Journal of Coastal Research 711-731 Fort Lauderdale, Florida Summer 1997

Contrasts within an Outlier-Reef System: Evidence for Differential Quaternary Evolution, South Florida Windward Margin, U.S.A. Barbara H. Lidz'[, Eugene A. Shinn]', Albert C. Hiner, and Stanley D. Locker'[ t U.S. Geological Survey tDepartment for Marine 600 4th St. South Science St. Petersburg, FL 33701, University of South Florida USA. 140 7th Ave. South St. Petersburg, FL 33701, U.S.A.

ABSTRACT ..

LIDZ, B.H.; SHINN, E.A.; HINE, A.C., and LOCKER, S.D., 1997. Contrasts within an Outlier-ReefSystem: Evidence .tllllllll:. for Differential Quaternary Evolution, South Florida Windward Margin, U.S.A. Journal of Coastal Research, 13(3), ~ 711-731. Fort Lauderdale (Florida), ISSN 0749-0208. euus~~# Closely spaced, high-resolution, seismic-reflection profiles acquired off the upper Florida Keys i i.e., north) reveal a =c platform-margin reef-and-trough system grossly similar to, yet quite different from, that previously described off the .-JUt lower Keys (i.e., south). Profiles and maps generated for both areas show that development was controlled by ante­ cedent Pleistocene topography (presence or absence of an upper-slope bedrock terrace), sediment availability, fluctu­ ating sea level, and coral growth rate and distribution. The north terrace is sediment-covered and exhibits linear, buried, low-relief, seismic features of unknown character and origin. The south terrace is essentially sediment-free and supports multiple, massive, high-relief outlier reefs. Uranium disequilibrium series dates on outlier-reef corals indicate a Pleistocene age (~83-80 ka). A massive Pleistocene reefwith both aggradational (north) and progradational (south) aspects forms the modern margin escarpment landward of the terrace. Depending upon interpretation (the north margin-escarpment reef mayor may not be an outlier reef), the north margin is either more advanced or less advanced than the south margin. During Holocene sea-level rise, Pleistocene bedrock was inundated earlier and faster first to the north (deeper offbank terrace), then to the south (deeper platform surface). Holocene overgrowth is thick (8 m ) on the north outer-bank reefs but thin (0.3 rn) on the south outlier reefs. Differential evolution resulted from interplay between fluctuating sea level and energy regime established by prevailing east-southeasterly winds and waves along an arcuate (ENE-WSW) platform margin.

ADDITIONAL INDEX WORDS: Florida Keys and reef tract (USA); arcuate paleomargin; antecedent Pleistocene bed­ rock topography; upper-slope bedrock terrace; stillstand beach-dune ridges; sea-level fluctuations; protective rock-island barriers; Pleistocene and Holocene coral-reef distribution and thickness; effects ofenergy regime, sediment availability, sedimentary patterns, and water quality on reef vitality; hydrocarbon traps.

INTRODUCTION The reef tract fringes a carbonate-bank windward margin. In the Florida-Bahamas region, several models have been In a comprehensive study to map Pleistocene bedrock to­ proposed for windward margins based on morphologies. I-IINE pography, thickness of overlying Holocene sediments and and NEUMANN (1977) and HINE and MULLINS (1983) de­ reefs, and extent and types of modern ecosystems along the scribed most Bahamian windward margins as either open (no Florida reef tract (LIDZ et al., 1996), geophysical data ob­ island or barrier) or protected (island and reef complex at the tained off north Key Largo in the upper Keys verify a mark­ margin edge). The South Florida morphologies do not strictly edly different subsurface (bedrock) platform-margin mor­ fit either model. In a study of Holocene sediment accumula­ phology and geometry than what is known off Key West in tions along the reef tract, ENOS (1977) proposed a model the lower Keys (Figure 1). The reeftract parallels the arcuate based on shelf morphology: shallow, outer-bank reefs backed margin for more than 385 km and comprises the seaward by a wide, open lagoon with islands set considerably back part of the new Florida Keys National Marine Sanctuary from the margin. A new model was recently developed by ~80 (FKNMS). The roughly rectangular study area covers LIDZ et al. (1991) based on offbank morphology seaward of ~8 km", measures km (from north Key Largo offshore to The Sand Key Reef, located ~ 12 km southwest of Key West (Fig­ ~ Elbow reef) by 10 km (from The Elbow southwest to Mo­ ure 2B). Their model is distinguished by multiple, discontin­ lasses Reef), and includes a portion of John Pennekamp Coral uous, fossil, outlier reefs located on an upper-slope terrace Reef State Park and NOAA's Key Largo National Marine and separated from the margin by deep, wide, backreef Sanctuary (Figure 2A). troughs containing little sediment. The width of the reef-and­ trough system varies but attains a maximum of ~2.5 km. 95077 received 26 July 1995; accepted in revision 15 March 1996. This model proposes that the platform progrades- by build- 712 Lidz et al.

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Figure I. Index map showing lower, midd le, and upper Florida Keys, majo r portion of Florida Keys Na tiona l Marine Sa nctuary (including parts of Florida and Biscayne Bays seawa rd to platform margin ), and primary ree fs discussed in text and shown on other figures. Dott ed line approximates margin esca rpment. San ctua ry boundaries ext end westw ard beyond Boca Grande Key to th e Dry Tortu gas. Bold arrows indicat e direction of water flow from the bay s an d Gulf of Mexico onto the reef tract thro ugh passes between th e Keys . Major passe s ind icated by broad arrow s.Insets A an d B shown in Figure 2A and B, respe ctive ly.

ing seaward "instantaneously" (in geologic time) whe n the ic provinces. All are Pleistocen e bedrock (Figu re 3A,B): (1 ) backreef troug hs become filled. The distance of prog ra dation emergent cora l-reef and oolite tidal-bar lithofacies th at form would equa l the width of the infilled troughs plu s that of the the discontinu ous chain of islands (keys); (2) a broad , trough­ outlier-ree f crests. like depression that forms Hawk Cha nnel; (3) a shelf-ma rgi n The genera l pu rp ose of this paper is to presen t new seismic ridge zone fronted by the massive ma rgin-escarpm ent reef, da ta from the upper Keys and to compa re the m with those both underl ying modern outer-bank reefs; and (4) an upp er­ obtained in the earlier lower Keys study (LIDZ et al., 1991). slope terrace. The island chai n is divided into three groups: In the compa rison, th e general purpose becomes more specif­ upper, middl e, and lower Keys (Figu re 1). The upper and mid­ ic: to exa mine the striking differ ences in platform -margin dle Keys are lump ed by orien tation (NE-SW) but sepa ra ted mor phologies, to consider problems with their interpretation, by differences in ti da l-pass width. They form an arc that ex­ a nd to relate th e role of fluctuatin g sea level to differ ences in te nds from Soldier Key near Miam i to Big Pin e Key to the growth a nd distribu tion of Quaternary reefs bet ween the ar­ southwes t, a distance of 177 km (HOFFMEISTER, 1974). The eas. One cave at pertains. Given the sho rtco mings of insu ffi­ middle Keys extend from Lower Matecumhe to Big Pine Key cient evidence other tha n high-quality seismic records, par­ and conta in the largest, deepest tida l passes tha t link th e ticularly lack of ra diometric control, the approac h to morpho­ mur ky waters of Florida Bay and th e Gulf of Mexico with the logic interpretation is to offer alte rnatives.Within this con­ clea re r reef-tract wate rs . The lower Keys are oriented nearly text, th en, the alte rnatives are in tended more to provide perpendicular (NW-SE) to the arc and exte nd from Big Pine ideas for future research in the region than to reach a firm Key to Boca Grande Key in the Gul f west of Key West, a conclusion on which is the more pla usible. Nonetheless, in ~ our prelud e we also present three previously docum ented el­ distan ce of 130 km. The reef tract an d FKNMS extend an­ ements (termed PDE ) that provide important clues as to othe r 80 km westward and include the Dry Tortugas. which alte rnative may be the more likely. The difference in orientation between the upp er and middl e Keys and the lower Keys is du e to paleoenvironm ent, which Physiographic Setting produced the old shore -pa ra llel cora l-reef complex flanked by The Florida Keys and offshore reef tract withi n th e sea­ active, shore-normal, ooid tidal bars at both en ds. The reef ward part of the FKi'\TMS consist of four genera l physi ogra ph- complex is know n as the Key Largo Limestone (SAN FORD,

J ourn al of Coastal Resear ch, Vol. 13, No.3, 1997 Differ ential Quaternary Evolution 713

Northeast Po rtion of Key Largo National \ Marine Sanctuary shelf 25 °10' margin N 1

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kill A Upper Keys Stndy Area (Thi s Study) N A Southwest I'ortion of 82' 5(/ Ftorida Keys National Marine San ctuary Beca Grande ,. .Key -.. ,-

14 15+ margin , I ~ , I , I J I '.'''' ~, <><; escarpment reef 16a km 10 • • multiple outlier reefs 40' 81"30' B

Figure 2. (A) Closeup of upp er Keys study area showing numbered Augu st 1991 tracklines for seismic profiling and additiona l reefs and featur es discussed in text. Bold parts of tracklines 13, 9, 28, and 32 ind icat e areas of profiles A-A', B-B' , C-C' , and 0 -0 ', respectively (in Figures 6 and 7), Tr ian gles indicate tracklin es along which profile sections at margin were used to construct block diagram s (Figure 14A-Dl. Note escarpment reef at margin (shaded), Also note reen tran t (pass) th rough esca rpment reef south of The Elbow, (8) Closeup of lower Keys study area showing nu mbered J uly 1989 trac klines, margin-escarpmen t reef, additiona l reefs and fea tures discussed , and areas of hig hly developed, multip le, offban k, outli er reefs (da rk sha ding, well developed; ligh t shading, less well developed), Bold parts of tracklines indicat e profile sect ions illu strat ed in LIDZet al. (1991, Figu res 2 and 3J. Triangles indicat e tracklines along which profile sections at margin were used to construct block diagrams (Figu re 13A-0l.

1909; th e Key Largo unit of the Miami Limestone of HOFF­ unknown. Its ext ent from east to west includes the area of MEISTER et al., 1967) and accumulated as coalesc ing patch Miami Beach to th e Dry Tortugas (SHINN et al., 1977). HOFF­ reefs during the last interglacial at - 125 ka, when sea level ME ISTER (1974) concluded that the Key La rgo Limestone reef was - 7.6 m higher than pre sent (HOFFMEISTER and MUL­ rests on an undulating surface and mu st have been extant TER, 1968; Hoffmeister, 1974). The exposed parts of the reef for man y thousands of years in ord er to have built a thickn ess (keys) vary in width (2.6 to 13 k m ), Reeft hickness also varie s of such magnitude. Th e lower Keys are composed of th e ce­ 0 8 to > 51 m ), Its subsurface extent from north to south is mented tid al-ba r sands th at origin ated as an east-west

J ourn al of Coas tal Resear ch, Vol. 13, No, 3, 1997 714 Lidz et al.

Terminology A • bank, shelf, platform t' t upper slope _ shelf-margin Florida ridge zone with earlier Keys present outer-bank reefs Holo cene ~ sea level & back reef trough sea level patch~ e ef - 1 1!f l A- fringing reefs at PaleOPlatfOnn!margin <, Hawk Channel ~ ./ margin-escarpment reef '>-- ""000.",.r / '1''''''/ o~~& ba ckreeftrough 3U;: ~ "' 2 '-1 " nt:'n dune-ridge underplnning(?) I' ~n . \. /," n for outlier development Pleistocene ,ptlrnrk 4 """ offbank bedrock "'-" terrace Pleistocene Physiographic Provinces G) Em ergent Key Largo Limestone (coralline) and Miami Lim estone (oolite tid al-bar lithofacies) ® Broad trough-like bedrock depression lined with non-coralline grainstone ® Cor alline escarpment r eef und erlying Holocen e accumulations o Upper-slope terrace (a) seismic facies indicat e sediment-covered dune ridges on north terrace (b) seismic facies indi cate high-relief outlier reefs and deep troughs on south terrace Types of Pleistocene Dune Ridges Formed at Arcuate Margin B margin escar pment ~,f,~,b' / E \ when cemented, both ___ dune types form a W ;!:.:<'.A beach-dune ridge ~wa ves --- ___ tran sverse dun e = longitudinal dune = ___Winds asy· m metri'cal dun e n'dge elongate sand dune waves orie~ted .per pendicular oriented in direction --- to direction of SW ili . ill of prevailing winds prevar ng WIn s

Figure 3. (AJSchem atic cross section (not to sca le) of four genera l bedrock physiographic provin ces along Sout h Florida pla tform, number ed as discussed in text , and te rminology used in this pa per. Progradation occur s when backreef troughs are filled. Aggra dation occurs when frin ging reefs at shoreline grow sea wa rd. (B) Aer ial-view diagr am showing types of dunes that would form along an arcua te paleomargin in an E-SE prevailin g-energy regime. Margin-escarpmen t lines follow margin orient at ions in the upper (NE-SW) and lower (E·W) Keys (compare with Figu re 2A,B). Diagram assumes a consta nt energy dir ection thro ugh tim e.

mound of unstabl e ooids and was shaped into northwest­ ated on pre-existing topogr aphic highs, usua lly the Pleisto­ southeast sa nd bars by strong tidal currents flowin g in those cene reefs, although a few corals beca me established on car­ directions. For merly called the Key West Oolite by SANFORD bonate sands, mu ds, and coral rubble (ENOS, 1977). At the (1909 ), the rock is now consid er ed part of the ooliti c Miami seawa rd edge of the shelf-margin ridge zone is the massive Limeston e on which the city of Mia mi is located. reef that forms the ma rgi n escarpment. The oflba nk terrace The bedroc k depression of Hawk Cha nnel (Figu res 2A,B, lies at the ba se of the escarpment. 3A) lies between the Keys a nd outer-ba nk reefs throughout Modern reefs are most luxurient and abundant off Key Lar­ the reef tract. The channel is several met ers deeper in the go, least developed and spa rse off tida l passes in the middle lower Keys than elsewhe re (Lrnz and SHINN, 1991). Recent Keys, and moder ately well developed in th e immediate area core drilling to a depth of 19.8 m below sea level ind icates of th e lower Keys. Westward , beyond Key West in the Gulf that the bedroc k is post-Key Largo Limestone grainstone and of Mexico wher e no rock-island barriers exist and north- south lacks corals (KINDINGER, 1986; SHINN et al., 1994). ti da l curre nts are strong (SHINN et al., 1990), reefs are also Seaward of Hawk Channel beneath the modern outer- bank senescent and spa rse. According to MARSZALEK et al. (1977), reefs, the shallow shelf-margin ridges are composed of mul­ thes e trends are related to land barrier s and tidal passes, as tiple, parallel, Pleistoce ne reefs with a discontinuous cap of well as to reef-tract orientation in relation to exposure to cold, Holocene corals. In most cases, Holocene growth was in iti- wind-driven winter currents.

Jo urn al of Coasta l Research, Vol. 13, No. 3. 1997 Differential Quaternary Evolution 715

METHODS (pass) just south of the reef (Figures 2A, 5). The reef extension at The Elbow is offset slightly seaward from that Seismic profiling provided the primary bedrock and sedi­ at French and Molasses Reefs. ment-isopach data. More than 100 km of high-resolution seis­ The Pleistocene margin-escarpment reef is the underpinning mic-reflection profiles were collected in August 1991 (Figure for the outermost Holocene platform-margin reefs that 2A) using an ORE Geopulse boomer system and a Benthos extend throughout the study area (Figures 2A, 6C, 7A-C, 8). 20-element hydrophone streamer that provided input to an Its dimensions are 28 m high by '""'-'0.5 km wide, roughly the EPC 1650 recorder. Navigation was recorded at 5-minute in­ same as those of the primary 57-km-long outlier-reeftrend off tervals using a Magellan Global Positioning System (GPS) Sand Key Reef (Figure 2B; LIDZ et al., 1991). The seaward side receiver and verified with an Apelco DXL 6600 Loran receiv­ of the escarpment reef becomes a nearly bare limestone cliff at er. Because the seismic equipment used to obtain the profiles a depth of about -15 m. The base of the cliff levels out to form could not be towed over areas shallower than 3 m, which a relatively wide (0.75 km) sediment-covered terrace at depths included many reefs, the seismic data are thus restricted of -35, -40, and -45 m. The sedimentary seismic facies mainly to areas of sediment. Likewise, seismic penetration is reveal poorly defined, low-relief ('""'-' 15 m) features that may often limited by reefal growth and debris, making determi­ represent backfilled reef flats, sand shoals, dune ridges, or nation of thickness difficult. In such cases, groundtruthing by incipient proto-outlier reefs (Figures 7B, 8). diving, probing, and coring was necessary to estimate sedi­ The fossil outlier reefs off Sand Key parallel the margin and ment thickness. rise up from a wider (1.75 km) terrace that produces a clearly Bedrock-surface and sediment-thickness data points were discernible bedrock reflection at shallower depths of - 30, - 35, read from the seismic records at 5-minute intervals. Inter­ and -40 m (Figure 9). The outlier reefs are discrete pretation was based on the speed of sound in sea water morphologic structures consisting, in part, of three parallel ('""'-' 1,500 m/s), Thickness of Holocene sediments and reefs was rock ridges separated from the margin by deep, wide, measured on the profiles as the difference between depth intervening troughs. The primary outlier formed at the from sea level to the seafloor surface and depth to the reflec­ seaward edge of the terrace, and smaller secondary or tion identified as the Pleistocene bedrock surface. These data "satellite" outliers (Figure 9, trackline 16a-d) developed were plotted on charts and contoured by hand to construct between it and the margin-escarpment reef. Off Sand Key, the the bedrock topography and isopach maps at the same scale satellite outliers always occur landward of the primary outlier. as an aerial photomosaic used in the modern ecosystem part A core drilled from the primary outlier (Figure 9, trackline of the study (LIDZ et al., 1996). Supplementary data were 16a) indicates that Holocene reefs are only 0.3 m thick (SHINN derived from core borings, groundtruthing, and probing. Be­ et al., 1992). Uranium disequilibrium series dates of Acropora cause the 1991 counterpart lower Keys paper was con­ palmata at a core depth of 0.1 m reveal an age of '""'-'8.2 ka strained by length, which limited the use of figures, the in­ (LUDWIG et al., 1996). Additional cores containing Holocene A. terpreted profiles from that study are included here (figures palmata were also recovered by TOSCANO et al. (1995). cited later) for comparison of complete data sets. The seaward sides of both the massive margin-escarpment reef and outlier reefs are exposed to the Straits of Florida. RESULTS Off Key Largo, reef orientation is roughly northeast to Comparison of Platform-Margin Morphologies off the southwest, whereas orientation of the reefs off Sand Key is Upper and Lower Keys roughly east to west. Relative to the backreef troughs in both areas, those to the north are narrower, shallower, and infilled Antecedent Pleistocene Topography (Depth to (note obscure acoustic returns in trough sands, Figure 7B), Bedrock) whereas those of the outliers to the south are wider, deeper, In the study area, depth to bedrock generally ranges from and mostly void of sediment. For the most part, the peak of o to 25 m but averages 8 to 12 m (Figure 4, Table 1). The the Key Largo escarpment reef is elevated above the present Key Largo Limestone extends seaward for several hundred platform surface by as much as 10 m and thus lies only 6 to meters as a prominent, ubiquitous, shallow (2-4 m) ledge 8 m below sea level, whereas the crest of the primary Sand bordering the seaward side of the linear island of Key Largo. Key outlier reeflies at or below the platform surface and thus It merges gradually with the gently sloping edge of Hawk is 10 to 15 m below sea level. A similar massive reef fitting Channel, where sediments begin to onlap the rock surface the description of an outlier reef, as defined by LIDZ et al. (Figures 5, 6A). Depth from water surface to channel bedrock (1991), exists off Carysfort Reef, the next major Holocene generally ranges from 2 to 16 m but averages 8 to 10 m. A platform-margin reef north of The Elbow (SHINN et al., 1989). channel reentrant, marked by the 10-m bedrock contour at The Carysfort outlier is also late Pleistocene and is inferred Mosquito Bank, occurs landward of White Bank (Figures 4, to be contemporaneous with the dated Sand Key outlier. 6E). At the seaward edge of the shelf-margin ridge zone, depth to bedrock ranges from 20 to 25 m within a relatively Holocene Sediment and Reefs (Accumulated narrow and shallow, sediment-filled trough behind a massive Thickness) fossil reef that forms what we regard as the modern margin escarpment (Figure 3A; others may place the margin farther The island of Key Largo generally has a veneer of modern seaward). The trough is deepest behind The Elbow and soil and caliche (soilstone crust) and supports hardwood for­ branches into a wider, deeper trough at a margin reentrant ests. Mangrove forests mark the waterline. Sands are thin or

Journal of Coastal Research, Vol. 13, No.3, 1997 716 Lidz et aZ.

/'f, No data

Rodr-iguez 2~ ~

~Y80 0 20' ~------© HAWK CJLU!~ \) Mosquito Bank Marker 1()...- :> Mosquito Bank c::::p c.<:> 2 5~'+80 024 Patch Reef ~Cil? c:= ~Q ~ o l e ~ /;r~~-----J '<-

fOfifi m-

Figure 4. Digitized structur al-contour map showing depth to Pleistocene bedrock ti.e; bedrock topography) seaward of nort h Key Largo, based on seismic proftling as close to shore as possible. Exposed land to north sha ded. Contours in meters below sea level. Latitude and longitu de given at four cross-hair positions. Shallow <6-12 m) contoms mar k locations of escar pment reefs. Seaward te rrace (also shaded) begins at base of escarpment reef at 30-m bedrock contour. Note deep margin reentran t (20-m contour ) behin d The Elbow. Lan d contours and no-da ta zones around perimeter match boun darie s on photomosaic (LlDZ et al., 1996 ).

ab sent along the shallow-wate r ledge (Figure 5). Most of th e (SHlNN et al., 1989), and only a vene er (0.3 m) is found on submerged part of the shelf sea ward of th e ledge is covered th e primary outl ier off Sand Key (SHINN et al., 1992 ). by mangrove peat, lime mud s, carbona te sediments, or coral reefs. Wher e sediments ha ve not accumulated, th e ba re Pleis­ A Synopsis of Contrasting Morphologies tocene limestone usu ally has a caliche coating and is popu­ Within th e two a reas of study, specifics for maj or physio­ lated by scattered corals, sponges, and species of th e brown graphic differen ces in platform-margin morphologies (Ta ble alga Sa rgass um . Sedim en ts generally become thicker in an 1) are summa rized her e. In gene ra l and relative to th e lower offshore direction, ranging from 0 to 2 m along th e west edge Keys margin , the nor th terrace is seve ra l meters deeper and of Hawk Cha nnel, to 2 to 4 m within th e cha nnel, and up to lacks outer-te rrace outlier reefs; the margin-escarpment reef 6 m at th e ea st edge. Seaward of the channel within th e shelf­ is on average elevated above th e platform surface; and th e margin ridge zone, reefs and sa nds ave rage 6 to 8 m thick. platform bedrock is sh allow er. Relative to the upp er Keys The thickest sands (18 rn) occur within th e trough landward margin , th e south ter race is sha llower and is characteri zed of The Elbow. A narrow band of bare or slightly sandy lime­ by massive outlier reefs; the outlier reefs and margin-escarp­ stone rim s th e massive margin reef before the gradient steep­ ment reef are on avera ge eleva ted at or below the platform ens into the escarpment. surface; and th e platform bedrock is deeper. Thus, during th e Off th e upp er Keys, moderately thi ck Holocen e reefs (- 8 Pleistocen e, th e region of the north terrace flooded earlier m) occur as a discontinuous chain of line ar features along th e and faster th an th at to the south, but during th e Holocene, 14-m bedro ck contour in th e area of Grecian Rocks, located th e region of the north platform flooded later and more slowly - 1 km landward of the margin-escarpment reef (Figure 4; than that to the south, and it was protected from wave action SHINN, 1980). Off th e lower Keys, Holocene reefs are gener­ by offshore rock-i sland barriers form ed by th e crest of th e ally thinner (- 6 mi. North of the study are a on Carysfort margin-escarpment reef. The vari ations in antecedent Pleis­ Reef, Holocene reefs and sediments are - 19 m thi ck, but tocene bedrock elevation resulted in different environmental there is no Holocene accumu lation on th e Carys fort outlier cond itions along the ma rgin as Holocen e sea level rose, and

Jo umal of Coasta l Resear ch, Vol. 1:3, No.3. 1997 Differen tial Quaternary Evolution 717

No data

Gar den Cove

+ ~~~~ _----2S-0~12~ © ~A W K H ANN EL 6 0~nrker o Ba k OpatS ql~Ck~

Figure 5. Digit ized sediment-isopach map showing thickness of Holocen e sediments a nd reefs sea wa rd of north Key Largo, hased on seismic profili ng as close to shore as possihl e. Expo sed land to north sha ded . Contours in meter s. Latitude and longitude given at four cros s-hair positions. Th ickest zones occur in topogr aphic lows and trough s behind escarpment reefs, es peci ally at Th e Elbow (compare with Figure 4). Note O-m contour at margin marking seaward band of bare escarpment-ree f rock and approximate poin t where dropoff begins. Also note margin reentrant south of Th e Elbow. Sinkhole on shelf (both map s) discussed elsewhe re (SHINN, in press), Land contours and no-data zones around peri mete r match boundaries on ph otomosaic (LIDZet al., 1996 ). No-da ta zone in vicinity of Key Lar go marks Key Largo Sound.

only some of those conditions were conduciv e to cora l growth. north part of th e reef tract, onshore winds a nd currents move Holocene outer-bank reefs beca me established ea rlier to the sediments an d reef rubble from th e platform-margin edge south tha n nort h but grew for a longer period of time to the into the troughs behi nd the esca rpment reefs. In t he south north tha n sout h. Th ese differences were a lso, in part , t he part, alongshore proce sse s move sa nds west an d south, par­ res ult of platform-ma rgin curvature. a llel to and seawa rd of the margin. An example of such trans­ port is a distinct, localize d lobe of sand > 15 m thick that has DISCUSSION partially buried a deep outer-bank reef in front of Looe Key Reef (LIDZ et al ., 1985). In addition, numerous seawa rd-flow­ Before examining the alternative interpretations, we must ing , tida l "sa nd aprons" occur in br eaks between lower Keys first consider the fundam en tal influence of prevailing win d­ reefs, but only a few such features occur between upper Keys and-wave energy on the arcuate margin, with regard to sed­ reefs (LIDZ et al., 1996 ). Further more, currents within the iment transport, accumulation, and to location and develop­ confines of the "channels" formed by the outlier-reef walls ment of the massive Pleistocene and lesser Holocene reefs. would be st rong (LIDZ and McNEILL , 1995), tending to re­ On the South Florida platform, modern sedime ntary pa tterns move sediments. provide a key to past patterns (PDE 1, Table 2), in t hat sed­ Contour maps of bedrock surfaces an d sediment/reef iso­ iment-transport directions and accum ulation during a low­ pachs have been used to reconstru ct ancient shore lines off the ered sea level should have been like those of today . Keys (LIDZ and SHINN, 1991) and to define sedime nt a nd reef Investigation of the effects of Hurricane Donna, which im­ dis tribution a nd thickness (SHINN et al., 1977). Corals and pacted the study area in 1960, led B ALL et al . (1966 ) to con­ cora l reefs have long been known to be proxy ind icators of clud e th at storm wind s an d currents were influenced (but not pa st positions of sea level (cf HUDSON et al., 1976 ; LIGHTY, to th e sa me degree) by the same local and regional topogra­ 1977; FAIRBAN KS, 1989, 1990); hen ce, they provide a long­ phies that control everyday waves and tides. Th us , in th e term record of associated changes in ma rine environments.

Journal of Coas ta l Resea rch, Vol. 13, No. 3, 1997 718 Lidz et al.

Table 1. Dissimilar regional platform-margin characteristics off the lower and upper Florida Keys. Data for area of Carysfort Reef, located north of the study area. not included.

Genera l Pbysical Differences in Regional Param eters

Param eter Lower Keys Upper Keys Offba nk terrace and morphologies depths sha llower (30-40 rn ) deeper 135-45 m 1 max . terrace width - 1.75 km - 0.75 km out lier reefs tH, W, LJ 30 m, 1 krn, 57 km probably none dune rid ges (H, W. LI probabl y, under out liers - 15 m , < 0.125 krn, < 3 km num ber parallel trends 3, ~ conti nuo us 4, discontinu ous orientation ea st-we st nort hea st- southw est in-situ Pleistocene growt h thi ck ( -30 m 1 probably negligible to none in-situ Holocene growth thi n (- 0.3 m ) probably none dept h of linear crests shallow 110- 15 m ), exposed deep 1-;:30 m I. buried age - 83- 80 ka Pleist ocene I'?1 trough s wider, deeper , -;: em pty narrower. shallower. infilled max. tr ough width - 1.5 km - 0.25 km Platform she lf general depth to bedrock o to 20 m la v. 10-1 6 m ) o to 25 m lav . 8- 12 101 dept h to Haw k Cha nne l bedro ck 6 to 16 m lav. 12-1 6 m l 2 to 16 m lav . 8- 10 10 1 oldest Holocen e outer-b ank reefs - 6.6 ka (Looe Key Reef) - 6.0 ka (Grecia n Rocks ) Holocen e thickness av o6 to 8 m (up to 12 rn) avo6 to 8 10 lup to 18 m l Sedimen t-trans por t direction a longshore onshore Nature of ma rgin progradational aggradational

In gene ra l, the principal element th at determines whether teria: (1) presen ce of an upper-slope bedrock terrace (Figure reef growth will keep pace with rising sea level is water qu al­ 3A); (2) later presence of lithified dune ridges on the terrace ity (HALLOC K and SCHLAGER, 1986 ), which has been shown (Figure 3B); and (3) th e energy regime and resulting sedi­ by numerous authors to have deteri orated with rising sea mentation du rin g a post-dune-r idge rapid rise in sea level. level (LANDON, 1975; LIGHTY, 1977; PORTER et al., 1982; Wher e th e bedrock te rrace is abse nt, as , for exa mple, in th e NEUMAN N and MACINTYRE, 1985, among oth er s). Becau se pass off Key West (Figu re 9, trackline 14), reefs and troughs cora ls in South Florid a recrui t preferentially to sediment-free are abse nt. Although the terrace has not been sa mpled, we bedrock highs, the primary eleme nt that controls their dis­ infer from its variable depths (dee per off Key Largo than off tribution is ante cedent topography (SHINNet al .. 1977; SHINN Sand Key, Table 1) th at it form ed afte r postulated pre-Pleis­ et al., 1989). Topographic lows flood first and fill with sedi­ tocene, westward, platform tilting (PARKER and COOKE, ment. Topogr aphic highs flood last, lack a sediment cover du e 1944; PARKE Ret al., 1955; KINDINGl': H, 1986) and is therefore to wave a nd current actio n, and thus form a hard substra te Pleistocen e. The va riable depths probably resulted from the suitable for coral recruitment (PDE 2, Table 2). Bedrock con­ undulating pre-Key Largo Limestone topogr aphy of HOFF­ tours also constrain tho se parts of th e substra te (highest el­ MEISTEH (1974), Although we do not know whethe r th e ter­ evation), now below sea level, tha t would have been island race is erosiona l (beachrock) or constructional (cemented barrier s during per iods of lowered sea level. Islands provide sands ) in origin, we do know th at for cora ls to have recruited protection from high-ener gy waves and the reby an environ­ to its surface or to th at of a ny surficial dun e ridges, the sur­ ment suitable for cora ls ada pted to low-en er gy conditions. As faces had to have been hard (SHINN et al., 1989), Further­ th e islands are submerged, th e protected water s in their lee mor e, given its sign ificant breadth (Table 1), we infer th at become high- energy surf zones, and the dom inant cora l spe­ the terrace likely formed during a rather substantial still­ cies change accordingly, from head corals (Montastraea an­ stand. Numer ous Plei sto cene subaerial-exposure horizons in nularis, Diploria strigosa, Siderast rea radi an s, S. bournai i to South Florid a (Pl': HKINS, 1977 ) and th e Bah am as (BEACH and branching (Acropora palmata, A. cervicornis, va rious Porites GINSBUHG , 1980 ) indicate th at sea-level rises and falls were spp.) an d sti nging cora ls iMillepora complanata, M. alcicor­ punctu ated. nis). Such coral zona tions are observed in Holocen e cores re­ Common to both th e sout h a nd north terraces is th e lin­ covered from Grecian Rocks (SHINN, 1963, 1980) and Looe ea rity of th e surface (outlier-reef) and subsur face seismic ex­ Key Reef (Lmz et al., 1985) as Acropora palmata replaced pressions, respectively, whi ch is consistent with th e concept Montastraea annularis. Species of Millepora are becoming in­ of a dune-rid ge origin. A dune-r idge or frin ging-reef origin creasingly abundant on both reefs today. These zona tions highli ght environmental qu ality (PDE 3, Table 2) as a critica l was proposed by LIDZ et al. (1991) for th e outlier reefs off element in our interpret ation. Sand Key. Although we ha ve no ha rd evidence for dune-rid ge underpinnings ben ea th th e outlie rs (Figure 3A), and sa nd Pleistocene: Topography, Regional Sedimentation, and dunes do not exist along Keys beaches today, thi s is not to Outlier-Reef Distribution say th at th e terrace surface remained bare under conditions Onset of outlier reef-and-trough developmen t a ppea rs to of exposure. Ind eed, subaeri al exposure would invoke sea ­ have been dependent upon three non-contemp oran eous cri- ward tra ns port of platform -derived sediment by fres hwate r

J ourn al of Coastal Research, Vol. ia, No. a, 1997 Differ ential Quaternary Evolution 719

A NW bare Pleistocene III bedrock shelf Hawk Cha nnel o

15

30

.~ . : 45 -.-: : .. L

~ - , 60 5 . ~. A

III White Ban k 0

15

30

45

60

B

o 0.25 0.5 SE A' I I I Ho locene km III vertical exaggera tion x-7 Holocen e outer-ba nk 0 sediment reefs Pleistocene escarpment reef -- I S at French Reef

30

45

60 C

Figure 6. Continuous high-resoluti on seismic-reflection profile A-A' (trackline 13) from Key Lar go across Hawk Cha nnel an d Whit e Bank to Fr ench Reef at margin (see Figure 2A for profile location and geographic localities). (A) Area where submerged, bare, bedrock ledge merges with west edge of Hawk Channel. Note thin, muddy , channel sa nds onlapping ledge (at left) and minor karsti c irre gu lari ties in bedrock . Also note pat ch reef on topographic high at left edge of karst depression. (B) Area of Hawk Chann el where White Bank sand shoals have accumula ted. (C) Area of French Reef showing exposed seaward face of escarpment reef, which exten ds from The Elbow to Molasses Reef. Note backreef trough infilled with (Pleistoccne'' ) sediments and surficial Holocene reefs.

J ourn al of Coas ta l Research , Vol. 13, No.3, 1997 720 Lidz et al.

B NW o 0.25 0.5 SE B' I I I kill m vert ical exa gge ration x -] Holocene I outer-bank reefs o

Pleistocen e escarpment 15 reef at T he Elbow

30

;.Q '

;; ~~ p~--¥r ': 60 A

C NW o 0.25 0.5 SE C' I I I kill m vertical exagge ration x-7 0

15

30 i Pleistocene t~<:,;-; ; i; bedrock ~~f~~~' f : .r'"'.:~3~-;"'":; ----- 45

60 B

D WNW o 0.25 0.5 E D' I I I Holocene kill m vertical exaggeration x - '] outer-bank 0 Ho locene sed ime nt r eefs

Pleistocene escarpment 15 reef at Molas ses Reef

30

60 c

Figure 7. Portions of high-resolution seismic- reflectio n profiles across esca rpme nt reef off Key Lar go (see Figure 2A for profile locati ons and geographic localities). (A) Pr ofile B-B'(trackline 91across The Elbow. Surficial Holocen e sedi ments and reefs cover landward-accreting slope above backreef trough . Note patch reefs on bedrock topographic high at left. (B) Pr ofile C-C' (tra cklinc 28) across Th e Elbow. Note broad terrace, lack of seismic expressio n with in low-relief terr ace features of unknown origin (dune ridges"), an d presence of seismic reflections betw een features, ind icating sediment, inferred

J ourn al of Coasta l Resea rch, Vol. 13, No.3, 1997 Differential Quaternary Evolution 721 runoff and shoreward transport of seafloor sands. Movement (in -situ growth? or merely the presence of terrace morpholo­ of relatively large amounts of both types of sediment load gies with relief"). Each depends on control by several factors: would be expected during respective paleowinter storms and antecedent topography (the terrace; an exposed, arcuate pa­ hurricanes. leomargin; the dune ridges), energy regime along the paleo­ Carbonate sands move shoreward from the surf zone. For margin, and resulting sedimentary geometries. Both assume sands to accumulate on the broad terrace above the surfzone, a constancy in direction of regional energy and sediment position of a stillstandi') sea level would be at or just below transport through time. Unfortunately, no seismic evidence the elevation of the seaward edge of the terrace. Coastal exists to indicate whether the north escarpment reef actually dunes are wind-blown features that form along low-lying sea­ initiated growth on the inner terrace or as a fringing reef on shores above high-tide level. In low-latitude carbonate the paleomargin. The terrace reflection cannot be traced be­ realms, cementation occurs early and rapidly in both marine neath the reef as it can, to some degree, under the south and non-marine settings (see MILLIMAN, 1974, for discus­ outliers (see LJDZ et al., 1991). sion). Cemented dunes form dune ridges. Along the exposed The first interpretation would regard the north escarpment terrace off Key Largo, onshore winds would blow sands into reef as a fully developed, inner-terrace, outlier reefanalogous transverse dunes that would front the paleomargin cliff (Fig­ to the smaller inner-terrace satellite outlier off Sand Key, but ure 3B). To the south, alongshore winds would blow sands with an infilled backreef trough. If this were the case, how­ into streamlined longitudinal dunes along the breadth of the ever, the outer-bank Holocene reefs at The Elbow, French, terrace. A modern analogue to a longitudinal-dune hypothe­ and Molasses Reefs would have no counterparts on the sat­ sis would have been the extensive, east-west quartz-sand ellite outlier. Even so, if the north reef is an inner-terrace dunes +5 m and higher that existed along the Florida Pan­ outlier, it could have begun growth on a transverse dune handle Gulf Coast until Hurricane Opal, a Category 3 storm, ridge close to the base of the paleoescarpment, at first ac­ made landfall on October 4, 1995 (USGS unpub. data). Late creting laterally to merge with the deeper part of the escarp­ Pleistocene analogues, recently sampled and dated by LOCK­ ment, later assuming a more vertical growth style to form ER et al. (in press), comprise a series of elevated, shore-par­ the shallow backreef trough. This would place the paleomar­ allel ridges as long as 24.5 km located at the seaward edges gin landward of the modern trough (see Figure 8). The inter­ of deeper terraces (50-125 m) south of the Marquesas Keys. mittent lack of seismic expression on the terrace (Figures Those authors interpreted the primarily oolitic grainstones 7B,C, 8) would be regarded as indicating backfilled reef flats as representing a subtidal shoal complex (~18.9 ka) and or sand shoals similar to that at White Bank (Figure 6B). younger (~14.5, 14.0, and 13.8 ka), backstepped, upslope pa­ This type of outlier reef-and-trough system would be defined leoshorelines of eolian-dune or beach origin. The ridges as mature, i.e.. the trough has filled and the margin has pro­ formed during a reduced rate of sea-level rise or a stillstand graded. Regardless of interpretation, with no high-relief and were subsequently drowned by rapid pulses of sea-level structures fronting the margin, this part of the system has rise approaching 9 m in <500 yr. These evidences, especially reverted to an aggradational stage ti,e.; it can only build up­ the widespread linearity of the features in our study, are a ward by coral growth) and would produce the classic margin strong argument for subaerial formation and cementation observed in the geologic record (for examples, see TOOMEY, (PERKINS, 1977; ROBBIN, 1981; ROSSI NSKY et al., 1992) of 1981 L In this rendering, the upper Keys margin would be coastal paleodunes along a pre-existing bedrock terrace. The more advanced than the lower Keys margin, where the hard, elevated surfaces would form ideal substrates for coral trough behind the analogous satellite outlier reef is not filled colonization and initiation of the outlier reef-and-trough sys­ and the margin has yet to prograde by its being filled. tem during the next rise in sea level. If our data and infer­ The alternative interpretation shifts the emphasis from or­ ences to this point are correct, the morphologies imply peri­ igin of the north escarpment reef to the unknown features ods of at least one stillstand (terrace formation), one high­ buried on the north terrace. Here, we view them as the dune­ stand (Key Largo Limestone formation), and another proba­ ridge underpinnings for outlier reefs that never developed. In ble stillstandtr: (sand-dune and dune-ridge formation; Table this case, (a) the dune underpinnings are analogous to the 3). The third criterion for outlier-reef development, energy prominent outliers to the south; (b) the north escarpment reef regime, becomes apparent in the data interpretations. is not an outlier reef but is equivalent to that along the south margin landward of the outliers: and (c) the escarpment reef The Interpretations along the entire margin initiated growth as fringing reefs on Lacking sufficient information to prove precisely when and a paleomargin located seaward of the modern backreef why the morphologies developed differentially along the plat­ trough. The Holocene outer-bank reefs to the north (Molas­ form margin, we offer two interpretations of the upper Keys ses, French, The Elbow) now correspond to those to the south data. Each depends on what one defines as an outlier reef (Sand Key Reef, Eastern Dry Rocks, Pelican Shoal; Figure

to be non-reefal i Pleistocene"! sands. Also note general lack of seismic resolution in deeper backreef trough sediments, inferred to be (Pleistocene?) reefal debris. (e) Profile D-D' (trackline 32) across Molasses Reef. Note indistinct contact between bedrock and Holocene sediment on landward side of'escarp­ ment reefcompared with more distinct contacts visible in (A) and (B). Unlike off the lower Keys, it is not evident from the seismic record if the paleomargin/ terrace-floor contact is located beneath the upper Keys escarpment reef. i.e., if the reef is an inner-terrace, offhank, outlier.

Journal of Coastal Research, Vol. 1~3, No.3, 1997 722 Lidz et al.

Upper Florida Keys Margin Morphology

NNE mean sea level WNW patch reef • E NW~~=-_ ,:"",;;:::::::-= trackline23 SE karst(?) """ margin t Holocene ---+' - 30 m topographic -40m sediment ~ .4 0m high - 45 m

SE NW~_~-.'-II tracldine 15 SE Pleistocene n« Elbow bedrock Ruf ~-~04rg backfilled reef flats(?), sand sho~ m m 0 dune ridges(?), incipient proto-outlier reefs(?) NW i trackline 27 • SE 15 -----: : N W~_: trackfine 34 SE :nfilled trough 30 -­ - 30m The Elbow Q-<:;.30 m ._ 45 b"t.:.. Ruf P l e i s~~ 45m ~ so751N r--.. _ tra.."''' • sss NNW ==:;;> ss ~ t /' Fren ch Pleistocene margin-/, ~ 40m Ruf ~-40m bedrock escarpment ~ ~-45m reef Pleistocene terrace

trackJine35 SE ______I trackline 26 • SE N~ =( 1----" patch reef ~ ....~~~5" m

NW ESE N~ trackline 20 SE

" esca rpment Holocene reef sediment - 30m =~gm

uu',,;;, " ....V ~\ flo 1, .....1.., ' 11:I 11\ l-"U\U"VUl ~I NW _ SE NVC=::: ----J trackllne 32 • S: Grecian Rocks outlier reef(?) ~ ~--.:- 4~.n ~5m40m m

NW --=- So> ..... I trackline 5 • SE N W~~ I trackline1 8 • SE

~- 4 0m - 45 m

~., trackline7 SE N'... escarpment ... reef '-"""':: ",",=30 m averagevertical exaggeration - 7x buried ~ - 4 0m reef flat(?) ssw

Figure 8.Interpreted seismic profiles of platform margin off upp er Keys showing escarpment reef-and-tro ugh syste m. Profiles aligne d at what we define as the modern margin (vertical lin e) in consecutive order from northeast to southwes t (see Figure 2A for locations and geographic localities). The alte rnative int erpretations would place th e paleomargin (a) landward of the backreef trough if the escarpment reef wer e an inner-terrace outlier reef, or (b) sea wa rd of the tr ough if th e escarpment reef initiat ed as frin gin g ree fs. Island of Key Largo to left; St raits of Florida to right. Note (a) broad offbank bedro ck ter ra ce at depths betw een - 35 and - 45 m and small, bu ried , unknown features (dune ridges"); (b) infilled backreef trough and lack of seawa rd outlier ree fs, ind icat ing north ma rgin is aggradati onal; (c) elevation of esca rpment reef above platform bedro ck sur face, indicating presence of a protectiv e rock-island barrier du ring platform flooding; (d) better resoluti on of Pleistocene platform bedrock surface than in lower Keys profiles (Figu re 9); and (e) poorer resolution of seismic facies in backr eef trough, possibly indicati ng Pleistocene reef-rubble infill. Distance along margin between end profiles is - 10 km . Diam ond s indicate profiles used in Figure 14A-D.

2A,B), an d the upper Keys margin is defined as less matu re carpment reef flourished thro ughou t the reef tract, and how than the lower Keys margin, where the outliers are highly did the escarpment-reef backreef tro ugh to the south become developed, filled? At first, a connection between water depth and rates This interpretation rai ses the questions, why did outer-ter­ of cora l growth and sea-leve l rise (Figu re 10) might be im­ race outl ier reefs become highly developed to the south but plied, i.e., greater antecedent-bedrock (terrace) depth to the not to the north, especia lly when the Pleistocen e ma rgin-es- north app ears to correlate with poorer "outlier-reef ' devel-

Journal of Coasta l Research, Vol. 13, No. 3, 1997 Differ en tial Quat ernary Evolution 723

Lower Florida Keys Margin Morphology

mean sea level ENE mean sea level E trackline 9 SW _ margin trackline 1 SSE I ~~~_..... trackli ne 16d • SSW ~margin I ..&"""1outlier-- crenulated surface ---... erosion / ~ 30 escarpment . / acoustically ·------f reef - ~40 m reel U~: dense coral ~.- 40 m ~ m landward-transported 40 n outlier reefs Holocene sediment ~ trackline 10 SE

30 trackline 14 SE NW

-40m ~ 45 pass through oulli er reef 60 t . off Key West Pleistocen e ~ NW trackline 11 SE "'J 75 bedrock Pleistocen e . terrace erosion eSCa"rpment reel

N

N trackline 12 SSE __ incipient proto-oullier - 30 m reel(?) -40m

SE

trackline 4 S -y __ ouIIier reel - 30 m subbo ltom outlier - 40 m reflection t'--V buried reef outlier reel

Pelican Slwal trackline 3 SSE N _ pinnacle <»: __ facies change - I ridge rnarqln- ~ ::=~~="",_",,~- I", - 30m t 30 m ~ trough Pleistocene t - 40 m esc arprnent'v ( t •- 40 m terrace trough reef ~ rubble buried outlier reel "satellite" .t outlier reefs pnmary outlier reef

trackline 16c SE N acoustically trans paren \ : :~ - 30m -"~ ~- 40 m wsw double Pleistocene terrace average vertical exaggeration - 15x

Figure 9.Interpreted seismic profiles of platform margin (vertical line) off lower Keys showing escar pment reef-and -trough syste m and multiple ofTh ank outlier reefs. Profiles in consecutive order from east to west (see Figu re 2B for locations and geogr aphic localities ). Lower Keys to left ; St raits of Florida to right. Note pass between outliers off Key West (trackline 14). Dat ed core obtained from primary outlier off Sand Key Reef (truckline 16a ). Not e (a) terr ace at shallower dept hs (- 30 to - 40 rn ) th an to th e north; (b) essentially empty outlier backreef troughs, indicating south margin is prograd ati onal ; (c) elevation of out lier reefs on average at or below platform bedro ck surface, indic ating lack of a protecti ve of/ha nk rock-island barrier during platform flooding; and (d) bet ter resolution of onbank and ofThank Holocene seismic facies than in upp er Keys profiles (Figu re 8). Dista nce along margin between end profiles is - 65 km . Diam ond s ind icat e profiles used in Figure 13A- D. Line 16c not used du e to oblique orie ntation (see Figure 2B), which distorts width of features.

Tab le 2. Previously determined elements (termed PDE in text) that pro­ opment. Whereas the north corals might have been less ef­ vide clues critical to interpretation of morphologies at the north margin. ficient in becoming established in a rapidly rising sea, those

Key Elements in Interpr etatioo on the shallower south terrace migh t have become esta b­ lished later, th ereby being ab le to keep pace with a slightly 1. Modern energy and sedimenta ry pat terns B A L L et al. (1966 ) less rapid rise. Two critical eleme nts (POE 1 and 2, Table 2) mimi c past patt erns 2. Topographic highs form hard substrates for SHINN et al. (1977) provid e clues to a more subtle but salient connection, how­ coral colonization SHINN et al. (1989) ever, between location along the margin, rate of rising sea 3. Differences in antecedent bedrock elevation, SHINN (1963 , 1980) level, an d effects of the preva iling-en ergy regime. Alt hough location, and orien ta tion influence environ­ LIDZ et al. (1985) lacking proof, we offer the following t heory for considera tion. ment and vitality of Holocene cora l growt h Subaerial dune ridges formed along the entire terrace and

J ournal of Coastal Research, Vol. 13, No. 3, 1997 724 Lid z et 01

Table 3. Relai n v S U("("PSSLOlI ofeven!s alon g So uth Florida platform mar gi ll mdicatu v 0/ iluctuonng sea level Estimat ed PO S/llOlIS of sea leoel ' I ll meters relan re to presenl i interred from dolo in text, Holocene PO S/IIOTlS based o~ sea lerel clln!' for reef tra ct ' Figu re 10 ,. Margin Evolution off Lower Florida Keys o E 10 Events. Paleosea Level, and Trrnmg Holocene sands ) Sand I0Reef ~ 20 E VPn! . Sea Lev el rm r l k a l n 30 sea level at present v Fl. Lauderdal e reef dies riSIng I - 4'). 5.0 outer-ba nk reefs backstep 40 Nortb plat form Hoods risin g 1· 8 , 7.0 lower-elevat ion corals on platform die 50 entire platform floods , South platf orm Hood. risin g I - 12 ' 8.0 outlier reefs die at -8.2 ka 60 Outlier-reef, drown n si ng r - 13?) < 8.2 upper 5.5 m of primary outUer - 83-80 ka " South terrace Hoods risin g I ' 301 ~ 9 . 2 Nort h terrace Hoods rr si n g I - 35 ' >9.2 ~-"""""'==;;;:::;;;:::~~1~ Out lier reefs growing high stand I - 20ry , 83-80 ~""''''~l< South out lier rppfs origmare rising I - 25'?) >83.0 In ;;:;;;;;;;;J ff _'" : North dun lo' rid ges buried rising 1·30 , >83.0 Dunlo' ridg es form su llstand? I > 45'! I < 100.0 40 Key La rgo Ls. form s high st and I • 7.6' 125.0 50 Terrace for ms stillsta nd I - 301- 45 I > 125.0 60

I I I 0 were drowned by a ra pid pulse of sea-level rise th at tr ans­ 10 ~ ported unconsolidated sa nds along th e margin in th e directions - ,-,-...= ccll- 20 "*'" 30 ~ they are moved today (POE 11. Off the south margin. sa nds o flowed south an d west. between th e ridges . allowi ng cora ls to 40 ~ ~ recru it to th eir hard surfaces and initiate outlier- reef forma­ sea Iev~I ' ~i ;20in; 50 patch reels grow on ptattorm ,': '.,..' .' 60 ~ tion (POE 2: Figure II I. Off th e north margin. san ds moved f ringi~g 'eelS g~v.; on oullier !~? f s and at paleo ".',~~gin landward. onto the terrace. burying t he ridges too quickly for cora ls to become established (Figure 12 1. The amount of In-situ B growth on th e north ridges is probably negligible to none. In thi s scenario. conditions for offbank cora l growt h were optimal

Depth Age (x 1ka) (m) 18 16 14 12 10 8 6 4 2 o iJ'i o A ---- posmon of mod ern ma rgin escarp ment 1 ____ posmon of paleornarqm at ~ 100 ka Florida reel - ~ 'g- h"" ~~ T'" tract age -.- I ~ --=---_...... cemen ted dun;;,dgeS(?) I I -125 ka(?)

- - 20 ---<. "'kt~ -, \I l\ terrace agel ?) sea level belowterrace platform lower than present -~ 1 4 c a ge 40 ~ - I~ Figure I I. Propo sed mode l for evolution of lower Keys platform ma rgin """ ..'? - ==- t sea level fluctuat ed man y times but positions correspon d to events ,f;Younger Dryas ~ Chronozone sho wn i , IAI Pleistocen e sti llsta ndt'n dune-ridge formation. ([3, Coloruza­ i 60 ,I': 1SJ ft" tion of sedime nt-free paleomargm an d dune ridges by Plei stocene coral s / that keep up wuh rap id rise in sea level. IC. 0 1Upwa rd building of ma r­ 7>- .s gin-es ca rpme nt reef a nd out lier reefs. I E I Dea th of cora ls on out liers at =f2, ,- -0- Lighty et al. (1982) - 8.2 ka and later backst ep ping of Holocene outer-bank reefs with plat­ 80 , - A Fairbanks (1989) form-wide flooding . : '" Fairb anks (1990) I -- ;.. -(( Broad Creek Peat ~ i. o Alligator Reel Peat 100 only along the south terrace st rictly beca use of different sedi­ 1-- Fowey Reef / /1 ¢.1-" o - Acropora palmala ment-transport directions resulting from different energy pat­ ,18'1.: terns impinging on the curved ma rgin. In addition, the thi ck ,./1 I 120 accumulations and type of coral (predominantly Montastraea Figure 10. Sea-level curves for Barbados corals (Fa irban ks. 1989, 1990) annularisl in the massive ma rgin-wide escarpment ree f attest plott ed agains t modifi ed . well -constrained curve for th e Florida reef tract to ubiquitous warm, quiet, low-nutrient, open-ocean conditions. based on reef-tract data (solid line , open symbols). Reef-t ract curve is us ed Therefore, corals shou ld have grown on the nort h terrace but in inte rpre ta tions in th is study. Query ind icates lack of reli ab le reef-tract data older than - 8.2 ka . could not follow the basic plan because there were no hard , sed­ iment-free sub strates avai lable for colonization . As th e platform

J oumal of Coastal Research, Vol. 13. No . 3, 1997 Differ en u al Quat ernary Evoluti on 725

Margin Evolution off Upper Florida Keys 0 10 20 30 40 50

o 0 10

sea'~vel at ·10 20 HoloCenereefs san s .. • 30 :',:' aii~ m u la t e on outer platfoim t'''--..'-===-=-=="",'" 40 oute(piatfonn floods, inner plalfonn dry land y- "'-! seaward fringing reefs shed debris landward ' 50 fringing reefs grow at modem margin

0 Q; c > 10 .!!! eo 20 :J: ~ 30 0 Q; 40 .c ~ 50 m OJ E 0 B 10 20 30 . ~_~~ !~;;~ ~.,!!.jgJ 9.9!!~9). !! ~9. :~g. !!l _ _._ . 40 .fringing reels grow at paleoplatform margin cOrals attempt colonization(?) of (?)dune ridges , ..... rapid bankward in!i/lin and burial of terrace ? dune rid es 0 A 1 position of modem margin escarpment 10 ______position of paleomargin at - 100 ka 20 30 ------....12=-=5::-:k-a-::(?::':) cemented dune ridges(?) sea levelbelow terrace 40 platfonn lower than present 50

Figure 12, Proposed model for evolution of upper Keys platform margin (sea level fluctuated many times but positions correspond to events shown). (A) Pleistocene stillstand(?) dune-ridge format ion. (B) Dune-ridge burial by bankw ard transport of Pleistocene sediments (non-reefal sands'') during ra pid rise in sea level. It is not known whether corals attempted colonization. (C, D) Backstepping, trough infilling, and upward building of fringi ng reefs at paleoescarpm ent. (E) Backsteppin g of Holocene outer-bank reefs with platform-wide flooding. Holocene sands on oRbank terrace and in trough at margin probably comparatively thin.

flooded with continued rise in sea level, the north escarpment­ nificant in-situ coral growth; and (3) th e north-terrace burial reef backreef trough became filled by landw ard transport of the sediments are non-reefal (i.e., th ey have seismic resolution; Pleistocene reefs own rubble, th ereby accounting for th e poor Figure 7H) and may be primarily Plei stocen e. Until all of seismic resolution within the deeper trough sediments (Figure th ese features can be inve stigated with tools oth er than seis­ 7B). The south trough was infilled with less acousti cally obscure mic profiling, the re is no way to know their true origin and (LIDz et al., 1991, their Figures 2 and 3) carbonate (versus ree­ composit ion, th eir relation to one anothe r, or which interpre­ fal) sands swept to the southwest from between Hawk Channel tation is correct. However , it is tempting to believe the sa lient and the margin by alongshore processes. conn ection between energy and location alon g the margin, The primary key, th en, to initi ation of differential Pleisto­ and th e rapid burial of dune-rid ge-lik e substrates th at would cene development is an energy regime like tha t of today act­ have been suitable for coral coloniza tion of the north terrace. ing, during period s of fluctuating sea level, on a readil y avail­ able sedim ent supply on a bedrock terrace th at fronted an Life and Death of the Massive Reefs arcuate margin. From the evidence, we infer th at: (l) th e margin-wide escarpment reef origina te d as cont emporan eous During th e late Pleistocen e, th e high est sea levels occurre d fringing reefs; (2) the north-terrace seismic expressions rep­ during substage 5e, when th e Key Largo Limestone form ed resent buried dune ridges that probably never harbored sig- at - 125 ka (S HACKLETON and M ATTH EW S , 1977 ). Although

Journal of Coasta l Research, Vol. 13. No, 3, 1997 726 Lidz et al. sea level has subsequently fluctuated many times, it has nev­ 1981). Influx of low-salinity waters via runoff from the Ev­ er been as high (HOFFME ISTER, 1974 ). The Pleistocene parts erglades would be particularly deleterious to cora ls in shal­ of th e massive reefs are t here fore pr obably cumulative and low env ironments . Th e prima ry key, the n, to differential Ho­ represent alternating peri ods of coloniza tio n, exposure and locene reef development is th e antecedent dissimilarity in ceme nta tion, and new colonization. Pleistocene cora ls even­ Pleistocene morphology (elevation, location, and orientation) tu ally form ed 28 to 30 m of relief a long the margin and in that produced corresponding differen ces in Holocene environ­ the south outlier reefs. men t and reef grow th along the ma rgin . Compa ratively spea king, Holocene coral s appear not to have fared as well as th e Pleistocen e cora ls th at bu ilt th e Holocene: Sea-Level Rise and Margin Morphology massive reefs. For unknown re as ons, Holocen e cora ls becam e established more readily and thrived more cons iste ntly on Th e first Holocene sea-level curve for South Florida was edges of shallow lagoonal-sh elf terraces (SHINN et al., 1989 ) cons tructe d from 14C dates of subae rially formed terrestrial than on the deeper outl ier reefs. Th e few Acropora palmata an d/or man grove peats now subme rged in Florida Bay t hat had colonize d th e outliers died shortly afte r - 8.2 ka (Ta­ (SCHOLL, 1964; SCHOLL et al., 1969 ), A more recent curve ble 3), and a very few, widely scatte red colonies ofliving Mon­ (Figure 10), based on llC ages of continuous peat sections tastraea ann ularis rem ain (EAS, pers. obseni). It is highly exposed in tid al passes through the Keys and of peats be­ unlikely th at poor colonization was related to rate of sea-level neath cora ls at Alligator Reef (Figure 1; ROBBIN, 1984), was rise. Growth rates of both cora l species are so great (as much modified by supplemental 14C ages of cored and excavated as 4.85 m/ka for M. annularis and twi ce that ra te for A. pal­ cora ls (LIGHTY, 1977; SHINNet al., 1977; SHINN, 1980; SHINN ma ta ; SHINN, 1976; SHINN et al., 1977 ) that th ey would have et al., 1981; SHINN et al.. 1991). Th e modified curve is pre­ kept pace with even a very ra pid rise. Differences in Holocen e ferred for calculating timing a nd positions of pal eosea levels grow t h occurre d within t he lagoonal-sh elf realm as well, with along th e reef tract. cora ls faring better to the north than to th e sout h. Three Projecti on of form er sea-level positio ns onto stable-platform fact ors inh erent to antecedent massive-r eef morphology (POE conto urs (Lmz and SHINN, 1991, the ir Figure 3) shows how 3; Ta ble 2), includi ng that of t he outl iers , may have played a ma rgin morphology changed through time. Generalities can role in the inequalities in Holocen e growt h: (1 ) surface ele­ be observed as we track rising sea level from paleodepths of vation relative to th at of the platform , (2) location along the - 40 to - 20 m: (1 ) th e area off Sa nd Key is at first defined margin , and (3) orientation relative to exposure to high en­ by a widely exposed, continuous , br oad , high , rock-island ergy and cold, lagoonal , winter-storm waters. Off the lower rid ge, th en by three cons iderably na rrower islands of mu ch Keys, the unprotected open -ocean exposure , si milar elevation lower eleva tion, all form ed by th e fossil outlier reefs before relative to the platform , and parallel orientation relative to th e outer platform flooded (Figure 13A-Dl; (2) in contrast, prevailing-en ergy direction mad e the outliers particularly th e area off Key Largo is at first defined by a series of low­ vulne ra ble to winter-storm conditions . relief, discontinu ous (dune-ridge'l) islands with widely flood­ Off the up per Keys, however, th e influence of high er ele­ ed, shallow, intervening a reas, the n by two narrow reef-rock vation and different location and orientation relative to land islands at the margin edge (Figure 14A-Dl; (3) at a paleo­ and energy proved positive, in that Holocene corals were less depth of - 10 rn, the escarpment reef form ed a relatively un­ affected by high-en ergy waves and influx of inimical waters broken , margin- wide island ba rrier off th e Keys. All the rock th an cora ls on th e lower Keys escar pment a nd outlier reefs. islands probably harbored fri nging reefs, and all provided Th e age of th e oldest Holocen e Acropora palrnata on the most protection from high-energy waves of t he Straits of Florid a. seaward reef off Ft. Lauderdale is 7 ka (LIGHTY, 1977 ). At Sma ller scale cha nges occurred as th e un even platform sur­ th e time of its gro wt h, position of sea level was at about -7 face was inunda te d a nd th e long protective islands at the m (Figu re 10). Th e massive escarpme nt reefin th e study area ma rgin disappeared (summa rized from LIDZ and SHINN, a nd those off Ca rys fort and Ft. Lauderdal e were isla nds with 1991 ), Th e area of the lower Keys platform flooded ea rlier the ir seaward sides in th e surf zone . Platform topogr aphic and fas ter th an th at off th e upp er Keys, which is opposite th e highs such as t hose at Grecian Rocks had not yet flooded. timing and speed of floodin g of th e terrace. The timing of Th e escarpme nt reefs were protected from th e effects of win­ platform floodin g suggests th at Holocene outer-bank reef ter storms by th e Key Largo land mass. We believe that Ho­ growt h began first to th e south an d is supporte d by the ages locen e A. palmata thrived on th e landwa rd sides of th e upper of reefs at Looe Key (6.6 ka J a nd at Grecian Rocks (6.0 k a), Keys escarpment reefs becau se bedrock eleva tion at th e sea­ As wat er flowed throughout Hawk Channel at - 6 ka, fring­ wa rd edge of th e margin was higher than that off th e lower ing reefs backstepp ed, some by as much as several hundred Keys, platform flooding occurred more slowly, influx of la­ met ers, and increased wave action on the outer bank create d goona l water s was blocked, and the high-en ergy surf zone conditions mor e suitable to the moose- a nd staghorn cora ls rem ained fairly close to th e margin for a longer peri od of th an to th e head cora ls. Exte ns ive, rigorous spurs and ti me. Whereas cora ls on the south outl iers died at - 8.2 ka, grooves developed on windward sides of all outer-b ank ree fs. those on th e massive north reefs survived for at least anothe r With continued flooding of th e inn er platform, seawate r in­ 3 kyr (the reef off Ft. Lauderdale died at - 5 ka; LIGHTY, vaded sma ller depressions in th e Key Largo Limestone th at 1977 ). Dem ise of th e upper Keys Holocene cora ls is likely a would become th e deep tid al passes between th e Keys. Larger response to flooding of the inner platform with increased tida l depressions, now Florida a nd Biscayne Bays (Figure 1), be­ excha nge of turbid, nutrient-rich waters (POE 3; HUDSON, ga n to fill at - 4 ka. By - 2 ka, sea level was - 0.5 m lower

Journal of Coastal Research. Vol. 13. No. 3, 1997 Differ ential Quaternary Evolution 727

east FLOODING OF PLATFORM MARGIN LOWER FLORIDA KEYS

Figure 13. Block diagrams of margin bedrock topography off lower Keys showing morphologies during Holocene sea-level rise. Distance betwe en profiles not to scale (see Figure 2B for geographic locat ions and scale; profiles from Figure 9). (A) Formation of long, broad, high island ridge on outer terrace with shallow, wide, landward, lagoonal trough. Direction of water flow probably westw ard . (B) Deepening oflagoon and formati on of smaller, marginward, rock-island rid ges by satellite outliers. (C) Narrowing of islands. (0) Flooding of Hawk Cha nnel and most of inn er platform off lower Keys (see depths to platform bedrock in Tabl e 1. Diagrams at same scale as Figure 14.

than present. Rodriguez Key (Figure 2A), a peat island sur­ macro- algae and coral diseases that began in the mid-1980s rounded by a mud bank, acquired a cap of branching coral is thought by many (cf HALLOCK and SCHLAGER, 1986) to and coralline algae (TuRMEL and SWANSON, 1976), and the be related to sea-level rise, urbanization, agriculture, and re­ Florida Keys looked much as they do today. Holocene Acro­ sultant deleterious effects of nutrient-rich water. The reefs pora palmata and other coral species that once had flourished are further impacted by occasional cold-water temperatures in the spur-and-groove zones began their decline . (MAYOR, 1914; ROBERTS et al., 1982) and hurricanes (BALL Today , the vigor of modem coral growth continues to de­ et al., 1967; PERKlNS and ENOS, 1968; SHINN, 1976). As in cline (DUSTAN and HALAs, 1987; WARD, 1990; PORTER and the past, the most diverse and best developed reefs occur in MEIER, 1992; HALLOCKet al., 1993), and the proliferation of protected locations such as off Key Largo (GINSBURG and

Journal of Coastal Research, Vol. 13, No. 3, 1997 728 Lidz et al.

FLOODING OF PLATFORM MARGIN north-northeast

UPPER FLORIDA KEYS

\'~(\O _ ~ or! \~~--Prench\

.-:--- - ~~Molass""es \Reef . .-F ";; ~r.I~ Reef _ . ::.~' ~.. v-: Reef

Figure 14. Block diagram s of margin bedrock topography off up per Keys showing morphologies during Holocene sea -level rise. Dista nce betw een profiles not to scale (see Figu re 2A for geogr aphic locations and scale; profiles from Figure 8). (A) Form ation of discontinuous series of linear, low-relief isla nds , possibly dun e ridges, with wide, shallow, intervening lagoonal areas, (B) Flooding of terrace islands. (C) Form ation of esca rpment-reef (outlier-reef") islands off Molasses Reef an d southwest of Th e Elbow. Note linea r embayment behind The Elbow and fork ed area of embayment, where margin reentrant, when flooded, would isolate th e offset reef (see Figure 5). (D) Narrowing of islands and flooding of outer platform. Most of Hawk Cha nnel and all of inner platform off upp er Keys remain dry lan d (see Tab le 1 and Figure 4). Diagrams at sa me scale as Figu re 13.

Journal of Coasta l Research, Vol. 13, No. 3, 1997 Diflt'rl'nti(t! Quaternary Evolution 729

SHINN, 1964, 1994), where Grecian Rocks is one of the few the deepest burial sands (Plcistocener! between the ridges, remaining reefs to harbor spurs of living A. palmuta (SHINN, and possibly the terrace, wherever it is most accessible along 1963), albeit they are poorly developed (SH INN, 19RO L the margin, for purposes of dating. Sampling the basal sedi­ ments (Pleistocene") in the north escarpment-reef trough Outlier Reefs in the Geologic Record might be accomplished by core drilling. During the Holocene, the south area of the platform flooded To date, no margins characterized by outlier reef-and­ earlier and faster than the north area due to lower elevation trough systems are known from other modern carbonate and fewer protective rock islands. Holocene reefs are thickest banks or the geologic record. It is well known that ancient on the north outer bank, thinner on the south outer bank, carbonates of all ages world-wide (KIN(;, 1948; Rot:HL and and very thin on the south offbank outlier reefs. Why corals CHOQUETTE, 1985; and DUNSll ( et (11., 19R8, among others) did not survive on the outliers is not known; they essentially are extensive hydrocarbon-bearing reservoirs. Given time died at '~'8.2 ka, whereas those on the north escarpment reefs and sufficient overburden, the outlier reefs off South Florida survived for another 3 kyr until the platform became fully could form similar discrete, parallel, hydrocarbon-bearing flooded, and cold, turbid, low-salinity lagoonal waters invad­ traps within a larger platform-margin complex. They un­ ed the shallow offshore regime. doubtedly contain numerous calcrete and/or cement seals formed during each fall in Pleistocene sea level. The fact that ACKNOWLEDGEMENTS they form significant structures implies that they probably exist off windward margins elsewhere, as well as throughout The authors are grateful to Captain Roy Gaensslen of the the geologic record, but have not been recognized. We suggest R / V Cu.pttun'» Lady, who skillfully maneuvered boat and seis­ that explorationists keep this example in mind when pros­ rnic equipment around shallow coral reefs. Christopher D. pecting any ancient platform carbonates. Reich, David Mallinson, and Steve Goodbred assisted in ob­ taining the seismic profiles. Michael W. Harris is acknowl­ CONCLUSIONS edged for his part in interpreting the seismic profiles. Appre­ ciation is also extended to Miss Jennifer C.W. Bexley for dig­ High-quality seismic-reflection profiles identify an exten­ ital cartographic work on Figures 4 and 5. Using ARC/INFO sive Quaternary reef-and-trough system along the South and MAPG EN computer software, her chief responsibilities Florida windward margin. Depending upon location, the sys­ incl uded development and revision of thematic data layers, tem exhibits contrasting morphologies and has a uniquely op­ graphic design, and preproduction of the contoured rnaps. posite, aggradational and progradational potential to huild Karen L. Monroe Morgan, Stephen P. Smith, and Christo­ seaward. Differential evolution of the morphologies was con­ pher D. Reich are thanked for their expertise and assistance trolled by different agents over time. During the Pleistocene, in creating the graphics for Figures 1, 2A and B, and 10-14. the primary influences were the effects of energy regime and We especially thank two anonymous reviewers whose excep­ fluctuating sea level on readily available terrace sediments tionally constructive comments greatly improved the manu­ at an arcuate, subaerially exposed margin. During the Ho­ script. locene, the primary influences were the disaimilarit.ics in the This project was funded in part by the USGS Center for antecedent Pleistocene morphologies that produced, with ris­ Coastal Geology and Regional Marine Studies and in part by ing sea level, corresponding, localized contrasts in environ­ the National Underwater Research Center at the University ment and reef vitality: the thickest Holocene reefs accurnu­ of North Carolina at Wilmington (grants UNCW-9325 and lated leeward of bedrock barriers in areas of calm. clear, oce­ UNCW-9225L anic waters. The massiveness of this system, particularly that of the outlier reefs, implies that similar systems should LITERATURE CITED exist elsewhere, and perhaps as parallel hydrocarbon traps in the geologic record. BALL. M.M.: SHINN, E.A.. and STOCKMAN. K.W., 1967. The geologic effects of Hurricane Donna in South Florida. Journal 0/ Geology, Pending confirmation by acquisition of radiometric data, 75(51. :')8:3-597. our preliminary interpretation of terrace morphologies off the BEA( 'J I, D.K. and GINSBI fl{(i. R.N., 1980. Facies succession of Plio­ upper Florida Keys is based on two key elements: palcocner­ cene-Pleistocene carbonates, northwestern Great Bahama Bank. gy and sedimentation patterns similar to those of today, and American Association 0/ Petroleum Geologists Bulletin, 64(10). 1634-1642. coral recruitment primarily to elevated, lit hified surfaces. We DllNSHI, Y., ZHON(i.JlAN. Q.: MIN(i(iAN(i, L.; GANSHI<:NC, L., and Rt T­ infer that: (1) the acoustically obscu re terrace seismic facies .11~, L.. 19~8. Zonation and oil and gas resource potential of oil­ represent Pleistocene stillstand beach-dune ridges; (2) the bearing regions of China. In: WA(iN~:R. H.C.: WA(;NER, L.C.: hard ridge surfaces were buried by onshore sediment trans­ WAN(i, F.F.H.. and WON(i, F.e. (eds.r. Petroleum Resources 0/ China and Related Subjects: Houston. Texas, Circum-Pacific port during a rapid pulse of sea-level rise; thus, (~J I Pleisto­ Council for Energy and Mineral Resources Earth Science Series, cene corals could not duplicate the outlier-reef blueprint of 10. pp. 2~1-39. the south, although conditions otherwise were optimal for DlJSTAN, P. and HALAS, J.C.. 1987 Changes in the reef-coral corn­ their growth as exemplified hy the margin-wide escarpment munity of Carysfort Reef, Key Largo. Florida: 1974 to 1982. Coral reef; and (4) the north part of the margin is less advanced Reef,;, 6.91-106. ENOS. P.. 1977. Carbonate sediment accumulat.ions of the South than the south part, where the outlier reefs are highly de­ Florida shelf margin, part I. In: PAUL ENOS and R.D. PEUKINS, veloped. Future research, perhaps by submersible. might be (eds. J. Quaternary Depositonal Framework of South Florida. Geo­ directed toward sampling the north-terrace (duner ' ridges, logical Society 0/ America, Memoir 147. pp. 1--130.

.Iournal of Coastal Research, Vol. 1:3, No. :3, H)97 730 Lidz et al.

FAIRBA NKS, R.G., 1989. A 17,000-year glacio-eu st atic sea level rec­ South Florida (3 map sheets including descriptive and interpret ive ord: Influ ence of glacial melting rates on the Younger Dryas event material and 14 figur es including 1 aerial photomosaici. and deep-ocean circulation. Nature, 342 ,(6250), 637-642. LIWI'I'Y, R.G., 1977. Relict shelf-edge Holocene cora l reef, southe ast FAIRBANKS, R.G., 1990 . The age and origin of the "Younger Dryas coas t of Florida. Proceedin gs. 3rd Int ernational Coral Reef Sy m ­ clim ate event" in Greenland ice cores. Paleoceanography, 5(6 1, posium. 2, Miami, Florida, pp. 215- 221. 937-948. LocK EH , S.D.; HINE, A.C.; TEDESCO, L.P ., and SHINN, E.A., (in GINSBUIHi, R.N. and SHINN , E.A, 1964 . Distribution of th e reef­ press ). Magnitude and timing of episodic sea-level rise during th e building community in Florida and the Bahamas (abs. J, American last deglaciation. Geology. Association of Petroleum Geologists Bulletin. 48, 527. LUDWIG , KR.; MUllS, D.R.; S IMM ONS, KR.: HALL EY, R.B., and GINSBURG , R.N. and SHINN, E.A., 1994. Prefer ential distribution of SHINN, E.A, 1996. Sea-level record s at - 80 ka from tectonic ally reefs in th e Florida reef tract : Th e pa st is th e key to th e present. stable platforms: Florid a and Bermuda. Geology. In: R.N. GINSBURG (compiler), Proceedings. Colloquium on Global MAHSZALI';K, D.S.; BAB ASIlOFF, JH., G.; NOEL, M.R., and WORLEY, Aspects of Coral Reefs: Hazard s and History, 1993, Rosensti el D.R., 1977. Reef dist ribution in South Florid a. Proceedings, 3rd School of Marine and Atmo spheric Scien ce, Miami, Florida, pp. Int ernat ional Coral Reef Symposium, 2. Miami , Florid a, pp. 223­ 21-26. 229. HALLOCK, P.; MULL EI1-KAHGER . F.E ., and HALAS , J. C., 1993 . Coral MAYOH, A.G., 1914 . The effects of temp erature on tropical marine reef declin e. Nati onal Geograph ic Research and Expl oration. 9(3J, animals. Carnegie Institution of Washin gton Publication 183. 6, 358-378. 1-24. HALLOCK, P. and SCH LAG ~;R , W., 1986 . Nutrient excess and th e de­ MILLIM AN, J .D., 1974. Recent Sedim enta ry Carbonates, Pt. I, Marin e mise of cora l reefs and carbonate platforms. PALAlOS , 1,389- 398. Carbonates. Springer-Verl ag Berlin, 375 pp. HINE, A C. and NEUMANN, A.C., 1977. Sh all ow carbona te bank ma r­ N ~; UMA N N , A C. and MACINTYH E. I.G., 1985. Response to sea level gin growth and structure, Little Bah am a Ba nk , Bah amas. Amer­ rise: Keep-up , catch-up or give-up. Proceedings. 5th Internati onal ican Association of Petroleum Geologists Bulletin , 61, 376-406. Coral Reef Symposium. 3. Tahiti, pp. 105-110. HI N~; , A C. and MULLINS, H.T., 1983. Modern carbonate shelf-slope PAHKEH, G.G. and COOKE, C.W., 1944. Late Cenozoic res ources of breaks. In : D.J . STANL ~; Y and G.T. MOOR ~; , (eds .I, Th e Sh elfbreak : southeaste rn Florida, with a discussion of the ground water. Flor­ Critical Interface on Continental Margins. Society of Economic Pa­ ida Geological S ur rey Bulletin . no. 27, pp. 1-119. leontologists and Mineralogists Sp ecial Publ ication 33. pp. 169­ PAHKEH , G.G.; Hoy. N.D., and S< 'IIHOEDEH, M.C., 1955. Geology. In : 188. G.G. PAI{KEH, G.E. FEHG USON, S.K Lovic, et al.. (eds. ), Water HOFFMElsn;H, J .E., 1974. Land from the Sea, the Geologic Story of resources of southeaste rn Florid a. U.S . Geological S urvey Water­ South Florida . Cora l Gables, Florid a: University of Miami Press, S upply Paper 1255. pp. 57- 125. 143p. P EH KI NS, R.D., 1977 . Depositional fram ework of Pleistocene rocks in H

Journal of Coast al Research , Vol. 13. No. 3. 1997 Differential Quaternary Evolution 731

SHINN, E.A., 1963. Spur and groove formation on the Florida reef bonate-sand accumulation, The Quicksands, southwest Florida tract. Journal of Sedimentary Petrology, 33, 291-303. Keys. Journal of Sedimentary Petrology, 60(6),952-967. SHINN, E.A., 1976. Coral reef recovery in Florida and the Persian SHINN, E.A.; REESE, R.S., and REICH, C.D., 1994. Fate and path­ Gulf. Environmental Geology, 1, 241-254. ways of injection-well effluent in the Florida Keys. Department of SHINN, E.A., 1980. Geologic history of Grecian Rocks, Key Largo Interior Open-File Report 94-276, 116p. Coral Reef Marine Sanctuary. Bulletin of Marine Science, 30, SHINN, E.A.; LIDZ, B.H., and HARRIS, M.W., 1992. Factors control­ 646-656. ling distribution of Florida Keys reefs (abs.). 1992 Symposium on SHINN, E.A., REICH, C.D., LOCKER, S.D., and HINE, A.C., 1996. A Florida Keys Regional Ecosystem, Miami, Florida (unpaginatcd), giant sediment trap in the Florida Keys. Journal of Coastal Re­ SHINN, E.A.; LIDZ, B.H.; KINDINGER, J.L.; HUDSON, J.H., and HAL­ search, 12(4), 953-959. LEY, R.B., 1989. Reefs of Florida and the Dry Tortugas: A guide SHINN, E.A.; HUDSON, J.H.; HALLEY, R.B., and LIDZ, B.H., 1977. to the modern carbonate environments of the Florida Keys and Topographic control and accumulation rate of some Holocene coral the Dry Tortugas. International Geological Congress Field Trip reefs, South Florida and Dry Tortugas. Proceedings, 3rd Interna­ Guidebook T176, American Geophysical Union, Washington, D.C., tional Coral ReefSymposium, 2, Miami, Florida, pp. 1-7. 55p. SHINN, E.A.; HUDSON, J.H.; ROBBIN, D.M., and LIDZ, B.H., 1981. TOOMEY, D.F., 1981. European Fossil Reef Models. Society of Eco­ Spurs and grooves revisited: construction versus erosion, Looe Key nomic Paleontologists and Mineralogists, Special Publication 30, Reef, Florida. Proceedings, 4th International Coral Reef Sympo­ 546p. sium, (Manila, Philippines), 1, pp. 475-483. TOSCANO, M.A.; LUNDBERG, J., and HINE, A.C., 1995. Age and pa­ SHINN, E.A.; LIDZ, B.H.; HALLEY, R.B.; HINE, A.C., and REESE, R.S., leoclimate of submerged late Pleistocene reef system, southeast 1993. Geology, tidal pumping, and nutrients in the Florida Keys. Florida shelf: Implications for substage 5a sea level. Geological Geological Society of America, Abstracts with Programs, 25(6), p. Society ofAmerica, Abstracts with Programs, p. A-448. A-289. TURMEL, R.J. and SWANSON, R.G., 1976. The development of Rod­ SHINN, E.A.; LIDZ, B.H., and HINE, A.C., 1991. Coastal evolution and riguez Bank, a Holocene mudbank in the Florida reef tract. Jour­ sea-level history: Florida Keys. SEPM Gulf Coast Section Confer­ nal of Sedimentary Petrology, 46(3), 497-518. ence on Gulf Coast Geology, pp. 237-239. WARD, F., 1990. Florida's coral reefs are imperiled. National Geo­ SHINN, E.A.; LIDZ, B.H., and HOLMES, C.W., 1990. High-energy car- graphic, 178( 1), 114-132.

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