Geologic History of Grecian Rocks, Key Largo Coral Reef Marine Sanctuary E

BULLETIN OF MARINE SCIENCE, 30(3): 646-656. 1980 CORAL REEF PAPER

GEOLOGIC HISTORY OF GRECIAN ROCKS, KEY LARGO CORAL REEF MARINE SANCTUARY E. A. Shinn ABSTRACT Two transects, consisting of seven 8- to 14-m deep core holes, were drilled across the major ecologic zones of Grecian Rocks (25°06'06"N, 80oI8'03"W) in the Key Largo Coral Reef Marine Sanctuary to determine its internal anatomy and age. Approximately 750 by 200 m in size, the reef accumulation was found to be controlled by a local Pleistocene topographic feature. Facies in the underlying Pleistocene correspond closely with the various coral facies in the overlying Holocene reef. Grecian Rocks Reef is composed of five major ecologic zones: (I) a deep seaward rubble zone ranging in depth from 6-8 m; (2) a poorly developed spur and groove zone composed of massive head corals and Mi//epora (4-6 m water depth); (3) a characteristic high-energy oriented Acropora palma/a zone extending from the surface down to 4 m; (4) a distinct broad reef flat composed of in situ A. palma/a and coral rubble, followed by (5) a narrow low- energy back-reef zone of unoriented A. palmata. thickets of A. cervicornis. and various massive head corals in water 0-3 m deep. An extensive grass-covered carbonate sand flat 3-4 m deep extends in a landward direction from zone 5. Cores revealed that all the zones except the massive coral head zone are superficial coat- ings over a carbonate sand and rubble accumulation. A thin I-m thick layer of lime mud and peat was found 11.5 m below sea level on the Pleistocene bedrock beneath the sand and rubble in the reef flat core hole. Carbon-14 analyses of coral from 7 m below the reef surface indicate that the reef began growing approximately 6,000 years before present.

Grecian Rocks, at 25°06'06"N, 80018'03''W within the recently created Key Largo Coral Reef Marine Sanctuary (Fig. 1), is the most popular and extensively studied reef within the sanctuary. In addition to being a favorite diving area with commercial tour boat operators, Grecian Rocks is frequented many times each year by educational groups, including both biologists from universities and high schools and geologists from universities and petroleum companies. Its popularity is due mainly to clear water, coral diversity, coral zonation, and relatively calm back-reef area, suitable for snorklers even during rough seas. The surface zonation of the dominant corals at Grecian Rocks was described by Shinn (1963; this paper, Fig. 2). The reef has been the site of other geological studies (Shinn, 1966, 1976; Ball et aI., 1967; Perkins and Enos, 1968). Most of these studies have been concerned with hurricane effects and recovery following these disasters. Because previous studies were directed toward surface features and little was known about the subsurface, U.S. Geological Survey scientists initiated a study of the three-dimensional aspects of Grecian Rocks, including not only zonation at depth but also the thickness and accumulation rate of the reef. To determine what controls the location of Grecian Rocks was of particular concern. With the permission of the Coastal Zone Management Office of the U.S. National Oceanic and Atmospheric Administration (NOAA), seven rotary core holes were drilled into the reef. The results presented in this report are based on the drilling of these holes.

METHODS

An underwater hydraulically openited drilling machine was suspended from a 4-m-high tripod and fitted with 1.5-m core barrels (BX and NX sizes) and 1.5-m drill rods (N rods). Using carbide core

646 SHINN: GEOLOGIC HISTORY OF A KEY LARGO REEF 647

______------7 •...... : " . •'\.."",t": , " I ,,----.::.;;---- "'------' ,-' ,I I • , I , I '.•• , .0 ... , I· .••" .. , I '.r-r ~ , I A- I ..••.•... : .•.. A. ." • '~l ,,------.- ~ .-_..J / I , I , -,"" / I ,.----- ~ I • I I .~, ------~ I ,,~--'-.:,. '------Figure I. Map of the Florida reef tract showing the boundaries of the Key Largo Coral Reef Preserve (dashed line), the Grecian Rocks reef, and the outline of area shown in aerial photo in Figure ]0. bits, the drilling reached depths of 12 m or more, penetrating the entire Holocene rock section as well as 1-2 m of the underlying Pleistocene limestone upon which the reef initiated. The drill, pat- terned after one described by Macintyre (1975), was used both above and below the water (Figs. 3, 4). Locations of the core holes are shown in Figures 5 and 6. Cores of reef material were examined in the field and later in the laboratory, where unaltered coral was selected for 14Cage determinations. All 14Canalyses were done at the University of Miami's Radiocarbon Dating Laboratory under the direction of J. J. Stipp. The amount of material that could be dated was limited because of poor core recovery; therefore, all dated material is from cores 3 and 4. Poor recovery was caused by unconsolidated reef sand, which was found to underlie most of the reef. Corals and coral rubble existed only in the upper 1-4 m, and only in core 4 did a complete section of coral extend down to the underlying Pleistocene bedrock. Although the core barrel used would not recover sand, the nature of the unconsolidated carbonate sand section could be determined in the field because in most cases sand was brought to the surface by drilling fluid, which in this case was sea water. Near the base of the Holocene section, silt and mud-sized carbonate were often encountered. In some cases, the mud was sticky enough that some was retained in the core barrel. In core 5, peat and soil-like materia] were recovered along with lime mud at a point less than I m above the Pleistocene bedrock. Unfortunately, there was an insufficient amount of peat for a 14Cage determination.

ACROPORA Za-JE Figure 2. Exaggerated block diagram of Grecian Rocks showing major zones discussed in text (from Shinn, 1963). Scale in feet. 648 BULLETINOF MARINESCIENCE,VOL.30, NO.3. 1980

Figure 3. (Left) Drilling on the reef flat at location 5. (See Figs. 5 and 6 for location.) View is toward the northwest. Note U,S, Geological Survey boat, HALIMEDA, which contains water pump and hydraulic power plant for drill. Boat in background is R/V SEA ANGEL, which served as base for entire one-month operation, Figure 4. (Right) Same drill as shown in previous figure being used underwater to drill core at location 4. (See Figs. 5 and 6 for location,) Paired hoses carry hydraulic power fluid. Single hose provides sea water used for drilling fluid.

SURFACE ZONATION Grecian Rocks has a distinct coral and hydrocoral zonation. Zonation of major reef-builders is shown in Figure 5 and core locations in Figure 6. Grecian Rocks has been divided into five zones by Shinn (1963; this paper, Figs. 2, 6). The first of these is a rubble zone lying in 7-8 m of water, and is composed of coral rubble in a matrix of Halimeda sand. Much of the rubble acts as a substrate for the holdfasts of gorgonians and other soft corals. Occasional large corals, mainly star corals (Montastraea sp,) and brain corals (Colpophyllia sp., Sider- astrea sp., and Diploria sp.), grow in the rubble zone. The rubble zone is flanked to the east and southeast by Halimeda sand containing only scattered fragments of dead coral. The detailed composition of carbonate sands in the Florida reef tract has been described by Ginsburg (1956) and Swinchatt (1965). A small rocky patch consisting mainly of dead coral lies a few meters east of core hole 7 (Fig. 5). The second, a Montastraea-Millepora Zone, populated by massive corals dominated by the stinging hydrocoral Millepora, begins abruptly in 5-7 m of water leeward of the rubble zone and extends landward to a depth of 3-4 m. Core hole 3 was drilled near the seaward edge of this zone, and core 4 was drilled near its leeward edge (Fig. 6). SHINN: GEOLOGIC HISTORY OF A KEY LARGO REEF 649

Figure 5. Oblique aerial view of Grecian Rocks looking north. Core locations 1-7 are shown by black dots. Seabottom in upper left-hand corner populated by turtle grass. White areas are grass-free sand holes or "blowouts" made during hurricanes. White area in bottom right corner of photo is carbonate sand. Note small dark rock patch to the right of core 7.

Zone three, the most spectacular one, is the zone of oriented Acropora palmata, which ranges in water depth from approximately 50 cm down to 3-4 m. The transition from the Montastraea-Millepora Zone to the Acropora Zone is generally abrupt and marked by a pronounced terrace (Fig. 2). In many places the seaward edge of this zone forms a vertical cliff 1-2 m high. The Acropora palmata Zone receives the brunt of sea swells and waves. The orientation of A. palmata is a response to the predominantly unidirectional wave attack. Branches of A. palmata in this zone are mostly oriented landward, i.e., away from the direction of oncoming waves. Any laterally protruding branch growth is periodically broken by storm waves (Ball et aI., 1967; Shinn, 1963). The A. palmata Zone was not drilled because of possible damage to live coral and divers. The largest coral zone at Grecian Rocks, the reef flat, is the fourth zone. The reef flat is composed predominantly of unoriented A. palmata, which has grown upward to spring low tide level. Most of the coral is dead because of overcrowd- ing; however, A. palmata forms a living band around the leeward side, where it is apparently extending the boundaries of the reef flat. On the seaward side, the reef flat is transitional with the oriented A. palma to Zone. Here and there, A. palmata colonies have been tipped on their sides, causing branches to protrude 30 or 40 cm above the water at low tide. In some cases, the protruding "rocks" are composed of completely overturned colonies that have been transported several meters during hurricanes. Zone five consists of scattered colonies of delicate, unoriented A. palmata and forests of A. cervicornis that thrive in the quiet waters immediately leeward of the reef flat. Large heads of the massive corals Montastraea annularis, Diploria strigosa, and Colpophyllia natans are also scattered throughout this back-reef area. Between coral colonies, the bottom is composed of storm-derived rubble 650 BULLETIN OF MARINE SCIENCE. VOL. 30. NO.3. 1980

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PERHHUGE TOTAL HOUH fROM EACH DIRECTIOH Figure 6. Map of Grecian Rocks showing major zones. drill locations, and two cross sections, NW- SE and NE-SW (shown in Figs. 7 and 8). Wind roses show wind direction based on U.S. Weather Bureau data (modified from Shinn, 1963). in a matrix of carbonate sand. Ball et aI. (1967) found that some of the back-reef rubble was transported from locations well seaward of the reef flat during Hur- ricane Donna in 1960.

Spurs and Grooves Because Grecian Rocks is located more than I km from the platform margin, it receives less wave action than the outermost reefs. Consequently, spurs and grooves, which are a response to continual wave action, are less well developed. Although reduced in size, the channel-like grooves oriented perpendicular to the overall trend of the reef are the most spectacular geomorphic features at Grecian Rocks. Grooves transect both the Montastraea-Millepora Zone and the oriented Acropora palmata Zone and in some cases extend a short distance into the reef flat (Fig. 5). Spurs and grooves shown in Figure 2 are somewhat exaggerated.

The Third Dimension: Results of Core Drilling Locations of all core holes are shown in Figures 5 and 6 and in the interpretive cross sections in Figures 7 and 8. The average thickness of the reef was found to be about 10 m, and approximately half of the material under the reef is carbonate sand, which was not recovered. The best core recovery was in core 4, which penetrated the Montas- traea-Millepora Zone. The principal corals encountered were Montastraea annularis and M. cavernosa. Similar corals were recovered from the upper half of core 3. Interestingly, the same species of Holocene corals were recovered from the Pleistocene in core 4, whereas elsewhere, except in the very bottom of core 7, the Pleistocene consisted of poorly cemented carbonate sand. In core 5 taken on the reef flat, coral, principally Acropora palmata, was SHINN: GEOLOGIC HISTORY OF A KEY LARGO REEF 651

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Figure 7. Cross section NW-SE at Grecian Rocks showing thickness of Holocene, depth to Pleis- tocene, facies changes, and radiocarbon ages. Solid portion of core holes indicates recovery. Clear area indicates no core recovery. Stippled pattern is carbonate sand. See text for description of coral zones. recovered only from the upper 3 m, suggesting that probably all of the reef flat overlies carbonate sand, as shown in Figures 7 and 8, Apparently, the reef flat has been extending laterally in a leeward direction. Although difficult to confirm in a small-diameter core, a combination of core and surface observation indicates that lateral accretion is accomplished mainly by corals establishing themselves on storm-derived rubble periodically transported onto the leeward side of the reef flat.

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Figure 8. Cross section NE-SW at Grecian Rocks (symbols same as in Fig. 7). 652 BULLETIN OF MARINE SCIENCE. VOL. 30, NO.3, 1980

Unfortunately, lush coral prevented drilling in the oriented Acropora Zone between the sites of cores 4 and 5. Because data from this area are lacking, the three-dimensional relationships between the Montastraea-Millepora Zone in core 4 and the Acropora palmata Zone and carbonate sand in core 5 are difficult to determine. As shown in Figure 7, Montastraea is thought to underlie the seaward part of the oriented Acropora palmata Zone. The transition from a section consisting of A. palmata over carbonate sand to A. palmata over Montastmea occurs somewhere between cores 4 and 5. Only more closely spaced core holes can determine where the change occurs. Although the oriented Acropora palmata Zone is growing seaward and has grown out over a portion of the Montastraea- Millepora Zone, most probably the bulk of lateral growth has been in a leeward direction. This interpretation is compatible with results of drilling other Florida reefs (Shinn et aI., 1977) and interpretation of reef growth at Alacran Reef (Logan, 1969). Cores 6 and 7 penetrated a section composed almost entirely of carbonate sand. Unfortunately, the underlying Pleistocene limestone at the site of core 7 was so poorly cemented that the precise contact between Holocene and Pleistocene rocks could not be determined accurately. The importance of this boundary location will be discussed later. One of the most surprising discoveries was the presence of lime mud and peat at the base of core 5. Although pollen content of the peat has not been analyzed, it may be of mangrove origin because of its appearance and relationship with lime mud. Similar associations of lime mud and mangrove peat can be found along the western side of the Florida Keys and within Florida Bay. Significance of the peat and lime mud will also be discussed later.

Age and Accumulation Rate

The oldest 14C date (5,950 ± 100 years) was obtained from a sample near the base of core 4 (Fig. 7). This date suggests that the reef started growing around 6,000 years ago. The youngest date (3,230 ± 75 years) was obtained from core 3, approximately 1.5 m down from the surface (Fig. 7). In Core 4 the dates for the upper two samples (see Fig. 7) are reversed, that is, the sample deeper in the core was 150 years younger than the sample above. The two samples are only 1 m apart, and the age difference of 150 years is within the margin of experimental error for the carbon dating method. Growth rates of Montastraea annularis determined by J. H. Hudson (oral communication) indicate that 1 m of upward growth of this species can occur in less than 150 years; however, reef accumulations seldom grow upward at a rate equal to the growth rate of the constituent corals. If Hudson's rate is valid, the 8-m section penetrated by core 4 could have accumulated in less than 1,000 years. Instead, 14C analysis indicates that accumulation took at least 6,000 years (which is still very rapid geologically). The great difference between the theoretical and actual accumulation rates is attributed to periodic die-offs and bioerosion (Hudson et aI., 1976; Hudson, 1977). Pleistocene bedrock in core 3 was dated to confirm a visual interpretation of Pleistocene age. The 22,740 ± 300-year date supports this interpretation (Fig. 7). However, the true age is probably around 100,000 years, as such rocks are generally contaminated with younger material. Contamination by a minute amount of young material generally gives a false date that is much younger than the true age. SHINN: GEOLOGIC HISTORY OF A KEY LARGO REEF 653

Figure 9. Upper surface of Pleistocene bedrock from cores 2, 4, 5, and 6. All show soil features. Note dark laminated crust at top of core 4 and brown soil-stone features in cores 2 and 6. Barely visible roots occur in upper portion of core 5. Note that only core 4 is coral.

Pleistocene Bedrock Pleistocene bedrock was generally easily recognizable in cores. A soilstone crust, indicating subaerial exposure and soil conditions, was recovered from core hole 4, and caliche-like patches and root marks were present in cores 2, 4, 5, and 6 (Fig. 9). The origin and meaning of such crusts in Florida have been described by Multer and Hoffmeister (1968), Perkins (1977), and by numerous other authors. Such crusts, 1-2 cm in thickness, require at least 5,000 years to form (Robbin and Stipp, 1979). Although from one to several meters of Pleistocene limestone was penetrated in each core hole, coral was found only in core 4 and in the lower portion of core 7. For the most part, the bedrock consisted of poorly cemented carbonate sand 654 BULLETIN OF MARINE SCIENCE. VOL. 30. NO.3. 1980

Figure 10. Aerial photomosaic showing the reef tract and White Banks in the area of Grecian and Key Largo Dry Rocks. Truncated pyramid outline with arrow shows direction of view of aerial oblique photo in Figure 5. White areas are carbonate sand. Most dark areas are populated by marine grasses. Dark spots with white circular halos are coral patch reefs. Note linear trend of Grecian Rocks, Key Largo Dry Rocks, and an unnamed reef in between. Several other trends are obvious to the seaward of Grecian Rocks. similar to that presently accumulating adjacent to coral reefs in the Florida Keys. Except for the presence of Acropora pa/mata in the Holocene section and its absence in the Pleistocene section, Holocene facies and biota appear to mimic those that existed during the Pleistocene.

DISCUSSION Closely spaced core drilling on other reefs in the Florida Keys (Shinn et aI., 1977) indicates that reef location and trend are controlled by underlying bedrock topography. High areas, such as ridges, provided a suitable surface for coral attachment following the last postglacial transgression, which flooded the area 6,000 to 7,000 years ago. Corals grew on the highs but could not become estab- lished in the low areas because of rapidly accumulating sediment. Lows filled with sediment while coral reef growth continued over the topographic highs. Grecian Rocks, Key Largo Dry Rocks to the north, and a smaller unnamed reef between the two form a distinct linear trend on charts and air photos (Fig. 10). Seaward of Grecian Rocks there are at least three other parallel reef trends. Hence, the underlying topography is suspected in controlling the position of both Grecian Rocks and Key Largo Dry Rocks. Cores 3 through 7 were drilled to test SHINN: GEOLOGIC HISTORY OF A KEY LARGO REEF 655 this hypothesis. Figure 7 shows that a topographic difference exists between cores 3 through 6, but the greatest relief might occur seaward of core 3. Core 7 was thus drilled for confirmation. Unfortunately, as mentioned earlier, a precise determination of where the drill bit entered the Pleistocene was difficult because the rock is poorly consolidated. Based on subtle differences in drilling rate and the "feel" of the drill, the contact was picked at approximately 6 m. This depth cannot be confirmed as no core was recovered from this interval, but it is deemed accurate at this time. If 6 m is correct, a distinct topographic change occurs between core 3 and core 7, as shown in Figure 7, and this linear "cliff' in the bedrock controls the trend of the three reefs mentioned above. The presence of lime mud and peat at the base of hole 5 requires a seaward barrier. Hydrographic barriers would have been necessary to prevent erosion and removal of the mud and peat as sea level rose and flooded the area. Drilling did not confirm the presence of a topographic barrier seaward of core hole 5 (Fig. 7). At least one core must be drilled between cores 4 and 5 to verify the existence of a topographic barrier. Nevertheless, such a barrier must be present.

Historical Reconstruction The above considerations show that, sometime before 6,000 years ago, the rising sea flooded a Pleistocene terrace composed mainly of a carbonate sand bank that had corals along its seaward margin. This bank was probably analogous to the present-day White Banks, a major sedimentary feature off Key Largo composed of carbonate sand with patch reefs dominated by Montastraea annularis scattered along its seaward margin. This linear feature probably became an island similar to the present Florida Keys, both during the initial stage of sea level fall and again as sea level rose. During exposure, it became capped in places (core hole 4) with a soilstone crust indicative of soil development (Perkins, 1977; Multer and Hoffmeister, 1968; Robbin and Stipp, 1979). In addition to the soilstone crust, roots and caliche patches were found in several other cores (i.e., cores I, 5, and 6), all giving evidence that the area was once land and supported soil and vegetation. As sea level rose, mangroves, mangrove peat and lime mud probably accumulated on the leeward side of the "island," just as they are presently accumulating on the leeward side of Key Largo. As the sea eventually flooded this feature, the soil and vegetation were washed away, leaving only the crusts and some roots. Peat and mud were preserved in the protected lee in the vicinity of core hole 5. Corals rapidly populated the high areas, and carbonate reef sand buried and preserved the mud and peat in the lee. The initial fauna was limited to the more resistant coral varieties, such as Montastraea, Porites, and other massive forms. Acropora palmata and A. cervicornis, both species that require open, clean, well-circulated water normally found near the outer reefs, did not emigrate into the area until a further rise in sea level converted the area into one of open circulation and a more stable temperature regime. Thus, A. palmata and A. cervicornis, which require more oceanic conditions, did not appear until the sea had risen nearly to its present level and at least 5 m of reef and reef sediments had accumulated. At that time, between 3,000 and 4,000 years ago, the reef composition shifted from being one dominated by Montastraea annularis to one of predominantly Acropora species. Upward and lateral reef accumulation from that time has been the result of Acropora growth, aided only minimally by the same kinds of massive corals that initiated the reef. This interpretation is compatible with that of Lighty (1977), although his work showed that the deeper, outermost reefs adjacent to the Florida Straits were initiated by A. 656 BULLETINOFMARINESCIENCE,VOL.30.NO.3, 1980 pa/mata over 7,000 years ago. When those reefs were growing, the inshore areas, such as Grecian Rocks, had not yet been flooded by the sea; thus, the outer shelf margin, where Lighty conducted his study, was at that time essentially the shore- line bathed by clean Gulf Stream waters. Inshore areas were dry land, and there was not a broad, shallow, back-reef area to provide turbid coastal water that could periodically flood and affect coral growth on the platform margin reefs.

ACKNOWLEDGMENTS

Funding for the field work described here was from NOAA's Office of Coastal Zone Management. Carbon-14 analysis was provided by J. J. Stipp of the University of Miami's Radiocarbon Dating Laboratory. The author is deeply indebted to J. H. Hudson and D. M. Robbin for invaluable assistance during drilling operations. Dan Robbin and Barbara Lidz drafted illustrations and Barbara Lidz edited the manuscript. Steve Earley provided underwater photography. The author also thanks Captain Roy Gaensslen of the R/V SEA ANGELfor navigation and assistance with diving operations and equipment. During drilling, we were also aided by C. Phipps, University of Sidney, Australia.

LITERATURE CITED

Ball, M. M., E. A. Shinn, and K. W. Stockman. 1967. The geologic effects of Hurricane Donna in south Florida. J. Geology 75: 583-597. Ginsburg, R. N. 1956. Environmental relationships of grain size and constituent particles in some south Florida carbonate sediments. Bull. Amer. Ass. Petrol. Geol. 40: 2384-2427. Hudson, J. H. 1977. Long-term bioerosion rates on a Florida reef. A new method. Pages 491-497 in Proc., Third Int'l Coral Reef Symposium, Miami, Fla., 2-Geology. --, E. A. Shinn, R. B. Halley, and B. Lidz. 1976. Autopsy of a dead coral reef: Abstract, AAPG-SEPM Annual Meeting Program, New Orleans, La. p. 76. Lighty, R. G. 1977. Relict shelf-edge Holocene coral reef. Southeast coast of Florida. Pages 215- 221 in Proc., Third Int'I Coral Reef Symposium, Miami, Fla., 2-Geology. Logan, B. W. 1969. Coral reefs and banks, Yucatan shelf, Mexico. Pages 129-196 in B. W. Logan, J. L. Hardin, W. H. Ahr, J. D. Williams, and R. G. Snead, eds. Carbonate sediments and reefs, Yucatan Shelf, Mexico. Amer. Ass. Petrol. Geol. Memoir I I. Macintyre, I. G. 1975. A diver-operated hydraulic drill for coring submerged substrates. Atoll Res. Bull. 185: 21-25. Multer, H. G., and J. E. Hoffmeister. 1968. Subaerial laminated crusts of the Florida Keys. Oeol. Soc. Amer. Bull. 79: 183-192. Perkins, R. D. 1977. Depositional framework of Pleistocene rocks in south Florida. Pages 131-198 in P. Enos and R. D. Perkins, eds. Quaternary sedimentation in South Florida. Oeol. Soc. Amer. Memoir 147. --, and p, Enos. 1968, Hurricane Betsy in the Florida-Bahamas area, Geologic effects and comparison with Hurricane Donna. J, Geol. 76: 710-717. Robbin, D. M., and J. J, Stipp. 1979. Depositional rate of laminated soilstone crusts, Florida Keys. J. Sedimentary Petrology 49: 175-180. Shinn, E. A. 1963. Spur and groove formation on the Florida reef tract. J. Sedimentary Petrology 33: 291-303. --. 1966. Coral growth rate, an environmental indicator. J. Paleontology 40: 233-241. --. 1976. Coral reef recovery in Florida and the Persian Gulf. Environmental Oeol. I: 241-254. --, J. H. Hudson, R. B, Halley, and B. Lidz. 1977. Topographic control and accumulation rate of some Holocene coral reefs. South Florida and Dry TOrJugas. Pages 1-8 in Third [nt'l Coral Reef Symposium, Miami, Fla., 2-0eology, Swinchatt, J, P. 1965. Significance of constituent composition, texture, and skeletal breakdown in some Recent carbonate sediments. J. Sedimentary Petrology 35: 71-90.

DATE ACCEPTED: March 3, 1980.

ADDRESS: U.S. Geological Survey. Fisher Island Station, Miami Beach, Fla. 33/39.