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Low marine terraces of Grand Cayman Island
Estuarine, Coastal and Shelf Science (x98x) x2, 569-578
Low Marine Terraces of Grand Cayman Island ~
K. O. Emery Woods lIole Oceanographic Institution, IVoods Hole, l~Iassachusetts 02543, U.S.A. Received 28 April I98o
Keywords: terraces; erosion; coastal erosion forms; limestone; radioactive dating; sea levels; Caribbean Sea Twenty levelling profiles across the shore zone of Grand Ca)man Island reveal the presence of six low marine terraces above present sea level. The lowest and youngest one at about 2 m elevation is the most widespread anti probably represents Sangamon interglaci,-d high sea level. Higher terraces are much eroded and probably much older, but none provides evidence of post-terrace warping. Similarities in age and elevation of the 2 m terrace on this island to those in north-western Yucatan, northern Jamaica, the Bahamas and Florida suggest that vertical movements at Grand Cayman Island have been minor during at least the past x25 ooo years, even with its nearness to the teetonically active Cayman Trough.
Introduction Grand Cayman Island is the largest of a group of three islands that form a British dependency in the north-western Caribbean Sea. While participating in a submersible study of the Cayman Trough, the author visited the island several times during the spring of I976 and made observations and measurements of shore characteristics on foot and from a helicopter. The study was initiated to learn what evidence of diastrophic movement could be provided by the land to supplement data obtained by surface ship and submersible in the adjacent tectonically active Cayman Trough. Even casual observation shows the presence of low terraces around the island, but the degree of correlation and possible warping of the terraces had not been determined. Warping might be expected, because the Cayman Ridge on which the island is situated plunges westward 4ooo m in xooo kin. The shape of the island (Figure 1) is suggestive of an emerged and tilted atoll; however, its geology shows it to be an erosional feature carved from the Miocene shallow-water Bluff Limestone and surrounded by fringing and barrier reefs. Tile limestone underlies the low interior and forms the highest seacliffs of the island [Plate x(c), (d)], thus giving rise to the name Bluff Limestone. Its fossil content (Matlcy, I924, I926; Richards, I955; Rehder, I962 ) suggests correlation with the White Limestone of Jamaica. Lapping atop the Bluff Limestone along much of the shore is the Ironshore Formation that is considered to be as young as Pleistocene and perhaps even Holocene by Matley, Richards, and Rehder and by Brunt et al. (x973). The name derives from its hardness and position. Thicknesses of the Iron.shore Formation reach 17 m at George Town, covering old ~ No. 3956 of the Woods Hole Oceanographic Institution.
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seacliffs and platforms cut in the Bluff Limestone (Brunt et al., I973). Five facies described by Brunt and his associates are: Reef Facies--large corals in growth position; Black Reef Facies--fragments of corals that had been transported and deposited in a matrix of sand with many molluscs, especially Strombus gigas Linn., which lives only at depths of 4 to 9 m in the region (Abbott, I958); Lagoonal Facies---chalky limestones and marls; Shoal Facies--ridges of crossbedded calcarenite as high as 7 m atop the lagoonal facies; and Beach Ridge Facies-low ridges of crossbedded calcarenite with casts of roots or burrows atop the Bluff Limestone. The first three facies are in the western part of the island, and the last two are common around most of the island. A modern counterpart of the lagoonal facies is now forming in North Sound (Roberts, x97ia, 197tb, x976). Calculations from drawdown in test borings ~Iather, x972) indicates that the lagoonal facies has a low permeability and that all other facies (high energy ones) have high permeabilities except where case-hardened by deposition of calcium carbonate in the shore zone. Grand Cayman Island is in the trade-wind belt, and the winds are almost invariably from the north-east (National Climatic Center, I974). These winds produce waves that are higher than x'8 m 25% of the time and higher than 3"4 m 2"5% of the time during January; lowest waves occur during October when they exceed x.8 m only 7% of the time. Roberts (1974), Roberts et aL (t975), and Rigby & Roberts (i976) used hindcasting of winds suppolted by field measurements to show that the reef morphology is closely related to wave power. For example, the highest waves and the best developed reefs occur along the eastern coast and the lowcst waves with reefs absent characterize the western coast. In addition, the high temperature (24 to 29 ~ and high rainfall (i5o cm year -1) (Hsu et al., t972 ) are conducive to rapid weathering of rock outcrops. Shore types
Mapping on foot and from a helicopter showed that the x27 km of shoreline around Grand Cayman Island consists of four main kinds of material (Figure 2). These are muddy peats
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Figure 3- Profiles across shore zone at positions shown by Figure t, using method described in text. Each profile begins near mean low water. Horizontal lines are at z m intervals. No vertical exaggeration. Large dots are measured elevations; small ones are syrnbolie of the intervening topography. are fronted by a series of rimmed pools as much as 5 m wide and z 5 m long, particularly near profiles M [Plate x(e)], and L. In the spray zone there are solution basins that are steepsided, fiat-floored, contain many marine plants and animals, and are light buff coloured; these are formed by solution of the limestone at night by water charged with carbon dioxide produced by plant and animal respiration (Emery., I946, z96z ). Also present are borings by marine animals which add to erosion by mechanical rasping (Neumann, z966 ). Where storm waves are particularly high, a few solution basins have been converted into potholes through abrasion by one or more cobbles swirled around inside them. The rate of intertidal solution of the general surface of the low terrace was found to be 2 to 6 mm year-1 by Worthin (x 959), who measured the lowering of the rock surface with respect to insoluble materials placed against the Ironshore Formation at known dates. Along some shores (profiles D, E, and H) the original seacliff has become protected from wave and spray attack by deposition of rubble or sand beaches. Shoreward of the spray zone, including areas covered by land vegetation, the rock surface of all low terraces is sharply jagged, case-hardened, and black or dark grey--typical phyto-
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karst [Plate z(0 ] due to solution by boring blue-green algae aided by rain and dew (Folk, Roberts & Moore, z973). Clearly, the effect of wave abrasion and seawater solution is shore recession, and the effect of rain and dew solution is lowering of the land side of the shore. Lowering of the land ~rfaee has gone so far that evidence of former shallow-water deposits atop the terraces higher than z m could not be found. In fact, the bottoms of the conical phytokarst arc commonly as deep as 0. 5 m below their uppermost tips, and exceptionally they are more than x m deep. Solution pits or shaft-like caves as deep as 8 m were noted at profiles J and P and may occur elsewhere. Even though the original terrace surfaces, their coral-algal growth and their sediments have been destroyed and lowered, the general shapes and profiles of the terraces appear to remain. Profiles Profiles were surveyed across the rocky shores at zo sites (Figure z) to investigate the degree of correlation and possible warping of the terraces. The instruments were two poles, each 6 or 9 ft long and having a yardstick nailed to the top patt. Each was held vertically one rod length (6 ft) apart and at right angles to the shore trend. The intersection of the line of sight to the horizon on each pole was read and recorded. Over karstic topography tile poles were set atop high points of the rock so far as possible. Successive landward pole positions and readings permitted construction of a profile as long as desired, usually to the point where rock outcrops were obscured by thick soil or dense vegetation. Correlation was later made for the depression of the horizon below local horizontal [see Emery (z96r) for details and accuracy of the method] and for tide level obtained from a tide gauge operated by the
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Mosquito Research Control Unit and located at the shore of South Sound (directly south of George Town). The tide-gauge recordings were adjusted to their best vertical fit with the predicted tide levels given by the U.S. Department of Commerce (x975). Mean tide range appears to be about 0.4 m. T h e profiles (Figure 3) converted to metres show details of the topography, but they are rather difficult to compare with each other. An interpretation of them is provided by Figure 4 separately for the north side and the south side of the island. A diagonal line pattern shows the elevation above sea level and the probable correlation of terraces crossed by the profiles. The 2 m terrace is most widespread, absent mainly or only where subsequent wave erosion has removed it, or where it was buried under later shore deposits. The higher terraces are less easily correlated, partly because some had been removed by erosion when lower terraces were formed, but also because higher terraces [Plate i(e), (d)] could be recorded only where the Bluff Limestone had been high enough to protrude above the once relatively higher sea levels. Thus only low terraces occur at the west ind. Additional terraces now below sea level are reported by local divers and described by Roberts (x974) and Rigby & Roberts (x976) at Grand Cayman Island. Common depths for these submarine terraces are 8 m, 15 m to the shelf break at 2o-25 m, and 73-75 m. Similar submerged terraces at Jamaica were described by Cant (x972), Roobel & Gibb (x973) , Goreau & Land (i974), and Moore et al. (I976).
Radiometrie dates Ages of the terraces are shown only to be post-Miocene by the species of fossils in the Bluff Limestone and the Ironshore Formation. Two attempts at radiocarbon dating of recrystallized shells in the lagoonal facies of the Ironshore Formation yielded only ages older than 4 ~ ooo years (Brunt et al., I973). Samples of conch (Strombus) shells embedded in reef and back reef facies of the Ironshore Formation at profiles C, R, and S (Figure 3) were selected for thorium-uranium dating because they are very dense and consist mainly of aragonite with some alteration to calcite, according to X-ray spectra. Isotope ratios foi" the dense central column of the least altered conch shell, from about 1-5 m above mean low water at profile R, yielded a 23~ age of 24o 000+3000 yeats. However, tile 2aIPa/2asu ratio was 1.2--higher than the theoretically attainable value of i.o; low uranium here is attributed to losses during weathering, so that the 24o ooo-year age must be too high. Another attempt was made on a sample of coral [Diploria cf. strigosa (Dana)] embedded in the reef facies about x.5 m above mean low water at profile C. X-ray data show about i5% calcite, indicating some alteration by weathering. Two allquots of the sample were analysed; both yielded a 2a~ age of x55 000110 ooo years. The activity ratios of 2a~U/z3sU (1.1o4-o.oi) and of 2alPa/2asU (0.994-o.o 3) for the same are consistent with this age. Loss of some uranium indicates that this age also is somewhat too high. Thus the terrace deposit probably formed during the Sangamon stage of interglacial high sea level, according to the time scale of Emiliani & Shacklcton (i974). The highly weathered and eroded condition of the higher terraces implies a considerably greater age for them.
Related regional data Both emerged and submerged terraces may have been controlled by changing custatic sea levels caused by waning and waxing of glaciers. However, tectonic movements of the island
Lozo marine terraces of Grand Cayman Island
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could also have occurred because of the island POsition atop the Cayman Ridge just north of the Cayman Trough [a crustal plate boundary (Molnar & Sykes, 1969)]. Holcombe et al. (t973) suggested that the trough is the scene of active sea-floor spreading on the basis of topography, heat flow (Ericson et al., 1972), seismic reflection data, free-air gravity anomalies (Bowin, 1968), numerous small earthquakes in the region (Barazangi & Dorman, 1969) , dredgings by Perfit & Heezen (1978) and our own investigations at sea (Ballard, 1976; CAYTROUGH, i979; Emery & ~'Iilliman, i98o ). A way of estimating tectonic stability of Grand Cayman Island is to compare the 2 m Sangamon terrace with the elevations and ages of terraces in surrounding land areas. Similar elevations and ages were found at north-eastern Yucatan (Szabo et al., 1978); on the north coast of Jamaica even though local displacements by post-terrace faulting are known (Cant, 1972; Horsfield, 1972); in the Bahama islands (Neumann & lXioore, 1975); and in Florida (Osmond et al., 1965). In another 'stable' area, Oahu of Hawaii, a terrace at 7.6 m elevation yielded 23 more dates having a mean of I22 ooo years (Ku et aL, 1974). Still others in the same elevation range of 2 to io m have been listed by Kern (1977) in the Pacific Ocean as 12o ooo to 14o ooo years old. At other places in the Caribbean Sea the general Sangamon age is reported for terraces at high elevations as though displaced by local diastrophic movements. For example, ~Iesolella et al. (1969) , Bender et al. (1973), iNIatthews (1973) and Fairbanks & ~'Iatthews (1976) obtained about 125 ooo years for terrace III between 24 and 59 m above sea level on Barbados Island. These higher levels may be due to local uplift associated with subduction at the eastern end of the Caribbean Sea. As many as 28 undated terraces reach maximum elevations of 4o0 m on Hispaniola and eastern Cuba, perhaps indicating an eastward upwarp of the region centering upon north-western Hispaniola (Horsfield, 1975). Living land snails on Grand Cayman Island (Pilsbry, 1931) are pertinent to the question of relative elewtion, because 78~o of the species are Jamaican and only 4~o Cuban, and because some of the snails are endemic with enough independent evolution to require at least part of the island to have remained subaerial since well back in the Tertiary Period. The present maximum elevation of about 2o m would seem to have been enough to allow part of the island to remain above sea level during this time. Evidence of some emergence of Grand Cayman Island has been cited by English (1912) whose remarks were repeated by ~Iiatley (1926), Richards (1955), and Brunt et al. (1973). English reported that he was shown rocks and trees 'many yards from even shallow water, marking places from which it was possible from my informant's memory to catch sizeable fish, or launch canoes'. Richards added that at Boddentown some streets now run where the oldest inhabitants remember shallow water and that shoaling caused abandonment of the town harbour. Accordingly, they concluded that the island was rising about a half inch per year; if continued, this would correspond with the enormous and totally unreasonable rate of 127o cm/tooo years, or about 6 m since Columbus' time.
Conclusions The modern shores of Grand Cayman Island consist of rock along seaeliffs and platforms, loose sand and rubble on beaches with local cementation into beachrock, and muddy sand in mangrove swamps. A terrace 2 m above mean low sea level is composed of reef facies having a probable San~amon interglacial age. Several higher terraces exist, but no marine materials remain on them owing to extensive subaerial weathering and erosion after emergence. Twenty topographic profiles surveyed across the rocky shores showed that the emergent
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terraces can be correlated. No warping, either transverse or along the length of Grand Cayman Island, can be recognized from the elevations of the 2 m terrace nor of the other slightly higher but probably much older terraces.
Acknowledgements Appreciation is due R. D. Ballard for the opportunity to participate in the Cayman Trough sttfdy and to Gideon Bain of Grand Cayman Island, Raymond Wright of the Jamaica Geological Survey, Walter Sullivan of the New York Times, and D. S. Hosum of the Woods Hole Oceanographic Institution for their help as rodmen, observers and photographers during the levelling surveys of the raised terraces. Special thanks is given the Mosquito Research Control Unit for a low altitude helicopter flight around the island that permitted mapping of shore types and identification of important sites to be profiled, and for collection of a coral sample for radiometric dating. Most important are the radiometric ages for samples from Grand Cayman Island kindly provided by T.-L. Ku of the University of Southern California. Ku, R. K. Matthews of Brown University, and J. D. Milliman of Woods Hole Oceanographic Institution read a semi-final draft of the manuscript and provided suggestions and additional data. Funding was by National Science Foundation grant 2o]185.33 to R. D. Ballard and by the Ocean Industry Program of the Woods Hole Oceanographic Institution.
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Folk, R. L., Roberts, H. H. & bloore, C. H. x973 Black phytokarst from Hell, Ca)man Islands, British West Indies. Geological Society of America Bulletin 84, 235i-=36o" Goreau, T. F. & Land, L. S. I974 Fore-reef morphology and depositional processes, north Jamaica. In Reefs in Time and Space (Laporte, L. F., ed.). Society of Economic Paleontologists and Mineralo. gists Special Publication x8, pp. 77-89. Higgins, C. G. z98o Nips, notches, and the solution of coastal limestone: an overview of the problem with examples from Greece. Estuarbze and Coastal Marine Science xo~ z5-3o. Holcombe, T. L., Vogt, P. R., Matthews, J. E. & Murchison, R. R. x973 Evidence for sea-floor spreading in the Cayman trough. Earth and Planetary Science Letters 2o, 357-37I. Horsfield, W. T. t972 A late Pleistocene sealevel notch, and its relation to block faulting on the north coast of Jamaica. ffournal of the Geological Society of ffamaica I2, z8-22. Horsfield, W. T. x975 Quaternary vertical movements in the Greater Antilles. Geological Society of America Bulletbz 86~ 933-938. Hsu, S.-A., Giglioli, bl. E., Reiter, P. & Davies, J. I972 Heat and water balance studies on Grand Cayman. Caribbeanffournal of Science x2~ 9-22. Kern, J. P. x977 Origin and history of upper Pleistocene marine terraces, San Diego, California. Geological Society of America Bulletin 88, x553-x566. Ku, T.-L., Kimmel, bl. A., Easton, x,V. H. & O'Neil, T. J. x974 Eustatic sea level z2o,ooo years ago on Oahu, Hawaii. Science x83, 959--962. blather, J. D. x972 The geology of Grand Cayman and its control over the development of lenses of potable groundwater. V1 Conference del Caribe--Isla de 3fargarita, Venezuela lllemories, pp. x54x57. iX,latley, C. A. x924 Report of a reconnaissance survey of the Cayman Island. Supplement toffamaica Gazette, Friday I8 June 47~ no. 9, 69--73. Matley, C. A. z9z6 The geology of the Cayman Islands (British ~Vest Indies) and their relation to the Bartlett Trough. ffournal of the Geological Society of London 8;~ 352--387. Matthews, R. K. x973 Relative elevation of late Pleistocene high sea level stands: Barbados uplift rates and their implications. Quaternary Research 3, x47-x53. blesolella, K. J., blatthews, R. K., Broecker, W. S. & Thurber, D. L. x969 The astronomical theory of climatic change; Barbados data. ffounml of Geology 77~ z5~ 9 Molnar, P. & Sykes, L. R. x969 Tectonics of the Caribbean and Middle American regions, from focal mechanisms and selsmicity. Geological Society of America Bulletin 8o~ z639-z684. Moore, C. H. Jr. x973 Intertidal carbonate cementation Grand Cayman, West Indies. Jourlml of
Sedimentary Petrology 43, 59 x-6o2. Moore, C. H. Jr., Graham, E. A. & Land, L. S. x976 Sediment transport and dispersal across the deep fore-reef and island slope (--55 m to --305 m), Discovery Bay, Jamaica. ffournal of Sedimentary Petrology 46, z74-x87. National Climatic Center z974 Summary of synoptle meteorological observations, Caribbean and nearby island coastal marine areas; Area 7--Cayman. U.S. Naval IVeather Service Command, Asheville, N. C. 79 PPNeumann, A. C. z966 Observations on coastal erosion in Bermuda and measurements of the boring rate of the sponge, Cliona lampa. Limnology and Oceanography ltx~ 92-xo8. Neumann, A. C. & bloore, W. S. 1975 Sea level events and Pleistocene coral ages in the northern Bahamas. Quaternary Research 5~ 215-224. Newell, N. D. & Bloom, A. L. x97 o The reef fiat and "two-meter eustatie terrace" of some Pacific atolls. Geological Society of America Bulletin 8z~ x88z-z894. Osmond, J. K., Carpenter, J. R. & x,Vindom, H. L. x965 Thea~ TM age of the Pleistocene corals and oolites of Florida.Journal of Geophysical Research 7o~ x843-x847. Pilsbry, H. A. x93z Results of the Pinchot South Sea Expedition; x. Land mollusks of the Caribbean islands, Grand Cayman, Swan, Old Providence and St. Andrews. Academy of Natural Sciences of Philadelphia Proceedings 8z, 22z-?6z. Rehder, H. A. x96z The Pleistocene mollusks of Grand Cayman Island, with notes on the geology of the island, ffournal of Paleontology 36~ 583-585. Revelle, R. & Emery, K. O. z957 Chemical erosion of beach rock and exposed reef rock. U.S. Geological Survey Professional Paper z6o-T. 699-709. Richards, H. G. z955 The geological history of the Cayman Islands. Academy of Science of Philadelphia, Notulae Naturae 284, x x pp. Rigby, J. K. & Roberts, H. H. x976 Geology, reefs, and marine communities of Grand Cayman Island, British West Indies. Geology Studies, Coastal Studies Institute, Louisiana State University Special Publication 4, z-'95. Roberts, H. H. x97za Environments and organic communities of North'Sound, Grand Cayman Island BAV.I. Caribbeanffournal of Science xx~ 67-7x. Roberts, H. H. x97xb Mineralogical variation lagoonal carbonates from North Sound, Grand Cayman Island (British West Indies). Sedimentary Geology 6~ 2ox-zz3.
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Roberts, H. H. x974 Variability of reefs with regard to changes in wave power around an island. Secm:d
Iutert~tional Symposium on Corals and Coral Reefs, June -°z-July e, z973, Great Barrier Reef Committee, Brisbane, Australia Proceedings. pp. 497-5 xx. Roberts, H. H. I976 Carbonate sedimentation in a reef-enclosed lagoon, North Sound, Grand Ca)man Island. Geology Studies, Coastal Studies Institute, Louisiana State Uni~'ersity Special Publication 4, 97-xzz. Roberts, H. H., bIurray, S. P. & Suhayda, J. N. x973 Physical processes in a fringing reef system. Journal of 2~larine Research 33~ 233-260. Roobol, J. J. & Gibb, J. S. x973 Submarine sea cliffs and caves at Negril, W. Jamaica.Journal of the Geological Society of Jamaica 13, x4-x8. Russell, R. J. x96z Origin of beach rock. Zeitschriftfiir Geomorphologie 6~ t - i 6 . Shepard, F. P., Curray, J. R., Newman, W. A., Bloom, A. L., Newell, N. D., Tracey, J. I. Jr. 5: Veeh, H. H. x967 Holocene changes in sea level: evidence in Micronesia. Science x57, 54z-544. Szabo, B. J., Ward, W. C., Weidie, A. E. & Brady, M. J. x978 Age and magnitude of the late Pleistocene sea-level rise on the eastern Yucatan peninsula. Geology 6~ 7x3-715 . U.S. Department of Commerce x975 Tide tables x976, high and low water predictions East Coast of North America and South America. National Oceanic and Atmospheric Administration I36, 239. Worthin, A. S. Jr. x959 Ironshore in some West Indian islands. New York Academy of Sciences Transactions Series e 2x~ 649-65z.
Plate x. Coastal views of Grand Cayman Island. (a) T w o - m e t r e terrace at shore near profile C (Figure I). N o t e surf from waves refracted around north-western corner of island. (b) Beachrock and loose beach sand off B o d d e n t o w n (Figure x). (c) Sixmetre terrace being eroded at seacliff near profile G (Figure i). (d) Eleven-metre terrace at profile P (Figure i). Note deep nip at base of seacliff. (e) T w o - m e t r e terrace during period of high waves at profile M (Figure x). T o the right and at the distant horizon is the 4-metre terrace. (f) Sharp phytokarstic reefroek at profile R (Figure i). H o s u m with one of the levelling rods.
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Low Marine Terraces of Grand Cayman Island ~
K. O. Emery Woods lIole Oceanographic Institution, IVoods Hole, l~Iassachusetts 02543, U.S.A. Received 28 April I98o
Keywords: terraces; erosion; coastal erosion forms; limestone; radioactive dating; sea levels; Caribbean Sea Twenty levelling profiles across the shore zone of Grand Ca)man Island reveal the presence of six low marine terraces above present sea level. The lowest and youngest one at about 2 m elevation is the most widespread anti probably represents Sangamon interglaci,-d high sea level. Higher terraces are much eroded and probably much older, but none provides evidence of post-terrace warping. Similarities in age and elevation of the 2 m terrace on this island to those in north-western Yucatan, northern Jamaica, the Bahamas and Florida suggest that vertical movements at Grand Cayman Island have been minor during at least the past x25 ooo years, even with its nearness to the teetonically active Cayman Trough.
Introduction Grand Cayman Island is the largest of a group of three islands that form a British dependency in the north-western Caribbean Sea. While participating in a submersible study of the Cayman Trough, the author visited the island several times during the spring of I976 and made observations and measurements of shore characteristics on foot and from a helicopter. The study was initiated to learn what evidence of diastrophic movement could be provided by the land to supplement data obtained by surface ship and submersible in the adjacent tectonically active Cayman Trough. Even casual observation shows the presence of low terraces around the island, but the degree of correlation and possible warping of the terraces had not been determined. Warping might be expected, because the Cayman Ridge on which the island is situated plunges westward 4ooo m in xooo kin. The shape of the island (Figure 1) is suggestive of an emerged and tilted atoll; however, its geology shows it to be an erosional feature carved from the Miocene shallow-water Bluff Limestone and surrounded by fringing and barrier reefs. Tile limestone underlies the low interior and forms the highest seacliffs of the island [Plate x(c), (d)], thus giving rise to the name Bluff Limestone. Its fossil content (Matlcy, I924, I926; Richards, I955; Rehder, I962 ) suggests correlation with the White Limestone of Jamaica. Lapping atop the Bluff Limestone along much of the shore is the Ironshore Formation that is considered to be as young as Pleistocene and perhaps even Holocene by Matley, Richards, and Rehder and by Brunt et al. (x973). The name derives from its hardness and position. Thicknesses of the Iron.shore Formation reach 17 m at George Town, covering old ~ No. 3956 of the Woods Hole Oceanographic Institution.
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seacliffs and platforms cut in the Bluff Limestone (Brunt et al., I973). Five facies described by Brunt and his associates are: Reef Facies--large corals in growth position; Black Reef Facies--fragments of corals that had been transported and deposited in a matrix of sand with many molluscs, especially Strombus gigas Linn., which lives only at depths of 4 to 9 m in the region (Abbott, I958); Lagoonal Facies---chalky limestones and marls; Shoal Facies--ridges of crossbedded calcarenite as high as 7 m atop the lagoonal facies; and Beach Ridge Facies-low ridges of crossbedded calcarenite with casts of roots or burrows atop the Bluff Limestone. The first three facies are in the western part of the island, and the last two are common around most of the island. A modern counterpart of the lagoonal facies is now forming in North Sound (Roberts, x97ia, 197tb, x976). Calculations from drawdown in test borings ~Iather, x972) indicates that the lagoonal facies has a low permeability and that all other facies (high energy ones) have high permeabilities except where case-hardened by deposition of calcium carbonate in the shore zone. Grand Cayman Island is in the trade-wind belt, and the winds are almost invariably from the north-east (National Climatic Center, I974). These winds produce waves that are higher than x'8 m 25% of the time and higher than 3"4 m 2"5% of the time during January; lowest waves occur during October when they exceed x.8 m only 7% of the time. Roberts (1974), Roberts et aL (t975), and Rigby & Roberts (i976) used hindcasting of winds suppolted by field measurements to show that the reef morphology is closely related to wave power. For example, the highest waves and the best developed reefs occur along the eastern coast and the lowcst waves with reefs absent characterize the western coast. In addition, the high temperature (24 to 29 ~ and high rainfall (i5o cm year -1) (Hsu et al., t972 ) are conducive to rapid weathering of rock outcrops. Shore types
Mapping on foot and from a helicopter showed that the x27 km of shoreline around Grand Cayman Island consists of four main kinds of material (Figure 2). These are muddy peats
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Figure 3- Profiles across shore zone at positions shown by Figure t, using method described in text. Each profile begins near mean low water. Horizontal lines are at z m intervals. No vertical exaggeration. Large dots are measured elevations; small ones are syrnbolie of the intervening topography. are fronted by a series of rimmed pools as much as 5 m wide and z 5 m long, particularly near profiles M [Plate x(e)], and L. In the spray zone there are solution basins that are steepsided, fiat-floored, contain many marine plants and animals, and are light buff coloured; these are formed by solution of the limestone at night by water charged with carbon dioxide produced by plant and animal respiration (Emery., I946, z96z ). Also present are borings by marine animals which add to erosion by mechanical rasping (Neumann, z966 ). Where storm waves are particularly high, a few solution basins have been converted into potholes through abrasion by one or more cobbles swirled around inside them. The rate of intertidal solution of the general surface of the low terrace was found to be 2 to 6 mm year-1 by Worthin (x 959), who measured the lowering of the rock surface with respect to insoluble materials placed against the Ironshore Formation at known dates. Along some shores (profiles D, E, and H) the original seacliff has become protected from wave and spray attack by deposition of rubble or sand beaches. Shoreward of the spray zone, including areas covered by land vegetation, the rock surface of all low terraces is sharply jagged, case-hardened, and black or dark grey--typical phyto-
573
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karst [Plate z(0 ] due to solution by boring blue-green algae aided by rain and dew (Folk, Roberts & Moore, z973). Clearly, the effect of wave abrasion and seawater solution is shore recession, and the effect of rain and dew solution is lowering of the land side of the shore. Lowering of the land ~rfaee has gone so far that evidence of former shallow-water deposits atop the terraces higher than z m could not be found. In fact, the bottoms of the conical phytokarst arc commonly as deep as 0. 5 m below their uppermost tips, and exceptionally they are more than x m deep. Solution pits or shaft-like caves as deep as 8 m were noted at profiles J and P and may occur elsewhere. Even though the original terrace surfaces, their coral-algal growth and their sediments have been destroyed and lowered, the general shapes and profiles of the terraces appear to remain. Profiles Profiles were surveyed across the rocky shores at zo sites (Figure z) to investigate the degree of correlation and possible warping of the terraces. The instruments were two poles, each 6 or 9 ft long and having a yardstick nailed to the top patt. Each was held vertically one rod length (6 ft) apart and at right angles to the shore trend. The intersection of the line of sight to the horizon on each pole was read and recorded. Over karstic topography tile poles were set atop high points of the rock so far as possible. Successive landward pole positions and readings permitted construction of a profile as long as desired, usually to the point where rock outcrops were obscured by thick soil or dense vegetation. Correlation was later made for the depression of the horizon below local horizontal [see Emery (z96r) for details and accuracy of the method] and for tide level obtained from a tide gauge operated by the
574
K. O. Emery
Mosquito Research Control Unit and located at the shore of South Sound (directly south of George Town). The tide-gauge recordings were adjusted to their best vertical fit with the predicted tide levels given by the U.S. Department of Commerce (x975). Mean tide range appears to be about 0.4 m. T h e profiles (Figure 3) converted to metres show details of the topography, but they are rather difficult to compare with each other. An interpretation of them is provided by Figure 4 separately for the north side and the south side of the island. A diagonal line pattern shows the elevation above sea level and the probable correlation of terraces crossed by the profiles. The 2 m terrace is most widespread, absent mainly or only where subsequent wave erosion has removed it, or where it was buried under later shore deposits. The higher terraces are less easily correlated, partly because some had been removed by erosion when lower terraces were formed, but also because higher terraces [Plate i(e), (d)] could be recorded only where the Bluff Limestone had been high enough to protrude above the once relatively higher sea levels. Thus only low terraces occur at the west ind. Additional terraces now below sea level are reported by local divers and described by Roberts (x974) and Rigby & Roberts (x976) at Grand Cayman Island. Common depths for these submarine terraces are 8 m, 15 m to the shelf break at 2o-25 m, and 73-75 m. Similar submerged terraces at Jamaica were described by Cant (x972), Roobel & Gibb (x973) , Goreau & Land (i974), and Moore et al. (I976).
Radiometrie dates Ages of the terraces are shown only to be post-Miocene by the species of fossils in the Bluff Limestone and the Ironshore Formation. Two attempts at radiocarbon dating of recrystallized shells in the lagoonal facies of the Ironshore Formation yielded only ages older than 4 ~ ooo years (Brunt et al., I973). Samples of conch (Strombus) shells embedded in reef and back reef facies of the Ironshore Formation at profiles C, R, and S (Figure 3) were selected for thorium-uranium dating because they are very dense and consist mainly of aragonite with some alteration to calcite, according to X-ray spectra. Isotope ratios foi" the dense central column of the least altered conch shell, from about 1-5 m above mean low water at profile R, yielded a 23~ age of 24o 000+3000 yeats. However, tile 2aIPa/2asu ratio was 1.2--higher than the theoretically attainable value of i.o; low uranium here is attributed to losses during weathering, so that the 24o ooo-year age must be too high. Another attempt was made on a sample of coral [Diploria cf. strigosa (Dana)] embedded in the reef facies about x.5 m above mean low water at profile C. X-ray data show about i5% calcite, indicating some alteration by weathering. Two allquots of the sample were analysed; both yielded a 2a~ age of x55 000110 ooo years. The activity ratios of 2a~U/z3sU (1.1o4-o.oi) and of 2alPa/2asU (0.994-o.o 3) for the same are consistent with this age. Loss of some uranium indicates that this age also is somewhat too high. Thus the terrace deposit probably formed during the Sangamon stage of interglacial high sea level, according to the time scale of Emiliani & Shacklcton (i974). The highly weathered and eroded condition of the higher terraces implies a considerably greater age for them.
Related regional data Both emerged and submerged terraces may have been controlled by changing custatic sea levels caused by waning and waxing of glaciers. However, tectonic movements of the island
Lozo marine terraces of Grand Cayman Island
575
could also have occurred because of the island POsition atop the Cayman Ridge just north of the Cayman Trough [a crustal plate boundary (Molnar & Sykes, 1969)]. Holcombe et al. (t973) suggested that the trough is the scene of active sea-floor spreading on the basis of topography, heat flow (Ericson et al., 1972), seismic reflection data, free-air gravity anomalies (Bowin, 1968), numerous small earthquakes in the region (Barazangi & Dorman, 1969) , dredgings by Perfit & Heezen (1978) and our own investigations at sea (Ballard, 1976; CAYTROUGH, i979; Emery & ~'Iilliman, i98o ). A way of estimating tectonic stability of Grand Cayman Island is to compare the 2 m Sangamon terrace with the elevations and ages of terraces in surrounding land areas. Similar elevations and ages were found at north-eastern Yucatan (Szabo et al., 1978); on the north coast of Jamaica even though local displacements by post-terrace faulting are known (Cant, 1972; Horsfield, 1972); in the Bahama islands (Neumann & lXioore, 1975); and in Florida (Osmond et al., 1965). In another 'stable' area, Oahu of Hawaii, a terrace at 7.6 m elevation yielded 23 more dates having a mean of I22 ooo years (Ku et aL, 1974). Still others in the same elevation range of 2 to io m have been listed by Kern (1977) in the Pacific Ocean as 12o ooo to 14o ooo years old. At other places in the Caribbean Sea the general Sangamon age is reported for terraces at high elevations as though displaced by local diastrophic movements. For example, ~Iesolella et al. (1969) , Bender et al. (1973), iNIatthews (1973) and Fairbanks & ~'Iatthews (1976) obtained about 125 ooo years for terrace III between 24 and 59 m above sea level on Barbados Island. These higher levels may be due to local uplift associated with subduction at the eastern end of the Caribbean Sea. As many as 28 undated terraces reach maximum elevations of 4o0 m on Hispaniola and eastern Cuba, perhaps indicating an eastward upwarp of the region centering upon north-western Hispaniola (Horsfield, 1975). Living land snails on Grand Cayman Island (Pilsbry, 1931) are pertinent to the question of relative elewtion, because 78~o of the species are Jamaican and only 4~o Cuban, and because some of the snails are endemic with enough independent evolution to require at least part of the island to have remained subaerial since well back in the Tertiary Period. The present maximum elevation of about 2o m would seem to have been enough to allow part of the island to remain above sea level during this time. Evidence of some emergence of Grand Cayman Island has been cited by English (1912) whose remarks were repeated by ~Iiatley (1926), Richards (1955), and Brunt et al. (1973). English reported that he was shown rocks and trees 'many yards from even shallow water, marking places from which it was possible from my informant's memory to catch sizeable fish, or launch canoes'. Richards added that at Boddentown some streets now run where the oldest inhabitants remember shallow water and that shoaling caused abandonment of the town harbour. Accordingly, they concluded that the island was rising about a half inch per year; if continued, this would correspond with the enormous and totally unreasonable rate of 127o cm/tooo years, or about 6 m since Columbus' time.
Conclusions The modern shores of Grand Cayman Island consist of rock along seaeliffs and platforms, loose sand and rubble on beaches with local cementation into beachrock, and muddy sand in mangrove swamps. A terrace 2 m above mean low sea level is composed of reef facies having a probable San~amon interglacial age. Several higher terraces exist, but no marine materials remain on them owing to extensive subaerial weathering and erosion after emergence. Twenty topographic profiles surveyed across the rocky shores showed that the emergent
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terraces can be correlated. No warping, either transverse or along the length of Grand Cayman Island, can be recognized from the elevations of the 2 m terrace nor of the other slightly higher but probably much older terraces.
Acknowledgements Appreciation is due R. D. Ballard for the opportunity to participate in the Cayman Trough sttfdy and to Gideon Bain of Grand Cayman Island, Raymond Wright of the Jamaica Geological Survey, Walter Sullivan of the New York Times, and D. S. Hosum of the Woods Hole Oceanographic Institution for their help as rodmen, observers and photographers during the levelling surveys of the raised terraces. Special thanks is given the Mosquito Research Control Unit for a low altitude helicopter flight around the island that permitted mapping of shore types and identification of important sites to be profiled, and for collection of a coral sample for radiometric dating. Most important are the radiometric ages for samples from Grand Cayman Island kindly provided by T.-L. Ku of the University of Southern California. Ku, R. K. Matthews of Brown University, and J. D. Milliman of Woods Hole Oceanographic Institution read a semi-final draft of the manuscript and provided suggestions and additional data. Funding was by National Science Foundation grant 2o]185.33 to R. D. Ballard and by the Ocean Industry Program of the Woods Hole Oceanographic Institution.
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Plate x. Coastal views of Grand Cayman Island. (a) T w o - m e t r e terrace at shore near profile C (Figure I). N o t e surf from waves refracted around north-western corner of island. (b) Beachrock and loose beach sand off B o d d e n t o w n (Figure x). (c) Sixmetre terrace being eroded at seacliff near profile G (Figure i). (d) Eleven-metre terrace at profile P (Figure i). Note deep nip at base of seacliff. (e) T w o - m e t r e terrace during period of high waves at profile M (Figure x). T o the right and at the distant horizon is the 4-metre terrace. (f) Sharp phytokarstic reefroek at profile R (Figure i). H o s u m with one of the levelling rods.
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