The current understanding of the bedrock geology of the Florida Keys derives from a series of benchmark papers by Hoffmeister and Multer in the 1960s (Hoffmeister and Multer, 1964a,b, 1968; Hoffmeister et al., 1964, 1967; Hoffmeister, 1974), and subsequent work that has been built on that foundation. The key facts are: (1) the long linear islands of the Upper Keys represent a reef tract formed during high sea levels of the last interglacial, and (2) the larger Lower Keys are fossil oolitic shoals, also formed during high sea levels of the last interglacial.

Early workers were well aware of the two kinds of islands. For example, Sanford (in Matson and Sanford, 1913, p. 61-62) described the arrangement as follows:

"The Florida Keys are separated by Bahia Honda Channel into two distinctly differentiated divisions. East of the channel the islands are narrow and lie along a sweeping arc curved toward the southeast. Outside this arc is the Florida Strait....

West of Bahia Honda the keys form an archipelago roughly triangular in outline. In this group, the westward prolongation of the arc in which lie Bahia Honda and the keys to the cast and northeast is found in the southern shoreline of the keys; but the keys themselves, instead of lying parallel to this are, have a prevailing north-northwest, south-southeast arrangement, perpendicular to the arc....

Bahia Honda and the keys east of it represent an uplifted coral reef more or less covered with sand and marl; hence their basement rock ridges have the trend of the coral patches of the old reef. The keys west of Bahia Honda consist of an oolitic limestone formed from deposits in a broad expanse of shallow water."

We should note that the early work in the Keys preceded knowledge of Pleistocene glacioeustasy and sea-level highstands. In particular, Alexander Agassiz considered the "elevated reef of Florida" (Agassiz, 1896) to be one of a number of examples posing problems for Darwin's subsidence theory for the origin of coral reefs.

Fresh from his visits to the Bahamas and Bermuda, Agassiz thought that the oolitic limestone, evident near Key West and near Miami, was formed as eolian dunes, capping the elevated reef throughout the higher keys. According to Agassiz (1896, p. 50),

"The keys are all built upon this elevated coral reef foundation, which crops to the surface, as we have seen, at many points, and from the beaches on the sea face of this elevated reef has been obtained the oolitic material which as aeolian sand has raised the keys to a height of sometimes ten to eighteen feet. This sand has been blown to the northward ... to form low aeolian hills and bluffs between the patches and stretches of the old coral reef, or it has accumulated upon the top of patches and stretches of reef to form the higher keys...."

As it turns out, the highest elevation is at Windley Key, one of the Upper Keys, which now are known to be composed of the reefal unit. Sanford (in Matson and Sanford, 1913), who argued for the subaqueous rather than subaerial origin of the oolite, called attention to the difficulties of interpreting the geology of the islands without exposures and the benefits he had from the exposures opened up by railroad construction. Referring to the reefal unit in Key Largo, Sanford wrote (Matson and Sanford, 1913, p. 185),

"Borrow pits expose the limestone, not only where it was lightly covered by leaf mold but where it was buried under several feet of marl and sand, and dredging has revealed its character where it lies, as in channels between the keys, 10 feet or more below sea level. Hence, the opportunities for observing the various phases of the rock and determining its origin are incomparably better than when A. Agassiz visited the keys in 1894."

The now-closed Windley Key quarry, which was first opened for railroad ballast, is the source of decorative coralline limestone used throughout South Florida.

Today, the reefal unit is known as the Key Largo Limestone; the oolitic rocks make up a facies of the Miami Limestone. The Upper Keys, which are elongated and oriented parallel to the shelf edge, consist of the Key Largo Formation; the Lower Keys, which are elongated perpendicular to the shelf edge, are composed of oolite of the Miami Limestone. Stratigraphic nomenclature and usage are summarized in Randazzo and Halley (1997). Unlike the Miami Limestone, which has direct modern analogs that aid in its interpretation, the Key Largo Limestone apparently does not have a modern counterpart. Its interpreted origin has evolved over the past two decades.

The oolite facies of the Miami Limestone in the Lower Keys consists of well-sorted ooids with varying amounts of skeletal material (corals, echinoids, mollusks, algae) and some quartz sand. Oolite in the Lower Keys has less quartz sand and fewer marine fossils than the Miami Limestone near Miami (Sanford, 1909; Hoffmeister and Multer, 1968; Weisbord, 1974) and was deposited as a marine oolitic bank or bar system (Hoffmeister et al., 1967; Halley and Evans, 1983). Thickness of the cross-bedded oolite facies is 3-5 m and is greatest along the seaward edge, both in Miami and in the Lower Keys.

Fig. 5-4. Geometry of the Lower Keys compared with topography of the southeast Florida coast west of Biscayne Bay. (A) Generalized shoreline of the Lower Keys with the islands (stippled) showing broad topography with elongation parallel to tidal channels. (B) Topography in the Miami-Homestead area west of Biscayne Bay showing areas higher in elevation than 2.7 m (9 ft) for comparison with A. (C) Index showing relation of areas in A and B to the South Florida Peninsula. The similarity of geometry derives from the fact that both areas are underlain by Pleistocene oolite that retains the topography of ooid sand shoals. [larger image]

The Key Largo Limestone in the Keys consists of hermatypic corals with intra-and interbedded calcarenites and thin beds of quartz sand. According to Hoffmeister and Multer (1968), the thickness of the Key Largo varies widely, but more than 60 m was identified from core borings at Big Pine Key. The upper part of the formation is visible in many cuts and quarries in the Upper Keys. In the terminology of Dunham (1962), the upper Key Largo consists of boundstones, grainstones, wackestones, and packstones (Harrison et al., 1984; Shinn et al., 1989). Coral geometries vary with phyletic groups and conditions of preservation, but, in general, the formation contains a surprising amount of coral in growth position (Hoffmeister et al., 1964). Coral boundstones are interbedded with and surrounded by grainstones and wackestones, with the highest concentrations of corals at the topographically higher positions beneath the islands. Seaward of the islands, grainstones are common in the Key Largo; in contrast, wackestones and packstones dominate on the Gulf of Mexico side of the Keys. Coral distribution appears to occur in large, elongate patches several hundred meters wide and several kilometers long, generally conforming to the geometry of the islands (Stanley, 1966; Hoffmeister and Multer, 1968).

Although there is general agreement that the Key Largo Limestone represents a Pleistocene reef tract, there are significant geometric and biologic differences between the Key Largo reef tract and the modern reefs. These differences gave rise to a small controversy in the 1960s about what kind of reef the Key Largo might have been. As shown on Fig. 5-1, the Key Largo trend is parallel to the present reef tract and set back from it by about 7 km. The Key Largo trend is wider than the present reef tract, which is simply a discontinuous line of linear shelf-margin reefs, none longer than a few kilometers. In contrast, the island of Key Largo itself is about 70 km long.

The differences between the Key Largo and the present reefs extend to faunal composition and implied wave energy. Most striking is the absence of Acropora palmata (Stanley, 1966; Hoffmeister and Multer, 1968), which is the principal reef-crest structural element of the modern outer reefs (Ginsburg, 1956; Shinn, 1963). The main framework element of the Key Largo is Montastrea annularis, implying lower wave energy (Stanley, 1966). In addition, Millepora and coralline algae are underrepresented in the Key Largo, and Halimeda is overrepresented in the Key Largo relative to the modern reef tract (Hoffmeister and Multer, 1968). Stanley (1966) proposed, therefore, that the Key Largo formed as a relatively deep water, outer reef tract, at water depths comparable to the modern Montastrea zone of West Indian reefs (6-15 m; Storr, 1964); he did not believe that the Key Largo formed as a line of coalesced patch reefs behind a now-absent outer reef tract, because he could not accept that any such outer reef tract could have been eroded in the time available post-depositionally while the Key Largo continued to be preserved.

Hoffmeister and Multer (1968) rejected the deep-water interpretation of Stanley (1966) on evidence of a strandline at an elevation of about 7.5 m (25 ft) in the Miami Limestone (in Miami) that was then (and now) thought to be contemporaneous with the Key Largo fossil reef tract. Hoffmeister and Multer (1968) argued strongly for a backreef interpretation, accepted the implied erosion, and added the possibility of structural tilting as a means of lowering the outer platform. Also, Hoffmeister and Multer (1968) encountered fragments of Acropora palmata at 17 m (58 ft) below sea level in a core near Looe Key Reef at the edge of the platform and considered them to be remnants of the missing outer reef tract contemporary with the Key Largo. Dodd et al. (1973) reviewed the arguments of Stanley (1966) and Hoffmeister and Multer (1968) and pointed out that reefs off Newfoundland Harbor Keys, although less extensive than the Key Largo Limestone, share many similarities with the Pleistocene reef.

The work of Perkins (1977) and Enos (1977) sharpened the dilemma. From considerations of regional relationships (South Florida) and what was known of late Pleistocene sea levels in general, the interpretation of shallow water for the Key Largo was acceptable (Perkins, 1977). On the other hand, the hypothesized coeval outer reef tract could not be accepted. According to Perkins (1977), regional isopachs, facies patterns, and shelf gradients also ruled out tectonic adjustments during the Pleistocene. According to Enos (1977), the highest culminations of the Pleistocene surface on the outer reef tract are 8-11 m below present sea level. Adding the ~7-m layer above present sea level, the implied erosion of the hypothetical outer reef and associated sediments would be 15-18 m - over an area 5 to 8 km wide and more than 100 km long (Harrison and Coniglio, 1985). In order to resolve the dilemma of shallow-water origin and absent outer barrier, Perkins (1977) proposed that the Key Largo is the leading edge of a complex of patch reefs and sand shoals that were initiated near the outer shelf edge and migrated laterally as sea level rose.

The presently accepted interpretation is that of Harrison and Coniglio (1985): the Key Largo Formation is a complex of shallow-water shelf-margin reefs and associated deposits along a topographic break; the absence of Acropora palmata and other biotic differences is due to environmental stress. The concept of environmental stress derives from observations of the modern depositional system (Ginsburg and Shinn, 1964; Marszalek et al., 1977). As discussed in the Case Study, modern reefs are best developed seaward of protective islands; the reefs are not developed opposite intervening passes. The islands shield the reefs from injurious waters generated in the shallow lagoon behind them. At the time the Key Largo reef was alive, there were no insular shields and the shoreline was 160 km or more farther north (Perkins, 1977).

Hoffmeister et al. (1964) found that the oolite facies of the Miami Limestone passes laterally into the Key Largo Limestone at the southeastern point of Big Pine Key (Fig. 5-5). They also found that the oolite in the rest of the island is underlain by the Key Largo Formation. The facies intergradation and superposition of the two units were mapped in detail by Coniglio and Harrison (1983a) using cores from nine shallow wells drilled by the U.S. Geological Survey (Fig. 5-5). As indicated by these cores, the contact between the Miami and Key Largo Limestones in most places in Big Pine Key lies at a depth of 4-6 meters (Hanson, 1980). The facies transition between the two units in southeastern Big Pine Key occurs laterally over a few hundred meters (Kindinger, 1986).

Fig. 5-5. Geologic map and cross section of Big Pine Key (adaped from Vacher et al., 1992, after Coniglio and Harrison, 1985). Inset shows relation of Key Largo and Miami Limestones to stratigraphic nomenclature of Perkins (1977). [larger image]

Harrison and Coniglio (1985) also studied the Key Largo Limestone on the island of Key Largo (Fig. 5-6), its type locality in the Upper Keys (Fig. 5-1). They confirmed the presence of subaerial discontinuity surfaces and traced Perkins' (1977) three shallowest units, Q5, Q4 and Q3, among ten shallow borings on the island. They noted the presence of a quartz sandstone layer about 10 m below the surface just above the boundary between Q3 and Q4. Harrison and Coniglio (1985) demonstrated that a topographic high beneath Key Largo persisted through several sea- level highstands. Recent drilling off both sides of Key Largo island suggests that the Q5 portion of the Key Largo Limestone is an isolated ridge, with Q4 exposed at the surface on either side of the Key. The underlying Q4 ridge served as a focus for reef growth and is a partial explanation for the location of the modern Florida Keys (Fig. 5-3). Antecedent topography is generally accepted as a fundamental control of reef position (Tucker and Wright, 1990, p. 204) and is likely to have influenced several episodes of reef growth in South Florida during various Pleistocene sea-level highstands.

The Key Largo Limestone and Miami Limestone have experienced a variety of alteration processes that are typical during early freshwater diagenesis of shallow-water marine carbonate sediments. The Keys have been exposed to subaerial diagenesis since they emerged from the sea about 125 ka (Hoffmeister and Multer, 1964). The highest elevations in the Keys are close to the estimated maximum sea level at that time, so exposure was practically coincident with sea-level lowering. Such exposure of the marine carbonates to meteoric weathering has produced widespread minor karstification (Dodd and Siemers, 1971). A peculiar and spectacular exception is a recently identified sinkhole seaward of Key Largo, 600 m in diameter and as much as 100 m deep, now filled with Holocene sediments (Shinn et al., 1996).

Fig. 5-6. Topographic map of Key Largo (after Harrison and Coniglio, 1985) showing positions of important coral reefs formed by living Acropora palmata at the shelf margin east of the island. [larger image]

Within the formations, diagenetic alteration is typical of what occurs when metastable marine carbonate minerals interact with meteoric pore fluids (Friedman, 1964; Land, 1967; Halley and Harris, 1979). Alteration processes include dissolution of aragonite, loss of magnesium from high-Mg calcite, precipitation of calcite cement, and the development of secondary porosity. In the upper portions of these formations, metastable minerals are still present (Q4 and Q5 units of Perkins, 1977). In the older units, metastable minerals are absent, and diagenetic processes altering high-Mg calcite and aragonite to low-Mg calcite have gone to completion.

An important result of these diagenetic processes is that net porosity in these formations has changed little, although the mineralogy and permeability are radically altered compared to those of modern sedimentary analogs. Wholesale chemical and pore-distribution changes are affected during early freshwater diagenesis without massive pore-volume reduction (Halley and Schmoker, 1983; Halley and Evans, 1983). The result is a rock with high porosity and permeability - as is characteristic of most carbonate islands throughout Florida and the Bahamas.

Substage 5e. Radiometric age dating (²³⁰Th/²³⁴U) has been attempted several times using corals and oolite from the Q5 unit of Perkins (1977), and the results have been reviewed in detail by Muhs et al. (1992). The first attempts were by Broecker and Thurber (1965) and Osmond et al. (1965), early in the development of U-series methods. As noted by Muhs et al. (1992), the samples from those studies that were 95-100% aragonite gave ages of 90 ± 9 and 120 ± 10 ka for oolite from the Miami Limestone and a range of ages from 95 ± 9 to 145 ± 14 ka for coral from the Key Largo Limestone. The latter range is comparable to the spread of comparable-vintage (alpha-spectrometric) values from corals of the Rendezvous Hill Terrace of Barbados [q.v., Chap. 11] as reported by Ku et al. (1990) - the unit in the classic Barbados stratigraphy that is unarguably associated with isotope substage 5e.

Later, a coral from Windley Key quarry was used by Harmon et al. (1979) as part of the Uranium-Series Intercomparison Project. The mean age of the coral was 139 ka. As noted by Muhs et al. (1992), the sample contained only 90% aragonite; thus, the relatively large age might reflect some loss of U because of recrystallization. Finally, Muhs et al. (1992) added an alpha-spectrometric result of their own: 144 ± 8 ka for a coral taken from a drill core on Key Largo island. According to Muhs et al. (1992), this coral, like the others before it, did not meet all the criteria that assure a closed system.

Without question, the Q5 unit of the Florida Keys formed during the interval of high sea levels associated with the last interglacial. As Muhs et al. (1992) note, however, the uncertainty in the ages prevents precise correlation of the oolitic shoals of the Miami Limestone with the Key Largo reefs and determination of how, exactly, these deposits fit with the ~125-ka peak of the deep-sea oxygen-isotope curve (substage 5e). Muhs et al. (1992) list three alternatives: (1) the Key Largo reefs possibly formed in an early peak, and the Miami oolite in a slightly later one; (2) the two may be correlative over a relatively long interval of high sea level; (3) perhaps the whole question in the Keys should await re-examination using mass-spectrometric methods and better samples (if they can be found).

Younger rocks. Lidz et al. (1991) have identified a series of reef ridges a few hundred meters seaward of the modern, shelf-edge reefs in the Lower Keys. These fossil features, termed outlier reefs, are 10-20 km long, up to 30 m high, and < 1 km wide. They crest at water depths of 12-30 m and are covered with thin Holocene coral rubble. Two cores from the ridges indicate that the veneer is less than 1 m thick and that it overlies a variety of massive corals in growth position, among which Montastrea annularis is the most common. Mass-spectrometric U-series dating of corals (Ludwig et al., 1996) indicate this reef grew at about 80 ka and is correlative with oxygen isotope stage 5a. These outlier reefs may be more similar to the Key Largo Limestone in composition and geometry than to any living reef.

Older rocks. Pre-stage-5 geochronology is more tenuous. A pre-stage-5 Quaternary section is certainly present in the subsurface, as indicated by the subaerial discontinuity surfaces and the stratigraphy proposed by Perkins (1977). There is a single radiometric date (Szabo and Halley, 1988; Muhs et al., 1992) from the core on Key Largo, where the 144-ka Q5 coral was obtained. According to Muhs et al. (1992), this older coral was the only aragonitic coral recovered from the Q4 unit in the drilling reported by Harrison and Coniglio (1985), and it gave an age of 361 + 120/ -60 ka. Muhs et al. (1992) also noted that addition of ²³⁴U was indicated, and thus the age must be considered a minimum. This result from the Keys, together with results from corals regionally distributed in southern peninsular Florida, led Muhs et al. (1992) to conclude that there was extensive deposition of marine sediment during the middle Pleistocene. They found no evidence, however, that any of their middle Pleistocene samples were as young as isotope stage 7 (ca. 200 ka).

The early work of Mitterer (1974, 1975), using amino-acid racemization (AAR) geochronology, is also relevant to the interpreted age of the Q units of Perkins (1977). D-alloisoleucine/L-isoleucine (or A/I) ratios on Mercenaria at a variety of South Florida localities defined three groups that Mitterer (1975) and Perkins (1977) correlated with the Q5, Q4, and Q3 units. Using the U-series age and A/I ratio for Q5 as a calibration point, Mitterer (1975) obtained the following ages: 170-191 ka for Q4, and 212-223 ka for the Q3. More recently, Mitterer has developed a new calculation technique for estimating ages from A/I ratios (parabolic kinetics; Mitterer and Kriausakul, 1989) and applied it to Bermuda (Hearty et al., 1989). Vacher used the new technique to recalculate ages from Mitterer's South Florida ratios and obtained the following results for units below the Q5 calibration horizon (Vacher et al., 1992): 184-216 ka for Q4, and 245 to >260 ka for Q3. These results are comparable to those in Bermuda [q.v., Chap. 2] for the Belmont Formation and the upper member of the Town Hill Formation, respectively, which are correlated by Hearty et al. (1992) with isotope stages 7 and 9, respectively.

The coral data, which do not reveal the presence of stage 7, and AAR data are not necessarily contradictory because these Pleistocene units can be extremely discontinuous. The stage-7 Belmont Formation in Bermuda is small and patchily distributed, in contrast to deposits in Bermuda of stage 5 and those interpreted as stage 9. In South Florida, Holocene carbonate deposits are not continuous, and large areas of Pleistocene "bedrock" are exposed offshore on both sides of the Keys (Enos, 1977). If stage-7 deposits are present in the Keys, they may well be missing in many drill cores. If five Q units occur in one drill core, and five Q units occur in another, then it may be too much to expect that the two sets correlate unit-to-unit. The possibility that the older Pleistocene units of the Keys are discontinuous illustrates the difficulty of resolving the stratigraphy in detail.