1971), and much of the original vegetation has now given way to development on the islands.
Topographically, the Keys are low and flat. Hoffmeister and Multer (1968) estimated that about one-half of the area of the Keys is covered by mangrove swamps. The rest is mostly <1 m in elevation; the highest elevation is 5.5 m (18 ft) at Windley Key (Hoffmeister and Multer, 1968). There are only a few sand beaches in the Florida Keys. Most of the shoreline is characterized by rock or muddy intertidal flats that border mangrove shorelines (Hoffmeister and Multer, 1968).
The Florida Keys occupy the southern portion of the Florida Platform, a 5-km thick sequence of shallow-water carbonates and evaporites with relatively minor components of terrigenous sediments. The platform represents continued sedimentation and subsidence on a passive continental margin that overlies transitional crust formed during the initial opening of the Atlantic Ocean (Klitgord et al., 1984). The Florida Platform was part of a great Mesozoic carbonate bank or "gigaplatform" (Hine, 1997), which stretched from Mexico and the Bahamas to Canada along the Atlantic side of North America (Hine, 1997).
During the Cretaceous and Tertiary, deep troughs developed in the platform and formed basins such as the Straits of Florida and the Tongue of the Ocean. Development of these basins reflect complex interactions of sedimentation and erosion rates of carbonate sediments, involving such factors as changes in sea level, circulation patterns, nutrient supply, and climate change (Hine, 1997). During the Miocene, Pliocene and, most recently, Pleistocene, some terrigenous clastic deposits extended from North Florida to the southern tip of the platform (Enos and Perkins, 1977; Warzeski et al., 1996). During the late Pleistocene, carbonate sediments dominated the Keys region as they do today.
The continental margin has experienced about 200 m.y. of cooling and subsidence since the formation of the Atlantic (Steckler et al., 1988). This period of rapid cooling and subsidence ended at about 150 Ma, and now the Florida Platform is near the distal end of its cooling and subsidence curve. As noted by Steckler et al. (1988), various cooling models converge to very slow subsidence rates after 200 m.y. such that the Florida Keys can be considered to be located on a very stable, old margin at the present time. As described later, the age and elevation of the coralline Florida Keys, along with correlation to other reefal limestones in the western Atlantic, suggest that less than a few meters of subsidence has occurred in the last 100 ky.
It should also be noted, particularly with regard to sea-level estimates for the Pleistocene, that the Florida Keys are slightly influenced by isostatic motions associated with the shift in load back and forth from continental ice to the global ocean. Peltier (1986) calculated that deglaciation-induced downward vertical motions of the Atlantic margin should be expected for the entire east coast of the United States, and that this subsidence may amount to about 0.1 cm y-1 in Florida.
U.S. Department of the Interior, U.S. Geological Survey
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