Evolution of Florida Bay from island stratigraphy PAUL ENOS Department of Geological Sciences, State University of New York at Binghamton, Binghamton, New York 13501 RONALD D. PERKINS Department of Geology, Duke University, Durham, North Carolina 27708 ABSTRACT Florida Bay with time. Progressively larger areas of the bay are building into the supratidal zone through both vertical and lateral The sedimentary record of most Florida Bay islands is an asym- accretion. If the present depositional trend continues without sig- metric cycle consisting of a transgressive sequence followed by a re- nificant increase in the rate of sea-level rise, the present area of gressive sequence, both formed during a continuous Holocene rise Florida Bay will eventually be accreted to the Florida mainland as a in sea level. The principal sedimentary environments of Florida Bay coastal mangrove swamp, a supratidal "marl prairie," and a series and the south Florida mainland are represented in the cycle by an of shallow lake basins. upward succession of (1) freshwater pond, (2) coastal mangrove swamp, (3) shallow bay ("lake"), (4) mud bank, and (5) island. Setting Some parts of the cycle may be missing, but the sequence is always the same. Supratidal carbonate sedimentation on islands may de- Sediments in Florida Bay are being deposited disconformably on velop from coastal mangrove swamps or by mangrove colonization an almost planar surface of Pleistocene pelletoidal lime packstone of mud banks. Islands have developed from mud banks at many and grainstone (Perkins, 1977). Extensive sediment probing and different times during the rise of sea level into Florida Bay, indicat- observations both from the air and underwater reveal no appreci- ing that mud banks must have existed throughout most of the his- able Pleistocene depositional relief. The Pleistocene surface is pitted tory of the bay. Present trends of island formation and growth by small solution holes several centimetres (less than 1 ft) deep, suggest that Florida Bay will evolve into a coastal carbonate plain producing a microkarst topography similar to that exposed farther with inland mangrove swamps and freshwater ponds, very similar north in the Everglades. The surface slopes southward and south- to the present southwest Florida mainland. westward from the Florida mainland at about 1:100,000 (0.5 ft/mi) until it rises abruptly along the Florida Keys, reflecting depositional INTRODUCTION morphology of coralline limestone. Florida Bay thus occupies a shallow, wedge-shaped basin that is deepest (3 m; 10 ft) im- Florida Bay occupies the inner part of the south Florida shelf mediately behind the Florida Keys and along its southwestern (Fig. 1) and covers an area of 1,550 km2 (600 mi2; Scholl, 1966, margin. Table 1). The boundaries of the triangular bay are defined by bar- Sea level in south Florida has risen at a slightly declining rate for riers that restrict circulation. On the southeast, a ridge of Pleis- at least the past 5,000 yr (Fig. 2; Scholl and others, 1969). Flood- tocene coralline limestone (Key Largo Formation) forms the nearly ing of Florida Bay began in the southwest, where the rock floor lies continuous barrier of the Florida Keys, interrupted only by a few 3 m (10 ft) below present sea level, about 4,500 yr ago and con- tidal passes. The southwest border of Florida Bay is delineated by a tinues to the present. Despite the continuous rise in sea level, parts series of shallow mud banks that extend from Cape Sable at the of the south Florida mainland have prograded seaward more than 8 southwestern tip of the Florida mainland southeastward to km (5 mi) through lateral accretion of marine lime mud and peat. Matecumbe Keys (Fig. 1) and comprise Sandy Key Bank near Cape The present bathymetry of Florida Bay is dominated by a series Sable, Ninemile Bank, and Peterson Key Bank. The triangular area of shallow sublinear mud banks, which divide the bay into slightly south of Ninemile Bank, extending from Lower Matecumbe Key to deeper basins, or "lakes" as they are locally known. Systematic dif- Schooner Bank to Bamboo Banks and Vaca Key, may be considered ferences exist in the width of mud banks, the relative area occupied either as part of Florida Bay or transitional to the Gulf of Mexico by basins, and the abundance of islands in different parts of the shelf to the west. This area has fewer banks and greater tidal ex- bay. The width of banks increases progressively from the north- change than Florida Bay proper. The south Florida mainland forms eastern part of the bay toward the west. The relative area occupied the northern boundary of Florida Bay, consisting of a supratidal by basins is concomitantly reduced. Islands are more abundant in carbonate flat ("marl prairie") from Cape Sable to Flamingo and a the northeastern and particularly the central part of the bay than in series of mangrove-fringed embayments and discontinuous low, the western part. In northeastern Florida Bay (north of lat 25°N shelly beach ridges east of Flamingo (Fig. 1). and east of long 80° 43'W), Stockman and others (1967, p. 644) Florida Bay is compartmentalized by a series of carbonate mud have estimated that basins occupy 90% of the area and are floored banks that are awash at low tide. Small, mangrove-fringed islands, by an average of 15 cm (6 in.) of Holocene sediment. Banks and some partially flooded at high tide, dot the mud banks. The dis- islands occupy 10% of the area and average 1 m (3 ft) in sediment tribution and stratigraphy of these islands with their mud-bank thickness. Scholl (1966, p. 283) has estimated the area of mud nuclei indicate an increase in the size and number of islands in banks for the entire bay to be 496 km2 (192 mi2) or 32% of the Geological Society of America Bulletin, Parti, v. 90, p. 59-83, 19 figs., 1 table, January 1979, Doc. no. 90112. 59 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/1/59/3434258/i0016-7606-90-1-59.pdf by guest on 28 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/1/59/3434258/i0016-7606-90-1-59.pdf by guest on 28 September 2021 EVOLUTION OF FLORIDA BAY 61 total area.1 He defined mud banks as areas less than 60 cm (2 ft) spite the limited range of the semidiurnal tides within the bay, the deep, thus excluding much of the gently sloping margins of the mud banks are awash at low tide. banks. The contrast in these two estimates primarily reflects the Prolonged winds may move larger volumes of water than the much greater area occupied by banks in the western part of the bay. lunar tides. Northeasterly winter storm winds tend to push water Mud banks of the western bay (west of long 80°50'W, north of out of the bay through the tidal passes, slightly reducing the bay lat 24°55'N) are broad, amoeboid-shaped shoals. Associated basins level. Prevailing wind direction and wind velocities are therefore ("lakes") of western Florida Bay are poorly defined and generally nontidal factors that significantly affect the water levels in Florida floored by as much as 1 m of sediment (Fig. 3). The basins are inter- Bay. connected by a few sluggish, poorly defined channels in the broad Currents within the open "lakes" of Florida Bay are less than banks. In contrast, basins of the eastern bay are interconnected by 1 cm/s (Gorsline, 1963, p. 131) but may reach 125 cm/s in tidal narrow, well-defined, active channels through most of the mud passes into the bay (Vaughan, 1935). banks. Salinities in Florida Bay range from 6°/oo to 70°/oo, depending Islands ranging in size from a few tens of square metres to about upon seasonal variations in rainfall and net evaporation within the 2 km2 (500 acres) are scattered along the mud banks, particularly in bay and in amount of mainland runoff (Ginsburg, 1956; McCal- central Florida Bay. Scholl (1966) estimated the total area of ap- lum and Stockman, 1961; Gorsline, 1963; Lloyd, 1964; Scholl, proximately 105 islands2 or keys at 67 km2 or about 4% of the bay 1966). Fluctuations are greatest in northeastern Florida Bay, the area. In contrast to the linear Florida Keys, which are underlain by area most isolated from normal oceanic water. Extremes diminish Pleistocene rock, the keys of Florida Bay are composed of muddy toward the southwest, so that salinities in most of Florida Bay are carbonate sediments extending a few centimetres to nearly 1 m between 35°/oo and 40°/no. Water temperatures within Florida Bay above the adjacent bank. Many of the islands are covered or range from 19 to 38 °C (Ginsburg, 1956). Even more extreme fluc- fringed by mangroves and are flooded at high tide. Other islands, tuations in both salinity and temperature occur in water a few cen- particularly those near the Florida mainland in western Florida timetres deep ponded on the islands. Bay, stand several decimetres above normal high tide and support Turbidity of Florida Bay water is generally high, imparting a salt grasses or palms and hardwoods. characteristic translucent milky-green color to the water. The floors Tidal deltas in the form of irregular or triangular-shaped banks of "lake" basins about 2 m (5 to 7 ft) deep are visible only after have accumulated along tidal passes through the rocky Florida calm periods lasting a day or more. Turbidity varies locally accord- Keys. Generally, the surfaces of the deltas are slightly submerged ing to the bottom conditions; turbidity is reduced in areas of thick except for natural levees along the tidal channels and for a few is- grass cover and shelly bottoms containing little interstitial mud.