Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies

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Quaternary Science Reviews 26 (2007) 536–559

Highstands during Marine Isotope Stage 5: evidence from the Ironshore Formation of Grand Cayman, British West Indies Morag K. Coynea,1, Brian Jonesa,, Derek Fordb a

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3 b School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario, Canada, L8S 4K1 Received 27 January 2006; received in revised form 8 May 2006; accepted 6 June 2006

Abstract The Ironshore Formation on Grand Cayman is formed of six unconformity-bounded packages (units A–F). Units A, B, C, and D, known from the subsurface in the northeastern part of Grand Cayman, formed during Marine Isotope Stages (MIS) 11(?), 9, 7, and 5e, respectively. Unconformities at the tops of units A, B, and C are highlighted by terra rossa and/or calcrete layers. Strata in core obtained from wells drilled in George Town Harbour and exposed on the west part of Grand Cayman belong to unit D, and the newly defined units E and F. Corals from unit E yielded Th/U ages of 104 ka whereas conch shells from unit F gave ages of 84 ka. Unit E equates to MIS 5c whereas unit F developed during MIS 5a. Th/U dating of corals and conchs from the Ironshore Formation on the western part of Grand Cayman shows that unit D formed during the MIS 5e highstand whereas units E and F developed in association with highstands at 95–110 ka (MIS 5c) and 73–87 ka (MIS 5a). Unit E, 5 m thick in the offshore cores, is poorly represented in onshore exposures. Unit F, which unconformably overlies unit D at most localities, is formed largely of fossil-poor, cross-bedded ooid grainstones. The unconformity at the top of unit D, a marine erosional surface with up to 2.5 m relief, is not characterized by terra rossa or calcrete in the offshore cores or onshore exposures. Unit D formed with a highstand of +6 m asl, whereas units E and F developed when sea level was +2 to +5 asl and +3 to +6 m asl, respectively. Thus, the highstands associated with MIS 5e, 5c, and 5a were at similar elevations. r 2006 Elsevier Ltd. All rights reserved.

1. Introduction Interpretation of the sea-level changes over the last 150,000 years has been based largely on uplifted reef terraces (e.g., Aharon and Chappell, 1986; Chappell and Shackleton, 1986; Schellmann and Radtke, 2004) and oxygen isotope ratios derived from benthic foraminifera (e.g., Shackleton, 2000; Waelbroeck et al., 2002; Lea et al., 2002). Isolated oceanic islands throughout the Caribbean Sea, adjacent areas, and the Pacific Ocean have been regarded as ‘‘tide gauges’’ (Land et al., 1967) or ‘‘dipsticks’’ (Hearty, 2002) because their Pleistocene deposits evolved during a period when sea levels oscillated in accord with the glacial Corresponding author. Tel.: +780 492 3074.

E-mail address: [email protected] (B. Jones). Present address: Engineering and Science Library, Queen’s University, Kingston, Ontario, Canada, K7L 5C4. 1

0277-3791/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2006.06.013

cycles. On tectonically active islands in the Caribbean Sea and the Pacific Ocean, uplifted reef terraces offer clear testimony of sea-level change (e.g., Broecker et al., 1968; Mesolella et al., 1969; Veeh and Chappell, 1970; Bloom et al., 1974; Chappell, 1974; Bender et al., 1979; Aharon, 1983; Aharon and Chappell, 1986; Chappell and Shackleton, 1986; Cutler et al., 2003; Schellmann and Radtke, 2004; Schellmann et al., 2004a, b). On tectonically stable islands, interpretations of Pleistocene sea levels have been derived from speleothems, paleosol sequences, patch reef locations, wave-cut notches, and interpretation of limestone successions that developed during the highstands (Harmon et al., 1978, 1981; Mylroie, 1988; Li et al., 1989; Jones and Hunter, 1990; Lundberg and Ford, 1994; Ludwig et al., 1996; Ve´zina, 1997; Hearty, 1998; Hearty, 2002; Hearty et al., 1998; Toscano and Lundberg, 1999; Ve´zina et al., 1999; Hearty and Kaufman, 2000; Hearty, 2002). There is, however, little consistency in the sea-level positions derived for each highstand. The sea-level position

ARTICLE IN PRESS M.K. Coyne et al. / Quaternary Science Reviews 26 (2007) 536–559

associated with the Marine Isotope Stage (MIS) 5a highstand, for example, ranges from 30 m (Bloom et al., 1974) to +10 m (Cronin et al., 1981) relative to present day sea level. The Cayman Islands, comprising Grand Cayman, Little Cayman, and Cayman Brac, are located in the northwest part of the Caribbean Sea (Fig. 1A). These islands are the summits of pinnacles that are founded on the Cayman Ridge, which stretches from the south of Cuba to the Gulf of Honduras (Fig. 1). As such, they are ideal ‘‘dip sticks’’ for determining the record of past sea levels in that part of the Caribbean Sea. These islands are located in a tectonically active area with the Mid-Cayman Rise, an active spreading centre, located southwest of Grand Cayman (e.g., MacDonald and Holcombe, 1978). South of the Cayman Ridge is the Oriente Transform Fault that

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forms the northern margin of the Cayman Trench where water depths are in excess of 6000 m (Fig. 1B). Left-lateral motion of the North American plate relative to the Caribbean Plate takes place along the Oriente Transform Fault (e.g., Rosencrantz et al., 1988). Despite its location close to a spreading centre and a major transform fault, Grand Cayman seems to have been tectonically stable given that the wave-cut notch that formed in association with the highstand that developed during MIS 5e is still at 6 m above present day sea level (Jones and Hunter, 1990). On Grand Cayman, the Pleistocene Ironshore Formation (Fig. 2) is known from exposures along the coastline, in quarries, and material excavated from shallow ponds. The internal stratigraphy of the formation has been difficult to establish because vertical successions are generally o5 m and commonly

Fig. 1. Location of the Cayman Islands; (A) Location of Cayman Islands relative to other locations cited in text (names in bold); (B) Locations of Grand Cayman, Little Cayman, and Cayman Brac relative to the Mid-Cayman Rise, the Cayman Trench, and the Oriente Transform Fault. Modified from Jones (1994).

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o2 m high and most attempts to drill and core these soft, extremely friable limestones have proven futile. Exceptional core obtained from wells drilled at Rogers Wreck Point (RWP) on the northeast corner of Grand Cayman (Fig. 2) showed that the Ironshore Formation is up to 18 m thick and includes four unconformity-bounded units (A, B, C, D) that formed 4400, 346, 229, and 131 ka in association with MIS 11(?), 9, 7, and 5e, respectively (Ve´zina, 1997; Ve´zina et al., 1999). This study shows that the Ironshore Formation found on the western part of Grand Cayman includes strata younger than unit D that formed during MIS 5c and 5a. The fact that these deposits are found on an island that has been tectonically stable for the last 125,000 years means that the highstand positions associated with each stage can be derived directly from the

Fig. 2. Geological map of Grand Cayman showing widespread distribution of the Ironshore Formation. Modified from Jones (1994).

sedimentary successions without having to factor in assumptions regarding uplift rates.

2. Material This study is based on four cores (BJC#1, BJC#2, BJC#3, BJC#4) obtained from wells drilled offshore from George Town in 1996 (Fig. 2). These cored wells, commissioned by the Port Authority of the Cayman Islands, were drilled by Fugro–McClelland Marine Geosciences Inc. and Moffatt & Nichol International where the water is 14–18 m deep and 7.6–12.7 m of unconsolidated sediment overlies indurated bedrock. The basal 10–15 m of each core includes good samples of the Ironshore Formation. Onshore collecting focused largely on the western part of the island where the Ironshore Formation is exposed in outcrop along the west coast of North Sound (MH1, 2, 3, 4; LSC1, LSCA, VDM2, VDM4), in coastal outcrop on the west coast of Grand Cayman (BL), and in scattered quarries (CLQ, BQ, PBQ, PBQA) (Fig. 3). In addition, one sample came from material excavated from shallow ponds (JP) and one sample from a well that produced poor core (TUF#1) (Fig. 3). Exposures of the Ironshore Formation in the central and eastern part of Grand Cayman are limited because of the low-lying land. Thus, samples could only be collected from scattered coastal exposures (SB) and from material dredged from small inland quarries (OG,

Fig. 3. Stratigraphy of the Ironshore Formation on the central and eastern parts of Grand Cayman with U/Th dates. Information for Rogers Wreck Point (RWP) from Ve´zina (1997) and Ve´zina et al. (1999).

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MAQ, TSE) (Fig. 3). Although not ideal, the dredged samples are the only material known from the area. 3. Methods Fieldwork focused on the measurement of stratigraphic successions, the identification of hiatial surfaces, and the collection of aragonitic fossils that could be used for Th/U dating. Such fossils included branching and head corals (e.g. Porites sp., Montastrea annularis, Diploria sp., Acropora palmata, Acropora cervicornis, and Madracis sp.), conch shells (Strombus gigas), and various bivalves. Eleven samples from offshore cores BJC#1, 3, and 4 were analysed by thermal ionization mass spectrometry (TIMS) at McMaster University. Thirty-two samples collected from various outcrops on Grand Cayman were analysed by induction-coupled mass spectrometry (ICPMS) at GEOTOP, Montreal, Quebec. The results of these analyses are consistent despite the fact that two different analytical methods were used in two different laboratories. Alteration of the fossil’s original aragonite to calcite indicates that the fossil has not remained a closed system and cannot be dated using the Th/U system. Thus, each fossil selected for dating was first subjected to XRD analysis in order to determine its calcite content. Following powdering in an agate mortar and pestle, a smear mount on a glass slide was analysed on a Rigaku Geigerflex Unit with a fine Cobalt focus tube from 251 2y to 401 2y to include the major aragonite (30.51 2y and 31.81 2y) and calcite (34.31 2y) peaks. The percentage of calcite was then determined by using the Jade 3.0 software that is associated with the XRD. 4. The Ironshore Formation The Ironshore Formation, originally named by Matley (1924a, b, c, 1925, 1926), is typically formed of friable, poorly consolidated limestone that commonly contains numerous corals, bivalves, and gastropods. On Grand Cayman, modern mangrove swamps overlie most of the formation and exposures of the formation are generally restricted to narrow coastal platforms, vertical sections up to 5 m high that are largely restricted to the west coast of North Sound, and scattered quarries (Fig. 2). General attributes of the Ironshore Formation have been described by Matley (1926), Brunt et al. (1973), Roberts (1977), Shourie (1993), and Jones and Hunter (1990). The formation has been extensively documented in terms of its bivalves and gastropods (Richards, 1955; Rehder, 1962; Cerridwen, 1989; Cerridwen and Jones, 1991), its general sedimentological attributes (Brunt et al., 1973; Hunter and Jones, 1990; Jones and Hunter, 1990), corals (Woodroffe et al., 1980; Hunter, 1994; Hunter and Jones, 1996), tunicates (Jones, 1990), ooids (Jones and Goodbody, 1984), rhodolites and stromatolites (Jones and Hunter, 1991; Hills, 1997; Hills and Jones, 2000), borings (Jones and Pemberton, 1988a, b), ichnology (Pemberton and Jones,

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1988; Jones and Pemberton, 1989), and diagenesis (Squair, 1988; Jones and Squair, 1989; Rehman, 1992; Rehman et al., 1994). Following Matley (1926), it was generally assumed that the limestones of the Ironshore Formation formed during a single depositional phase associated with the Sangamon highstand, 125,000 year ago. That notion was supported by the presence of a well-defined wave-cut notch cut into the dolostones of the Cayman Formation, 6 m asl, that formed during that highstand (Jones and Hunter, 1990). Dating of the Ironshore Formation was, for a long period, limited to those dates obtained by Emery (1981) and Woodroffe et al. (1983). Emery (1981) considered his 230 Th/234U ages of 155710 ka (coral) and 24073 ka (Strombus shell) too old because 231Pa/235U dating indicated that some of the uranium had been lost to weathering. Woodroffe et al. (1983) obtained an average age of 12478 ka from four different corals, three of which came from the ‘‘+2 m terrace’’ on Grand Cayman’s coast. The fourth was from Cayman Brac. These dates supported the notion that the Ironshore Formation developed during the Sangamon highstand, 125,000 years ago. The internal stratigraphy of the Ironshore Formation has been difficult to determine because only limited thicknesses of the succession are visible in outcrop (maximum of 5 m, typically o2 m) and its basal boundary is only visible in Paul Bodden’s quarry (PBQ, Fig. 3) and at Spotts Bay (SB, Fig. 3). The notion that the Ironshore Formation was a single unconformity-bound unit was dispelled when good core was recovered from 15 wells drilled at RWP (Figs. 2, 3), on the northeast coast of Grand Cayman (Ve´zina, 1997; Ve´zina et al. 1999). The cores revealed four-unconformity bounded packages, designated units A, B, C, and D, with the unconformities highlighted by terra rossa and/or calcrete (Ve´zina, 1997; Ve´zina et al., 1999). Th/U dating of aragonitic fossils showed that units A, B, C, and D had formed 4400 oo1,500,000, 346, 229, and 131 ka, respectively and were thus correlated to Marine Isotope Stages (MIS) 11(?), 9, 7, and 5e, respectively (Ve´zina, 1997; Ve´zina et al., 1999). The facies in each of these units are similar (Table 1) and in core, the different units are evident only because of the terra rossa and calcrete layers that highlight the unconformities. 5. Th/U dating of fossils from the Ironshore Formation Erroneous Th/U ages result when post-depositional uranium or thorium migration from or into the system changes the isotope activity ratios. Thus, under ideal situations, dates were only deemed reliable if the following criteria were met: (1) The skeletal aragonite showed little evidence of recrystallization as shown by low calcite content (e.g., Hillaire-Marcel et al., 1986; Muhs et al., 2002; Omura et al., 2004; Dumas et al., 2006; Muhs et al., 2006) and

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Table 1 Lithological and biological characteristics of units A–D in the Ironshore Formation in the Rogers Wreck Point area of Grand Cayman. Modified from Ve´zina et al. (1999, their Table 2)

et al. (1977) and Cutler et al. (2003) and gave preference to samples that had a 230Th/232Th ratio of at least 20.

Unit

Lithological features

Biota

A

Grainstones common; packstones, wackestones, mudstones common in south; rudstones, framestones, bafflestones locally. Skeletal sands formed of grains derived from gastropods, bivalves, benthic foraminifera, echinoid spines, coral, serpulid worm tubes, barnacles. Grains commonly micritized. Rudstones, packstones, grainstones, floatstones. Skeletal sands formed of grains derived from benthic foraminifera, coral, red algae, echinoid spines, bivalves, gastropods. Grains commonly micritized. Packstones, grainstones with minor wackestones. Grainstones rare to south; framestones and rudstones common in south. Skeletal sands formed of grains derived from foraminifera, Halimeda, red algae, echinoid spines, bivalves, gastropods. Grains commonly micritized. Dominated by grainstones, framestones, with subordinate amounts of packstone and wackestones. Skeletal sands formed of grains derived from foraminifera, Halimeda, red algae, bivalves, gastropods. Grains commonly micritized.

Head corals, branching corals, commonly encrusted by red algae and foraminifera

The greatest reliance was placed on dates that were derived from samples that complied with all of these criteria because it implies that the aragonite had not been subjected to modification of the U or Th contents through diagenetic change. Dates obtained from samples that did not meet these criteria are considered unreliable and, in general, were not used. Such dates were only used if no other dates were available and if most of the criteria were met. These dates, however, were used with great caution.

Head corals, branching corals, commonly encrusted by red algae and foraminifera

5.1. Evaluation of ages obtained from fossils from the Ironshore Formation

B

C

D

Head corals, branching corals, commonly encrusted by red algae and foraminifera

Massive corals (commonly encrusted by red algae and foraminifera and bored by bivalves), branching corals.

little change to the skeletal texture (Omura et al., 2004). In this study, preference was given to samples that contain o3% calcite (cf, Thurber et al., 1965; HillaireMarcel et al., 1986). (2) The initial 234U/238U ratio of the sample equals that of seawater. Values used for this criteria include 1.145–1.165 (Dumas et al., 2006), 1.13–1.16 (Omura et al., 2004), 1.145–1.159 (Muhs et al., 2002), and 1.14970.008 (Muhs et al., 2006). In this study we followed Ku et al. (1977) and Cutler et al. (2003) and gave preference to samples that had an initial 234U/238U ratio of 1.1470.03. (3) The uranium content of the fossil being dated is similar to that of its modern counterpart (e.g., Ku, 1976; Ku et al., 1977; Muhs et al., 2002; Cutler et al., 2003; Omura et al., 2004; Dumas et al., 2006; Muhs et al., 2006). (4) There should be little inherited 230Th as indicated by low 232Th and a high 230Th/232Th ratio (e.g., HillaireMarcel et al., 1986; Muhs et al., 2002; Omura et al., 2004; Muhs et al., 2006). In this study we followed Ku

Th/U methods were used to date bivalves, corals, and conchs collected from the Ironshore Formation found in 24 surface outcrops, one onshore core, and four offshore cores (Tables 2, 3). To a large extent, the fossils that were available dictated the type of fossil collected for dating purposes. For example, the scarcity of corals in the upper parts of the Ironshore Formation on the western part of Grand Cayman meant that S. gigas (i.e. conch) provided the only option for Th/U dating. Bivalves were only used in the absence of any other fossils. Overall, the corals gave the most reliable ages, whereas the bivalves yielded poor ages. Some of the conchs provided acceptable ages. 5.1.1. Corals Throughout the development of Th/U dating, researchers have found that corals consistently give reliable results (Broecker, 1963; Broecker et al., 1968; Mesolella et al., 1969; Bloom et al., 1974; Bender et al., 1979; Dodge et al., 1983; Edwards et al., 1987). Veeh and Burnett (1982) demonstrated that unaltered fossil and modern corals have similar mean uranium contents of 2–3 ppm, and that most unaltered corals have calculated initial 234U/238U activity ratios within 2s (0.04) of 1.14, the 234U/238U activity ratio of seawater. Corals will yield reliable dates if a closed system model had been maintained and uranium migration or uptake is negligible. Reliable dates were obtained from nine of the 12 corals taken from the offshore cores (Table 2). Three corals had initial 234U/238U ratios that exceeded the expected seawater concentration of 1.1470.03, and were therefore considered unreliable. Of the remaining nine corals, three contained 5–10.4% calcite. Although such values are high, the dates are used because the isotope activity ratios and concentrations are within the closed system limits, and the stratigraphic order is logical. Corals collected from 10 onshore sites, with 0–5.8% calcite, yielded six ages that fulfill the closed system requirements (Table 3). The remaining four samples have initial 234U/238U ratios within 0.06 of 1.14 and could probably be deemed reliable.

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Table 2 230 Th/234U data and ages for corals from George Town Harbour cores. The age error is derived from TIMS counting method statistics. Ages in italics are considered valid. Analyses by TIMS Well

BJC#1 BJC#1 BJC#1 BJC#1 BJC#3 BJC#3 BJC#3 BJC#4 BJC#4 BJC#1 BJC#3 BJC#3

Depth in core (m)

15.8 16.2 18.9 21.2 7.6 12.2 13.3 13.7 9.6 14.5 22.3 16.8

Coral species

A. cervicornis M. annularis M. annularis M. cavernosus M. annularis M. annularis M. annularis M. annularis M. annularis M. annularis M. annularis A. palmata

Calcite (wt%)

10.4 1.9 5 3.9 4.1 7.4 0 10.0 1.8 1.9 6.6 4.7

[U] (ppm)

3.10 2.13 2.52 2.48 2.59 2.48 2.38 2.61 2.85 2.49 2.46 2.32

234

U/238U

1.12 1.12 1.12 1.11 1.15 1.12 1.12 1.12 1.14 1.14 1.09 1.13

234 U/238U (initial)

1.15 1.16 1.16 1.15 1.16 1.17 1.16 1.15 1.15 1.36 N/A 1.22

5.1.2. S. gigas (Queen conch) Living molluscs, unlike corals, generally contain less uranium (Broecker, 1963; Blanchard et al., 1967). Their low initial 234U/238U ratios make molluscs susceptible to post-depositional uranium migration (Veeh and Burnett, 1982) and the tendency for molluscs to become open systems is well known (Szabo and Rosholt, 1969; Kaufman et al., 1971; Szabo and Vedder, 1971; Hoang and Hearty, 1989; McLaren and Rowe, 1996). The reliability of U-series dating of Strombus, for example, has been widely debated because of the propensity of these shells to absorb U following their death (McLaren and Rowe, 1996). It has been suggested, however, that U-series dates may be reliable if the shells were encased by well-cemented beachrock soon after their demise and thereby rendered a closed system with respect to U migration (Szabo and Rosholt, 1969; Hoang and Hearty, 1989; Belluomini et al., 2002). Gustavsson and Ho¨gbert (1972) suggested that the inner parts of large, dense, well-preserved and unfractured molluscs would remain closed systems because their high density ensured little isotope migration. Kaufman et al. (1996) demonstrated that for shells less than 130,000 years old, reliable U-series dates could probably be obtained from samples taken from the central parts of thick shells because the absorption of U there was less than that in the exterior parts of the shells. Hillaire-Marcel et al. (1996) also argued that that samples from the thickest parts of the shell, including the columella of Strombus, might yield reliable U-series dates, especially if the shell contained little calcite and little incorporated detrital Th (Hillaire-Marcel et al., 1986). Even so, it has been suggested that such dates should only be used following implementation of a rigorous sampling protocol (Belluomini et al., 2002) and if a comparative chronology can be used to verify the U-series dates (Hillaire-Marcel et al., 1995). Clearly, dates derived from these shells must be treated with caution. Sixteen conch shells were collected from various parts of the Ironshore Formation that lacked corals or contained

230

Th/234U

0.53 0.62 0.65 0.68 0.05 0.68 0.72 0.53 0.08 0.51 1.44 0.83

230

Th/232Th

306 539 118 1395 21 486 389 69 52 261 468 538

Age (yr BP)

80,140 103,730 109,750 121,150 5710 120,920 134,520 79,690 8330 69,101 N/A 181,660

Error (2 s)

Plus

Minus

1610 2090 1540 10,370 190 2050 2310 710 125 1450 N/A 3190

1580 2050 1520 9480 190 2010 2260 700 125 1430 N/A 3090

only recrystallized corals. The conch shells were selected because they were well preserved and unbroken. Indeed, some of the original pink colouration was still evident on the exteriors of some shells. Following the suggestion of Gustavsson and Ho¨gbert (1972) and Hillaire-Marcel et al. (1996), samples for U-series analysis were taken from the lip or columella where the shell is thick (lip up to 1 cm thick in some shells) and formed of tightly packed aragonite crystals. Eleven conchs contained 0% calcite, but only four had initial 234U/238U ratios that matched that of seawater. Three of those conchs came from CLQ, (Fig. 3). Of the other eight conchs, four yielded 234U/238U values within 0.03 of 1.14, and only one of the conchs contained less than 3% calcite (Table 3). Surprisingly, the two conchs with the highest percentages of calcite (10.8% and 10.2%) yielded ages that appeared reasonable. Conversely, some shells that had retained their original pink colouration yielded ages that are considered invalid. 5.1.3. Bivalves Three bivalves were collected from the upper part of the Ironshore Formation near Salt Creek for dating because they were the only fossils present. They yielded ages of 196, 679, and 320 ka years (Table 2, 3). Although the 230 Th/232Th ratios are 420, the calculated initial 234U/238U ratios of 1.41, 3.81 and 2.17, clearly indicate that these samples did not remain a closed system, and either gained uranium or lost thorium. These ages were therefore deemed unreliable. 6. The Ironshore Formation in the George Town Harbour Cores 6.1. Facies The Ironshore Formation in the George Town harbour cores is formed of head coral floatstone, branching coral

Taxa

M. annularis Coral M. annularis Porites sp. M. annularis M. annularis M. annularis M. annularis M. annularis M. annularis Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Strombus gigas Bivalve Bivalve Bivalve

Sample #

TUF1,10.50 -140 VDM4-1-99B JP-2-99 PBQA-13-2001 CLQ-3-2001 MH1-20 asl TSE-12-2001 JP-8-2001 VDM4-1-99 MH1-600 asl LSC1-3-99 PBQA-2A-99 PBQA-2B-99 PBQ-6-99 MAQ-9-2001 CLQ-3-2001 CLQ-5-2001 CLQ-8-2001 SB-4-99 BQG-99 PBQ-1-99 PBQ-1-99 TSE dredge JP-2-2001 LSCA-3-2001 BL-2-2001 PBQA-1-2001 TSE-5-2001 PBQ-9-2001 Omega dredge VDM2 unit 3 MH2 unit 1 2957.60 2623.78 2757.73 3353.05 3095.23 2795.46 2701.27 2878.02 2793.22 3041.72 776.91 2351.24 922.30 1347.54 1467.74 1265.95 1058.64 1059.91 1092.77 1465.27 2756.90 3373.50 2986.60 1138.78 1623.21 921.26 385.26 1298.62 1305.79 640.53 952.92 285.74

238 U conc. (ppb)

1.11 1.12 1.10 1.10 1.09 1.12 1.13 1.12 1.13 1.14 1.12 1.10 1.09 1.12 1.12 1.14 1.09 1.11 1.32 1.24 1.20 1.23 1.23 1.06 1.14 1.42 1.07 1.18 1.17 1.24 1.41 1.47

U/238U

234

0.01 0.01 0.01 1.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02

234 U/238U error

1.17 1.17 1.15 1.15 1.13 1.17 1.20 1.18 1.18 1.20 1.16 1.13 1.15 1.16 1.17 1.17 1.11 1.14 1.52 1.29 1.29 1.33 1.31 1.07 1.20 1.80 1.08 1.26 1.24 1.42 3.82 2.17

Initial 234 U/238U

8.72 4.69 25.86 1.61 1.52 3.42 24.53 2.70 21.76 1.58 1.83 1.91 8.71 2.13 4.21 1.27 0.90 1.65 2.06 2.26 4.62 4.78 2.11 0.68 4.28 0.26 0.29 22.89 1.95 4.66 3.74 1.11

Th (ppb)

232

0.74 0.73 0.75 0.73 0.71 0.69 0.79 0.72 0.71 0.69 0.56 0.61 0.56 0.56 0.64 0.50 0.52 0.59 0.83 0.44 0.71 0.71 0.61 0.61 0.68 0.95 0.46 0.70 0.67 0.87 1.12 1.05

Th/234U

230

857.34 1383.75 270.78 5084.17 4854.86 1923.93 298.60 2624.43 312.77 4604.59 811.88 2489.15 196.65 1212.32 765.35 1727.07 2043.08 1283.79 1775.88 1083.49 1543.65 1882.11 3243.09 3303.45 892.41 14597.94 1995.77 142.98 1601.91 453.79 1233.33 1216.81

Th/232Th

230

142,530 135,700 147,440 136,340 132,620 123,270 160,050 133,160 129,030 122,000 86,930 98,200 87,380 87,100 107,860 73,970 79,330 94,970 168,370 62,180 126,160 127,010 97,970 100,530 118,350 230,190 66,000 124,030 115,130 196,200 678,620 319,450

Age (yr BP)

3350 5220 4080 3530 3610 2710 5030 2900 3090 2610 3330 3470 6570 2940 2820 1180 1980 2780 4620 1410 4870 4620 1730 3050 2950 8460 2250 7000 2500 8880 447,940 23,960

Plus

Error (2 s)

3220 4960 3910 3380 3460 2630 4770 2810 3000 2540 3220 3340 6180 2850 2740 1170 1940 2700 4410 1880 4650 4410 1700 2940 2860 7840 2200 6590 2430 8170 169,530 19,900

Minus

542

5.8 2.1 4.2 3.3 0.0 0.0 0.0 0.0 3.6 0.0 o3.0 5.8 10.6 10.2 0.0 0.0 0.0 0.0 4.6 7.5 3.9 4.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.9

Calcite (wt%)

Table 3 Th/U data and ages for samples collected from Grand Cayman. Ages in italics are considered valid ages

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Fig. 4. Facies in core from George Town Harbour wells. (A) Montastrea cavernosa with packstone (PK) layers. 11.9 m bsl, BJC#3. (B) Montastrea cavernosa. 21.5 m bsl, BJC#1. (C) Diploria sp. 12.8 m bsl, BJC#4. (D, E) Branching coral (BC) floatstone. 8.5 m bsl, BJC#3 and 12.0 m bsl, BJC#1, respectively. (F) Halimeda floatstone–rudstone with most Halimeda plates leached. 17.7 m bsl, BJC#3. (G) Halimeda floatstone–rudstone with most Halimeda plates intact, intercalated with A. palmata. 10.7 m bsl, BJC#3. (H) Halimeda floatstone–rudstone with some leached Halimeda plates. 22.6 m bsl, BJC#3. (I) Skeletal grainstone to packstone. 21.6 m bsl, BJC#3.

floatstone, mixed coral floatstone, Halimeda rudstone/ floatstone, and skeletal grainstone/packstone/wackestone (Figs. 4, 5). Head coral floatstone: Dominated by M. annularis along with fewer Diploria sp. and M. cavernosa (Fig. 4A–C), the large corals (up to 0.5 m high) have their corallites oriented upwards, display minimal boring (mainly Lithophaga), have empty chambers, and red algae encrusting their upper surface. Intermixed coral fragments are abraded, extensively bored, partially encrusted with red algae, and have micrite-filled chambers. The matrix is a skeletal grainstone, packstone, or wackestone formed of poorly sorted, sub-

angular to sub-rounded, very fine to very coarse sand-sized grains derived from red algae, benthic foraminifera, echinoderm spines, mollusc, Halimeda, serpulids, peloids, and other indeterminate micritized allochems. Branching coral floatstone: This facies is characterized by numerous fragmentary A. cervicornis and A. palmata along with fewer Porites and Manicina (Fig. 4D, E). None of the corals appear to be in life position. The A. cervicornis fragments (1–4 cm long, 0.5–2 cm diameter), for example, usually lie horizontal. Some coral pieces have leached interiors filled with micrite and outer surfaces bored, micritized, and encrusted with red algae and Homotrema.

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Fig. 5. Correlation of facies and Units D, E, and F in cores BJC#1, BJC#2, BJC#3, and BJC#4 (see inset map and Fig. 2 for locations) with U/Th ages superimposed.

Other corals, however, show minimal alteration and/or encrustation. The matrix is like that in the head coral floatstone. Mixed coral floatstone: This facies contains mixed branching and large head corals, including A. cervicornis, A. palmata, M. annularis, Porites sp., Diploria strigosa, D. labyrinthiformis, D. clivosa, Agaricia fragilis (?), and Siderastrea sp. These fragmented corals, o8 cm long, are variously bored and encrusted. The matrix is like that in the head coral floatstone. Halimeda floatstone–rudstone: This facies is comprised of 40–70% Halimeda plates (Fig. 4F–H). In BJC#3, from 25 to 30 m bsl, the 0.4–1 cm long, well-preserved whitish plates have internal pores filled with clear equant cement or micrite. Below 30 m, at least 70% of the Halimeda plates are evident only as molds. The loosely packed plates lie at angles of 0–701 and are locally imbricated. The matrix is formed of fine- to medium-sand sized grains. Disarticulated bivalves (1–2 cm long) are common. Skeletal grain/pack/wackestone: This facies is formed of poorly sorted, sub-rounded to angular, very fine to very

coarse sand-sized skeletal grains derived from red algae, echinoid spines, mollusc, Halimeda, Homotrema, and benthic foraminifera (Fig. 4I). The muddier sediments are locally bioturbated. Scattered disarticulated and fragmented bivalves (up to 5 cm long) and gastropods are present. This facies is like the matrix found in the fossil floatstones and rudstones. 6.2. Facies architecture and age The facies from the four cores, which came from wells drilled along a 2 km transect that is parallel to the present-day shoreline, are laterally and vertically variable (Fig. 5). BCJ#3, located at the north end of the transect, is formed of interbedded mixed coral floatstone and Halimeda rudstone-floatstones. Branching coral floatstone forms the upper part of the core (Fig. 5). BCJ#2, located 750 m south of BJC#3, is formed largely of skeletal grainstones, packstones, and wackestones (Fig. 5). BCJ#1, located 550 m south of BJC#2, is formed of head coral floatstone interbedded with branching coral floatstone

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Fig. 6. (A–C)Thin section (A, B) and SEM (C) photomicrographs of ooid grainstones that are typical of units D and F in the Ironshore Formation on the western part of Grand Cayman. (D) Thin section photomicrograph of skeletal grainstone from unit D on the western part of Grand Cayman.

(Fig. 5). The basal part of BJC#2, located at the south end of the transect, is formed of Halimeda rudstone–floatstone whereas the upper part is formed of skeletal grainstones, packstones, and wackestones, branching coral floatstone, and head coral floatstone (Fig. 5). The matrices in the coral facies are skeletal sands akin to those in the skeletal grainstone–packstone–wackestone facies. Thus, the facies variations evident in the four cores largely reflect variations in the distributions of the head corals and branching corals (Fig. 5). An age of 5710 years was obtained from a coral just below the core/unconsolidated sediment interface in well BJC#3 (Fig. 5). Dates of 121 and 134 ka were obtained 4–5 m below the unconsolidated sediment/bedrock interface. Corals from well BJC#1 yielded dates of 80, 104 and 110, and 121 ka (in order of depth) (Fig. 5) within 7 m of core. The 80 and 104 ka dates were o1 m apart, indicating a time gap of 24,000 years. Two dates, 80 and 8330 ka, were obtained from BJC#4 (Fig. 5). The three oldest dates from the offshore cores ranged from 121710 to 13572 ka, with an average age of 125 ka. 7. The Ironshore Formation on the western part of Grand Cayman Limestones in surface exposures of the Ironshore Formation on the western part of Grand Cayman are formed largely of ‘‘egg-shell ooids’’ (Fig. 6) that are characterized by large nuclei encased by thin cortices (Jones and Goodbody, 1984). The diverse array of facies in the formation largely reflects the distributions of different fossil groups, the type of trace fossils present, the degree of

bioturbation, and sedimentary structures (Brunt et al., 1973; Woodroffe et al., 1980; Jones and Goodbody, 1984; Hunter and Jones, 1988, 1989, 1990, 1996; Jones and Pemberton, 1989; Shourie, 1993; Jones, 1994; Coyne, 2003). The different facies classification schemes implemented in these studies reflect the different focuses of each study. The facies names used in this study are based largely on the scheme provided by Hunter and Jones (1996) (Table 4). For ease of description, the localities are geographically divided into (1) Canary Lane Quarry (CLQ), (2) Morgan’s Harbour to Salt Creek, (3) Paul Bodden’s Quarries (PBQ), and (4) central Grand Cayman (Fig. 3). 7.1. Canary Lane Quarry Facies: Scattered blocks of dolostone on the quarry floor, which is 1–2 m above sea level, indicate that the upper boundary of the Cayman Formation is just below the surface. Numerous head corals and conchs held in oolitic grainstone characterize poor exposures on and just above the quarry floor (Fig. 7A, B). This fossil-rich bed is overlain by planar to low angle cross-bedded oolitic grainstone that contains widely scattered S. gigas in its lowermost part. Ophiomorpha nodosa and Conichnus are found in the oolitic grainstones exposed in the quarry walls up to a level that is  3 m above present day sea level (Fig. 7C, D). No fossils or trace fossils were found above that level. Age: An age of 13373.5 ka was obtained from a M. annularis colony whereas three conch shells yielded ages of 7471, 7972, and 9573 ka (Table 3). Although

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Table 4 Classification of facies in the Ironshore Formation exposed on the western part of Grand Cayman. Modified from Hunter and Jones (1996, their Fig. 3) Facies

Bivalve A

Bivalve B

Coral A Coral B Skeletal grainstone Well-sorted grainstone Moderately burrowed grainstone

Lithotype

Skeletal wackestones to packstones Skeletal packstones to grainstones Coral floatstone Coral boundstone Skeletal grainstone Skeletal grainstone Ooid grainstone

Sorting

Grain size

Sedimentary structures

Fossils

Trace fossils

Major

Minor

Poor

Silt to very fine sand

None seen

Bivalve

Gastropods foraminifera

None seen

Moderate to poor

Very fine to fine sand

None seen

Bivalves

None seen

Poor

Fine to coarse sand None seen

Corals

Poor

Fine to coarse sand None seen

Corals

Moderate

Fine to coarse sand Moderate burrowing Fine to coarse sand None seen

None

Corals Halimeda Gastropods Bivalves Gastropods Gastropods Red algae Corals bivalves

None

Corals

Well Well

Fine to medium sand

Laminated, moderately burrowed

None

None

Highly burrowed Ooid grainstone grainstone

Well

Fine to medium sand

Highly burrowed None

None

Unidirectional cross-bedded grainstone

Ooid grainstone

Well

Fine to medium sand

Unidirectional None high-angle crossbedding, little burrowing

None

Multidirectional cross-bedded grainstone

Ooid grainstone

Well

Fine to medium sand

None

Lithoclast rudstone

Moderate

Fine to medium sand

Multidirectional None high-angle crossbedding, moderate burrowing High burrowing None

Bioclast floatstone

Moderate

Fine to medium sand

Well

Fine to medium sand

Laminated grainstone

Ooid grainstone

exposures in the lower part of the quarry are poor, it appears that the coral came from the fossil-rich unit whereas the conch came from the overlying strata. 7.2. Morgan’s Harbour to Salt Creek area 7.2.1. Facies The Ironshore Formation, well-exposed in coastal exposures between Salt Creek and Morgan’s Harbour is divided into lower and upper units by an unconformity that has a relief of up to 2.5 m (Figs. 8–10). Tabular lithoclasts formed of cemented oolitic grainstones, up to 1.0  0.65  0.1 m in size, commonly rest on the unconformity

High-angle cross-bedding, moderate burrowing Laminated to low-angle crossbedding

None

None

Corals, bivalves

None

None

Gastrochaenolites, Entobia, Trypanites None seen Polykladichnus, Skolithos None seen Ophiomorpha, Polykladichnus, Skolithus, Conichnus Ophiomorpha, Polykladichnus, Skolithus, Planolites Polykladichnus, Skolithus, Conichnus, Bergaueria, Psilonichnus Ophiomorpha, Skolithus

Ophiomorpha, Polykladichnus, Conichnus Ophiomorpha, Polykladichnus, Skolithus None

(Fig. 9D–F). The lack of disturbance features in the grainstones beneath the lithoclasts indicates that the unconformity surface was firm to hard when the lithoclasts were deposited. The lower unit is characterized by patch reefs, 5–15 m diameter, that are formed of a diverse array of well-preserved corals (Figs. 8A, B, 9A–C, 10). Highly bioturbated oolitic grainstones, which lie between the patch reefs, are characterized by numerous Ophiomorpha, Conichnus, Bergaueria, Planolites, Polykladichnus, Psilonichnus, and Skolithus (Pemberton and Jones, 1988; Jones and Pemberton, 1989). The intense burrowing commonly obliterated the original sedimentary structures of the grainstones (Figs. 9A, B).

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Fig. 7. Field photographs of units D and F in the Ironshore Formation in CLQ (see Fig. 3 for location). (A) General view of quarry showing Unit D exposed on quarry floor overlain by unit F that is exposed in quarry wall. Note hammer for scale. (B) Numerous colonies of Diploria in unit D exposed on quarry floor (close to hammer shown in Fig. 8A). (C) Laminated ooid grainstones in unit F. Note Ophiomorpha (black arrows) and Conichnus (C) in lower part of unit. Uppermost trace fossil is 3 m above present day sea level. (D) Conichnus and Ophiomorpha in unit F exposed at entrance to quarry. Arrow indicates uppermost occurrence of trace fossils at 3 m above present day sea level.

The upper unit is formed largely of cross-bedded and planar laminated oolitic grainstones that are largely devoid of fossils (Figs. 8A, 10). High-angle cross bedding dominates the lower part of the unit whereas planar and low-angle cross-bedding is more common in the upper part of the unit. Long vertical Ophiomorpha and large Conichnus are scattered throughout this unit (Fig. 9B). Widely scattered conchs, bivalves, and corals are present in this unit. 7.2.2. Age Two corals from the lower unit in the Morgan’s Harbour area gave ages of 12373, and 13675 ka, whereas one conch from the upper unit near Little Salt Creek yielded an age of 8773 ka (Table 3). Corals and bivalves in the upper unit were badly recrystallized and could not be used for Th/U dating. 7.3. Paul Bodden Quarries Facies: At PBQ, the Ironshore Formation rests unconformably on the Cayman Formation. Although not exposed at PBQA, this unconformity probably lies just

beneath the quarry floor. At both localities, large Porites thickets in the lower part of the formation are overlain by oolitic grainstones that are locally cross-bedded and contain scattered conch shells. No obvious physical discontinuities are evident at PBQ or PBQA (Figs. 10, 11). Age: At PBQA, Porites from the lower part of the succession yielded an age of 13673 ka whereas conchs from the upper part gave ages of 8776 and 9873 ka (Table 3). One conch from the upper part of the sequence at PBQ gave a date of 8773 ka. 7.4. Central Grand Cayman Facies: Along the coastline near SB (Fig. 3), the Ironshore Formation is separated from the Cayman Formation by a vertical unconformity that represents an old coastal cliff-face (Fig. 12). There, the Ironshore Formation is formed of limestones that contain a diverse array of well-preserved corals. Apart from this exposure, there is little or no exposures of the Ironshore Formation in the central part of the island and the only samples that are available come from areas where the limestones are being

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Fig. 8. Field photographs of units D and F in the Ironshore Formation. (A) Cross-bedded ooid grainstones of unit F separated from underlying bioturbated ooid grainstones of unit D by an unconformity (arrows). West coast of North Sound, near Salt Creek. (B) Units D and F separated by unconformity (arrows). Note vertical Ophiomorpha in cross- and planar-bedded ooid grainstones of unit F. West coast of North Sound, near Salt Creek. (C) Unconformity surface separating units D and F. West coast of North Sound, just north of Little Salt Creek, looking south.

excavated for construction purposes. Samples collected from localities OG, MAQ, and TSE (Fig. 3) came from bivalve-rich facies that are characterized by thousands of well-preserved bivalves and numerous conchs (Cerridwen, 1989; Cerridwen and Jones, 1991). Many of the conchs from MAQ, for example, were still characterized by some of their original colouration. Age: Although seemingly well-preserved, the corals from SB and the conchs from localities OG, MAQ, and TSE generally yielded unreliable dates. Only one conch from MAQ yielded a reliable date of 10873 ka (Table 3).

8. Revised stratigraphy of the Ironshore Formation At RWP on the northeast corner of Grand Cayman (Figs. 2, 3), the Ironshore Formation is formed of four unconformity-bounded units (A–D) that onlap a coastal shelf eroded into the underlying Cayman Formation (Fig. 12). The unconformities between these units are highlighted by terra rossa and/or calcrete layers and the ages of units B, C, and D (346, 000, 229,000, and 131,000 years) are well constrained by Th/U dating (Ve´zina, 1997; Ve´zina et al., 1999).

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Fig. 9. Field photographs of units D and F: (A) Highly bioturbated ooid grainstone with numerous Ophiomorpha, unit D, west coast of North Sound, near Salt Creek: (B) Moderately bioturbated ooid grainstone with large open Ophiomorpha, unit D, loose boulder, Batabano Quarry; (C) Numerous in situ head corals in unit D, Boat Launch, northwest coast of Grand Cayman; (D) Large tabular lithoclasts resting on unconformity that separates units D and F, west coast of North Sound, near Salt Creek; (E, F) Lithoclasts of variable size (arrows) resting on unconformity, shown in Fig. 8C, that separates units D and F.

The cores from the Ironshore Formation in the wells drilled in George Town Harbour do not exhibit any obvious physical discontinuities and lack any evidence of terra rossa or calcrete. Indeed, the facies boundaries appear conformable and the sequences seem to be the product of continuous sedimentation. Th/U dates from corals in these cores, however, do not support this viewpoint. Ages of 121,1507 10,000 years from a coral at 21.2 m below the sediment/water interface (BSWI) in BJC#1 and 120,92072,000 years and 134, 52072300 years from corals at 12.2 m and 13.3 m BSWI, respectively, in BJC#3, indicate strata that can be correlated with unit D of the RWP succession (Fig. 5). In BJC#1, ages of 103,73072,000 years and 109,75071,500 years were obtained from corals at 16.2 and 18.9 m BSWI, respectively (Fig. 5). Corals that yielded ages of 80,1407 1550 and 79,6907700 years came from 15.8 m BSWI in BJC#1 and 13.7 m BSWI in BJC#4, respectively. The dates from these corals indicate the presence of two units, herein designated units E and F, that are younger than those found in the RWP sequence (Fig. 5).

In CLQ, an age of 132,62073500 years obtained from a coral from poorly exposed strata on the quarry floor shows that the coral-rich limestones belong to unit D. Although poor exposure precludes accurate positioning, three conchs that yielded ages of 73,970 71200 years, 79,33071950 years, and 94,97072700 years probably came from the lower part of the cross-bedded limestones that overlie the coral-rich limestones. The ages of these conchs indicate that the oldest conch came from unit E whereas the two younger conchs came from unit F. Although not evident in the field, the boundaries between units D and E and units E and F are probably unconformities. Exposures of the Ironshore Formation in coastal exposures between Morgan’s Harbour and Salt Creek (Fig. 2) are characterized by an unconformity that separates the lower and upper units (Fig. 10). The lower unit, with an average age of 12974 ka, indicates correlation with Unit D (Fig. 10). Dating the upper unit is difficult because of the scarcity of fossils. Rare corals and bivalves found in the upper unit are extensively recrystallized and

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Fig. 10. Facies distribution in units D and F on the western part of Grand Cayman. Note topography on unconformity that separates unit F from unit D.

Fig. 11. Field photograph units D and F exposed in west wall of quarry at locality PBQA; (A) Large Porites thicket of unit D overlain by cross-bedded and planar laminated ooid grainstones of unit F. Boundary between units D and F poorly defined, possibly at top of Porites thicket denoted by white dashed line; (B) Basal part of outcrop shown in Fig. 11A, showing Porites thicket overlain by ooid grainstones that contain scattered Strombus shells. Strombus indicated by arrow is same as that indicated in Fig. 11A.

unsuitable for Th/U dating. A conch shell collected from an exposure near Little Salt Creek yielded an age of 86,93073250 years. Such an age indicates that the strata above the unconformity should be assigned to unit F (Fig. 10). South of Salt Creek (locality VDM2, Fig. 3) the Ironshore Formation is characterized by a complex array of facies and three erosional surfaces (ES1, ES, and ES3,

Fig. 13) that Jones and Pemberton (1989) called channel diastems. Traced southward, the ooid grainstone below ES1 and ES2 abuts against a coral patch reef (locality VDM4, Fig. 3). This part of the succession is assigned to unit D because a coral from that patch reef yielded a Th/U age of 135,698 ka (Fig. 3). The stratigraphy of the overlying beds is more difficult to determine because their ages are unknown. Corals and bivalves from an ooid grainstone

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that overlies ES1 at Salt Creek are extensively recrystallized and unsuitable for Th/U dating. Also, it is not obvious if ES1 correlates with ES2 or ES3. Jones and Pemberton (1989) suggested that ES1 correlates with ES2 rather than ES3 because: (a) the two surfaces are found at similar positions in the sequence; (b) the ichnofossil assemblages above ES1 and ES2 are similar; and (c) the ichnofossil assemblage above ES3 is different from that found in the lower part of the succession. Interpretation of these units is complicated by the lack of outcrop at the mouth of Salt Creek and the poor, low-lying outcrop just north of Salt Creek. Other outcrops further to the north are characterized by only one erosional surface that invariably has tabular lithoclasts resting on it. If correlation were based on the erosion surface with overlying lithoclasts, then ES3 would be correlated with ES1 (Fig. 13). Such a correlation might mean that the strata overlying ES1 and ES2 belong to unit F. In turn, that correlation opens the possibility that the strata between ES2 and ES3 belong to unit E. These correlations must remain hypothetical until accurate dates can be obtained for these strata.

Fig. 12. Field photograph showing coral-rich facies of the Ironshore Formation abutting an oold cliff face in dolostones of the Cayman Formation. Arrows denote top of exposed vertical unconformity between the two formations.

551

The Ironshore Formation in Paul Bodden’s Quarries (PBQ, PBQA) unconformably overlies dolostones of the Cayman Formation. Although there are no obvious physical discontinuities in the limestones exposed at these localities (Fig. 11), Porites collected from one of the Porites thickets yielded an age of 136,34073400 years whereas three conch shells from grainstones that overlie the Porites thickets, yielded ages of 87–98 ka. Such ages indicate that strata belonging to units D and F are exposed in walls of these quarries. Although not obvious, the boundary between the two units may be coincident with the Porites/grainstone facies change. Most fossils collected from SB and excavated material in the island’s interior produced unreliable ages (Table 3). The single reliable age of 107,86072750 years from a conch shell collected from locality MAQ indicates that some of the strata from the central interior part of the island may belong to unit E. 9. Deposition regimes for units D, E, and F on the western part of Grand Cayman The Ironshore Formation found in the George Town Harbour cores and exposed on the western part of Grand Cayman belong to units D, E, and F (Fig. 5). Little is known about unit E because it has only been found in the offshore cores, in poor exposures in CLQ, and from one excavation at MAQ in the central part of the island. The limestones in units D and F, however, developed in significantly different depositional regimes. Surface exposures of unit D on the western part of Grand Cayman are characterized by extensive reef development along the west coast with patch-reefs and highly bioturbated oolitic sediments in the back-reef area (Fig. 9C). Bivalves and gastropods were common inhabitants of the reefs and the inter-reef sediments (Cerridwen, 1989; Cerridwen and Jones, 1991). These limestones probably developed during the Sangamon highstand when sea level was 6 m asl (Jones and Hunter, 1990; Ve´zina, 1997; Ve´zina et al., 1999). The diversity of corals, bivalves, and gastropods, the superb preservation of the fossils, and

Fig. 13. Facies distribution and erosional surfaces in Ironshore Formation exposed south of Salt Creek (Fig. 3). Modified from Jones and Pemberton (1989, their Fig. 4A).

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the paucity of sedimentary structures indicative of high energy conditions indicates that the patch reefs and their associated sediments thrived in relatively quiet water that was probably o10 m deep. Unit D in the George Town cores encompasses coral- and Halimeda-rich limestones that must have developed in deeper waters. Given that there is no evidence for fault displacement of unit D in these cores and that the island has not undergone any vertical tectonic movement over the last 125,000 years, it must be assumed that those limestones accumulated at the depths where they are now found. This suggests that these limestones developed in water that was 40–50 m deep. Limestones belonging to unit E are poorly represented in surface exposures on the western part of Grand Cayman. It is possible that most of unit E was removed during the erosional event that produced the unconformity between units D and F. This suggestion is feasible given that unit E is o5 m thick in the George Town Harbour cores and there is at least 2.5 m of erosional relief on the unconformity that separates units D and F (Fig. 10). Unit F is formed of cross-bedded and planar laminated grainstones that contain few fossils, especially when compared to the limestones in unit D (Fig. 10). The basal unconformity, which represents a time gap of 40,000 years, commonly has large tabular lithoclasts lying on its surface (Fig. 9D–F). As noted by Jones and Pemberton (1989) , the genesis of these large lithoclasts is not obvious because they could have originated through the breakdown of beachrock (e.g., Ball, 1967; Inden and Moore, 1983; Garrett and Gould, 1984; Strasser and Davaud, 1986), coastal limestone cliffs (Strasser and Davaud, 1986), or subtidally cemented carbonates (e.g., Shinn, 1968; Halley et al., 1983). A beachrock origin can probably be discounted because the oolitic grainstones that form the lithoclasts do not contain any cements indicative of beachrock (cf. Jones and Pemberton, 1989). Likewise, derivation from a coastal cliff is discounted because there are no obviously coastal cliffs in the vicinity. This process of elimination suggests, therefore, that the lithoclasts were ripped up from an area of the seafloor that had undergone subtidal lithification. This notion is supported by the similarity of the ooid grainstone in the lithoclasts and unit D. The lack of rhizoliths, terra rossa, or calcrete on the unconformity indicates that it did not form subaerially. This viewpoint is supported by the Conichnus (possibly formed by sea anemones) and Skolithus (probably formed by worms or worm-like organisms) that have their apertures at the unconformity exposed near Salt Creek (Jones and Pemberton, 1989). The high relief on the unconformity and the fact that it sharply truncates laminations in the underlying sediments points to an erosive origin (Jones and Pemberton, 1989). Modification of this surface while on the seafloor is indicated by: (1) the lack of soft sediment disturbance features in the sediment beneath the tabular lithoclast; and (2) the fact that smalldiameter Ophiomorpha borneensis that penetrate unit F

commonly terminate against the unconformity whereas the larger-diameter O. nodosa pass through the unconformity surface (Jones and Pemberton, 1989). Although these observations indicate that the erosional surface evolved into a firmground (cf. Ekdale et al., 1984), there is no evidence (e.g., overhangs, borings, encrusting organisms) that a hardground ever developed. Given the geographic setting, it is possible that a hurricane may have generated the strong currents that must have been responsible for erosion of the sediments and development of the topography on the unconformity. Unit F contrasts sharply with unit D because it contains few fossils and is formed of sediments that accumulated in a high-energy environment. The restricted biota of conch shells, scattered bivalves, and corals clearly indicates a depositional regime that did not favour luxuriant growth of a diverse biota. The high angle, cross-bedded ooid grainstones found in the Salt Creek area dip to northeast and appear to have formed under unidirectional highenergy conditions. The origin of the overlying and laterally adjacent cross-bedded ooid and planar bedded grainstones may have formed under lower energy conditions associated with tidal influences. Woodroffe (1988) suggested that the cross-bedded grainstones in unit F formed under subaerial conditions. The cross-bedded ooid grainstones in the lower 3 m of unit F are, however, penetrated by numerous Ophiomorpha and Conichnus (Pemberton and Jones, 1988; Jones and Pemberton, 1989). Attribution of these burrows to burrowing ghost shrimps and sea anemones (Pemberton and Jones, 1988; Jones and Pemberton, 1989) implies that the lower 3 m of unit F must have formed under subtidal conditions. The upper part of unit F, which is generally characterized by low-angle and planar laminated ooid grainstones that are largely devoid of any fossils may have formed in the low intertidal zone. 10. Discussion The Ironshore Formation at RWP (Fig. 14), on the northeast coast of Grand Cayman, is formed units A, B, C, and D that formed during highstands 4400, 346, 229, and 131 ka, respectively (Ve´zina, 1997; Ve´zina et al., 1999). Strata dated between 121 and 135 ka in the George Town Harbour cores (Fig. 5) and onshore exposures on the western part of Grand Cayman correlate with unit D that formed during MIS 5e. Strata in Unit E on the western part of Grand Cayman, dated between 95 and 110 ka correlate with the 103–105 ka highstands recorded on Barbados (Broecker et al., 1968; Mesolella et al., 1969; Matthews, 1973; Bender et al., 1979), the 108 ka highstand from Haiti (Dodge et al., 1983), and the 105 ka highstand known from the Huon Peninsula, New Guinea (Bloom et al., 1974; Aharon, 1983). Unit F, found in the George Town Harbour cores and widely exposed on the western part of Grand Cayman, which formed 74–87 ka correlates with the 85 ka highstand on the Bahamas (Carew and Mylroie, 1987), the 81 ka highstand on Haiti (Dodge et al., 1983),

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the 82 ka highstand in Barbados (Broecker et al., 1968; Mesolella et al., 1969; Matthews, 1973; Bender et al., 1979), and the 80 or 85 ka highstand known from the Huon Peninsula, New Guinea (Bloom et al., 1974; Chappell, 1974; Aharon, 1983). Units A, B, C, and D, known from the RWP succession were correlated with MIS 11(?), 9, 7, and 5e, respectively (Fig. 15). The high amplitude highstand associated with substage 5e, at 125 ka BP (Imbrie et al., 1984) on the SPECMAP time scale, was succeeded by two smaller scale Pleistocene highstands (5a and 5c). Evidence for the 125 ka highstand on Grand Cayman comes from the +6 m wave-cut (Jones and Hunter, 1990, their Fig. 8) and the coral-rich limestones of unit D (Fig. 9C; Ve´zina 1997; Ve´zina et al., 1999). Correlation of units E and F with the 5c and 5a highstands is supported by the their stratigraphic positions above unit D and their Th/U ages determinations. The highstand associated with development of the coralrich limestones of unit D was 3–6 m asl (Ve´zina, 1997; Ve´zina et al., 1999). On Grand Cayman evidence of a welldeveloped wave-cut notch located at 6 m asl and cut into the Miocene dolostones of the Cayman Formation is found near RWP (Fig. 3; Jones and Hunter, 1990, their Fig. 8) and at Old Man Village (Fig. 16). A similar wave-cut notch is also found at many localities on Cayman Brac (Jones and Hunter, 1990). Correlating a wave-cut notch with a particular succession of strata or a particular highstand is fraught with problems as shown by the debate engendered by ideas proposed by Neumann and Hearty (1996) with respect to the +6 m wave-cut notch found on the Bahamas (Carew, 1997; Mylroie, 1997; Kindler and Stsser, 1997; Hearty and Neumann, 1997; Neumann and Hearty, 1997). The assertion that the +6 m wave-cut notch on Grand Cayman formed during the highstand that accompanied

Fig. 14. Stratigraphic relationships between units A, B, C, and D of the Ironshore Formation in the Rogers Wreck Point area with superimposed Th/U ages. Unconformities between units are highlighted by lithified terra rossa and calcrete layers. Modified from Ve´zina et al. (1999, their Fig. 3).

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the development of unit D in the Ironshore Formation is based on the following. First, at RWP, coral-rich limestones of unit D abut against the foot of the cliff face in which the wave-cut notch is cut. The top of those limestones lie 2–3 m below the wave-cut notch. Second, isotopic signatures of speleothems associated with the wave-cut notch on Cayman Brac, also at 6 m asl, imply that the notch formed during the 5e highstand (Tarhule-Lips and Ford, 2004). Third, the elevation of the notch on Grand Cayman and Cayman Brac is the same as the MIS 5e wave-cut notch found on Bermuda (Harmon et al., 1981), the Yucatan Peninsula (Szabo et al., 1978), Haiti (Dodge et al., 1983), the Bahamas (Neumann and Moore, 1975; Neumann and Hearty, 1996), and Jamaica (Boss and Liddell, 1987). The corals and conchs from the western part of Grand Cayman that yielded Th/U ages indicative of the 5e, 5c, and 5a highstands came from exposures that are located between 3 and +3 m relative to present day sea level (Fig. 17). The conch shells found 1–2 m asl in CLQ record the lowest possible highstand position for units E. This assertion must be treated with some caution because the Strombus were mobile while alive and therefore not indicative of any particular range of water depths. Today, they can be found in the modern lagoons around Grand Cayman to depths of at least 6 m. Furthermore, conch shells may be thrown on shore during storms. The possibility that these conch shells were eroded from other strata (cf. Belluomini et al., 2002) before being included in the limestones of unit E at CLQ can be discounted because there are no other strata in the area from which these shells could be derived. Collectively, however, these considerations indicate that the minimum highstand position for unit E was probably 1–2 m above present day sea level. The depth of water above that minimal position is unknown because of the paucity of exposures of this unit. Water depths of 2–3 m do not, however, seem unreasonable. If that is accepted, the highstand position for unit E would have been on the order of 2–5 m as indicated. Determining the highstand position associated with unit F is difficult because of the general lack of in situ fossils. Conichnus and Ophiomorpha are found up to 3 m asl in unit F exposed in CLQ (Fig. 7C, D) and along the west coast of North Sound. Burrowing shrimp probably formed the Ophiomorpha whereas sea anemones were probably responsible for the Conichnus (Pemberton and Jones, 1988). Both are known from shallow-water lagoons around Grand Cayman. Conichnus, in particular, are restricted to subtidal settings. Thus, the highest occurrence of Conichnus and Ophiomorpha at +3 m in unit F must be taken as the lowest highstand position for that unit. The depth of water that existed when those burrows were formed is unknown and cannot be determined from the rocks themselves. As with unit E, water depths of 2–3 m do not seem unreasonable. If so, the highstand position for unit F would be on the order of +3 to +6 (?) m. It is possible, therefore, that the highstands associated with units D, E,

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Fig. 15. Correlation of dates for units A, B, C, D, E, and F in the Ironshore Formation with SPECMAP time scale (modified from Imbrie et al., 1984; Ve´zina, 1997; Ve´zina et al., 1999). d18O scale represents isotopic fractionation of 18O/16O against standard ocean water ( ¼ 0). Negative values represent ocean water enriched in 16O (interglacial periods) whereas positive values represent ocean water enriched in 18O (glacial periods).

Fig. 16. Wave-cut notch, 6 m above present day sea level, cut into the Cayman Formation at Old Man Village on the central north coast of Grand Cayman. Available evidence indicates that this notch formed during MIS 5e.

intriguing possibility that the wave-cut notch found at +6 m on Grand Cayman (Fig. 16) may be polycyclic with erosion related to the 5e, 5C, and 5a highstands. Unfortunately, this possibility is impossible to assess in detail because the wave-cut notch is found in areas where units E and F are absent and units E and F are found in areas where there are no preserved wave-cut notches. Estimates for the 5a highstand position range from 30 m below present-day sea level (bsl) on the Huon Peninsula (Bloom et al., 1974) to 10 m asl on the Atlantic Plain of the USA (Cronin et al., 1981) with many estimates being in the 10–20 m bsl range (Fig. 18). The discrepancy between these estimates and the 3–6 m attributed to the 5a highstand on Grand Cayman could be due to a number of different reasons.

 and F may all have been at approximately the same position (Fig. 17). Such similarities in the highstand positions probably explain the past confusion in assessing the paleogeographic development of the Ironshore Formation on the western part of Grand Cayman (e.g., Brunt et al., 1973; Jones and Hunter, 1990). This also raises the

Given the position of Grand Cayman close to the Orient Transform Fault, the island may have been tectonically uplifted following the deposition of unit F. There is, however, no evidence of tectonic uplift of the western part of Grand Cayman over the last 125,000 years where the wave-cut notch, attributed to the 5e highstand, is found at 6 m asl. On Cayman Brac, tectonic activity is evident from the westward tilting of the strata in the

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Fig. 17. (A) SPECMAP with superimposed Th/U ages determined from corals and conchs in the Ironshore Formation from the George Town Harbour cores and various onshore exposures. (B) Elevations of dated corals and conchs shown in Fig. 16A.



Bluff Group that forms the core of the island. Despite this, there is no evidence of vertical movement or tilting of the +6 m wave-cut notch, which is clearly evident at many locales around the island. As on Grand Cayman, unit D of the Ironshore Formation commonly abuts the foot of the cliff face in which the notch has been cut. The+6 m elevation of the notches on Grand Cayman and Cayman Brac are consistent with the position of MIS 5e wave-cut notches found elsewhere in the Caribbean. The age of unit F exposed on the western part of Grand Cayman is based on Th/U dating of conch shells. These ages could not be verified by dating other fossils because the associated corals and bivalves were recrystallized and unsuitable for dating. It should be noted, however, that corals from unit F in the George Town Harbour

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cores yielded similar ages to those derived from the conchs. The elevation of the corals and conchs that yield Th/U dates give the minimum sea-level position because those animals lived on the seafloor, not at sea level. For example, the 7 to 10 m highstand position estimated for the MIS 5a reef seaward of the Florida Keys was based on a date obtained from a M. annularis colony that was deemed to have lived in water 3 m deep (Ludwig et al., 1996). As noted by Ludwig et al. (1996), M. annularis can grow to depths of 80 m and enjoys optimal growth from 3 to 45 m (cf., Shinn et al., 1989). If, however, the dated coral had lived in water that was 10 m deep, the computed sea-level would be 0 to +3 m rather than the 7 to 10 m advocated by Ludwig et al. (1996).

Globally, the highstand positions associated with MIS 5a can be divided into those between 10 and 30 m asl (group I), and 0 and +10 asl (group II) (Fig. 18). Those in group I come from tectonically unstable islands (e.g., Barbados, Huon Peninsula, New Guinea) whereas those in group II come from areas that have been deemed tectonically stable (e.g., Bermuda, Bahamas, Cayman Islands). The highstands derived from tectonically active islands like Barbados and New Guinea are based on calculations that try to account for the tectonic component of the current elevation of the dated corals (e.g., Schellmann and Radtke, 2004, their Fig. 18). Such calculations, however, rely on assumptions regarding both the rate of island uplift and its consistency through time. Any error associated with these assumptions is propagated through the calculations and produces false estimates of highstand positions. In contrast, the sea-level positions in group II are based on the present-day position of the dated fossils and their associated biota. The 40 m range associated with the estimates for the MIS 5a highstand position (Fig. 18) clearly shows the uncertainty that surrounds this issue. The +3 to +6 m elevation derived for the MIS 5a highstand on Grand Cayman (this study) is, for example, significantly different from the 19 to 23 m elevation for the MIS 5a-1 and 17 to 20 m elevation for the MIS 5a-2 highstands derived from Barbados (Schellmann and Radtke, 2004). If the Cayman elevation is correct, then the calculations used to determine the highstand position on Barbados must contain false data or mistaken assumptions. If the Barbados elevations are correct, then Grand Cayman would have to have been uplifted by 25 m over the last 75,000 years. The fact that the wave-cut notch considered to have formed during the MIS 5e highstand is still at +6 m on Grand Cayman indicates that the island has undergone little or no uplift during this period. 11. Conclusions The Ironshore Formation found in the George Town Harbour cores and exposures on the western part of Grand

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Fig. 18. Sea-level elevations reported for highstands associated with MIS 5a and MIS 5c.

Cayman belong to unit D as defined by Ve´zina (1997) and Ve´zina et al. (1999) and units E and F, as defined in this study. Analysis of these units has led to the following important conclusions.

   

Unit D, as shown by Ve´zina (1997) and Ve´zina et al. (1999) was deposited during the MIS 5e highstand, 123–147 ka, when sea level was 6 m above present day sea-level. Unit E, which formed 95–110 ka, was deposited during the MIS 5c highstand when sea level was 2–5 m above present day sea level. Unit F, which formed 74–87 ka, was deposited during the MIS 5a highstand when sea level was 3–6(?) m above present day sea level. The highstand sea levels for units D, E, and F may have been about the same.

 

On the western part of Grand Cayman, unit E is found in the George Town Harbour cores and in poor exposures at one or two onshore localities. In most onshore localities, unit F unconformably overlies unit D. The unconformity is an erosional surface with up to 2.5 m relief. Unlike the unconformities between units A and B, B and C, and C and D, this unconformity is not highlighted by terra rossa or calcrete.

This study, in conjunction with that of Ve´zina (1997) and Ve´zina et al. (1999), shows that the Ironshore Formation of Grand Cayman is a complex succession that encompasses six unconformity-bounded packages. Each package provides the record of sedimentation that took place during successive highstands throughout the Pleistocene.

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