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Holocene lagoonal development in the isolated carbonate platforms off Belize
Sedimentary Geology 159 (2003) 113 – 132 www.elsevier.com/locate/sedgeo
Holocene lagoonal development in the isolated carbonate platforms off Belize Eberhard Gischler * Geologisch-Pala¨ontologisches Institut, J.W. Goethe-Universita¨t, Senckenberganlage 32-34, 60054 Frankfurt am Main, Germany
Abstract Thirty-one vibracores were taken in interior lagoons of Glovers Reef, Lighthouse Reef, and Turneffe Islands—three isolated carbonate platforms offshore Belize, Central America. Holocene facies successions overlying the Pleistocene limestone bedrock begin with soils, followed by mangrove peats, and marine carbonate sediments of lagoonal origin. The soils formed on top of subaerially exposed Pleistocene limestone before the Holocene transgression. Mangrove peats developed during initial flooding of the platforms (Glovers ca. 8.5 ky, Lighthouse ca. 7 ky, Turneffe ca. 6 ky BP). As water depths increased, reefs colonized platform margins, lagoonal circulation improved thereby promoting carbonate production. The basal lagoonal carbonate sediments are characterized by shell beds and/or Halimeda packstones – grainstones. Mollusk-dominated wackestones and packstones follow upsection in Glovers and Lighthouse Reefs. At present, open circulation prevails in Glovers and Lighthouse lagoons. In contrast, organic-rich Halimeda wackestones and packstones dominate the Turneffe Islands Holocene succession. The main lagoon area of Turneffe is enclosed by mangroves, and restricted circulation prevails. Factors that explain the differences in geomorphology, circulation, and facies are variations in depth of antecedent topography and degree of exposure to waves and currents. The thickness of Holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls. Holocene sedimentation rates average 0.6 m/ky, with highest rates in Turneffe (0.82 m/ky), followed by Lighthouse (0.53 m/ ky), and Glovers (0.46 m/ky). Like in many other isolated carbonate platforms and atolls, lagoon floor sedimentation did not keep pace with rising sea level, leading to unfilled accommodation space. At present, Glovers has an 18 m deep lagoon, while Lighthouse and the main Turneffe lagoon are 8 m deep. It is unlikely that the lagoons will be completely filled during the Holocene sea level highstand cycle. This observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbonate platform; Atoll lagoon; Coral reef; Holocene; Belize; Carbonate
1. Introduction Meter-scale shallowing-upward cycles of lagoonal and peritidal sequences in the interior of Paleozoic and Mesozoic carbonate platforms have been used as
* Tel.: +49-69-798-25136; fax: +49-69-798-22958. E-mail address: [email protected] (E. Gischler).
records of high-frequency (104 – 105 year) eustatic sea level fluctuations (e.g., Fischer, 1964; Goldhammer et al., 1987; Read and Goldhammer, 1988). In these studies, cycle thickness is assumed to roughly represent accommodation space and, after correcting for tectonic subsidence, cycle thickness is used as a proxy for sea level change. Goldhammer et al. (1990) acknowledged, however, that high-frequency, meterscale shallowing-upward cycles may well be missing
0037-0738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0037-0738(03)00098-8
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in settings of high-frequency and high-amplitude (>120 m) glacio-eustacy such as the Pleistocene where large differences in relative amplitudes of different orders of sea level fluctuations (eccentricity: 100 ky, precession: 20 ky) exist. Whereas the 100-ky cycle highstands submerged Pleistocene platforms and also recorded some of the 20-ky cycles, the 100-ky cycle lowstands kept the platforms exposed so that two or more superimposed 20-ky cycles were missed on the platform, according to these authors. Similarly, Read (1985) and Koerschner and Read (1989) also noted the differences between the abundance of Paleozoic high frequency, peritidal platform cycles and their scarcity in Quaternary carbonate platforms. They concluded that modern platforms are poor analogs for most ancient cyclic platform sequences because the large amplitude of eustatic sea level fluctuations produced accommodation space, which could not be subsequently filled (Koerschner and Read, 1989, p. 685). Indeed, several studies on the Holocene development of modern isolated carbonate platform and atoll lagoons have shown that unfilled accommodation space is a common feature (e.g., Purdy and Winterer, 2001). In the north lagoon of the Bermuda platform, for example, Kuhn (1984) discovered Holocene successions of soils, peats, restricted marine and eventually open marine sediments. He found that sedimentation rates generally lagged rates of Holocene sea level rise. Rasmussen (1989) reported a transition form terrestrial, brackish, hypersaline, and marine phases in the Holocene successions of the Bight of Abaco, Bahamas. Based on average sedimentation rates, it would take another 30,000 years to completely fill this lagoon. Boss and Rasmussen (1995) showed that Holocene cycle thickness and accommodation space across the Great Bahama Bank are uncorrelated. They concluded that, due to low sedimentation rates and the limited duration of the interglacial sea level highstand, unfilled Holocene accommodation space will probably result. MacKinnon and Jones (2001) found a transition from fresh to brackish to marine sediments in the North Sound of Grand Cayman and noted an upward increase in accommodation space through time. In the Indian Ocean, an investigation of a mixed siliciclastic-carbonate lagoon around the island of Mayotte by Zinke (2000), found strong lateral variability of facies and
accumulation rates depending on terrigenous or carbonate predominance. In all locations studied, however, lagoonal sedimentation rates lagged behind sea level rise, creating unfilled accommodation space. These detailed studies of Holocene lagoon development support the contention that unfilled accommodation space is common to modern platforms and atolls. Even so, there are considerable differences with respect to Holocene facies successions and interpreted flooding scenarios. Controlling factors include topography of the Pleistocene bedrock surface, rates of Holocene sea level rise, exposure to waves and currents, size of platform interior lagoons, carbonate productivity in lagoons and surrounding marginal reefs, and sediment export. This study further examines the processes which control the development of modern platform lagoons by investigating Holocene facies sequences in three interior lagoons of the carbonate platforms off Belize. These three localities are well suited to study lagoon development as (1) there is significant morphological and environmental variation among the three platform interior lagoons, and (2) a framework of reference for this study had been provided previously by rotary core drilling on marginal areas and reefs of the platforms (Gischler and Hudson, 1998; Gischler and Lomando, 2000).
2. Setting The isolated carbonate platforms of Glovers Reef, Lighthouse Reef, and Turneffe Islands are located east of the Belize Barrier Reef (Fig. 1). The structural basement of the platforms and the barrier reef itself are situated on NNE-trending ridges interpreted as tilted normal fault blocks along the passive margin (Dillon and Vedder, 1973). Exploration wells on Turneffe Islands and Glovers Reef have recovered 1030 m of largely Tertiary reef and shallow water limestone, underlain by late Cretaceous to early Tertiary clastics and metavolcanics (Dillon and Vedder, 1973; Gischler and Lomando, 2000). Belize lies in the trade wind belt, and consequently winds from the E and NE predominate for most of the year. Accordingly, the mean wave approach is from ENE (75j). In the winter, winds from the N to NW are common. Average air temperatures range from 24 jC
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in the winter to 27 jC in the summer. Precipitation rates on the mainland amount to 150 –200 cm/year in the low north and may exceed 400 cm/year in the mountainous area of the Maya Mountains (Purdy, 1974; Purdy et al., 1975). Water temperatures range from 27 (winter) to 29 jC (summer). The tide range is 30 cm. The passing Caribbean currents produces N – S flowing countercurrents both on the Belize shelf
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(Purdy, 1974; Purdy et al., 1975), and within the open lagoon of Glovers Reef (Wallace and Schafersman, 1977). All three platforms are surrounded by deep water and have breakwater reefs encircling lagoons (Stoddart, 1962). The interior lagoons of the three platforms are significantly different with regard to geomorphology, water circulation, and sediment distribution
Fig. 1. (a) Map of Belize with locations of the investigated atolls Glovers Reef, Lighthouse Reef, and Turneffe Islands. Fault blocks after Dillon and Vedder (1973). (b) Map of the Belize atolls with locations of 31 vibracores. Fourteen rotary core hole locations from Gischler and Hudson (1998) and Gischler and Lomando (2000).
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Fig. 1 (continued).
(Gischler, 1994; Gischler and Lomando, 1999). Glovers Reef covers 260 km2 and has a lagoon, which is up to 18 m deep and contains f 860 patch reefs that
are, for the most part, randomly distributed. There are five small marginal sand – shingle cays. Lighthouse Reef covers 200 km2 with a lagoon that has an 8 m
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deep eastern part and a shallow 3 m deep western part, divided by a linear trend of patch reefs (‘‘Middle Reef’’). Two large mangrove – sand cays are developed. Lagoon floors in Glovers and Lighthouse Reefs are covered by patches of seagrass (Thalassia testudinum, Syringodium filiforme) and calcareous algae (Penicillus sp., Halimeda sp., Udotea sp., Caulerpa sp.). Sediment facies include mollusk –foram – Halimeda facies in the deep lagoon and mixed peloidalskeletal facies in the shallow parts (Gischler, 1994; Gischler and Lomando, 1999). The existence of abundant coral patch reefs and clear waters in the lagoons of these atolls supports the contention that open circulation prevails. Turneffe Islands has an area of 525 km2 and 25% of the platform is covered by mangroves. These mangroves encircle a lagoon, which is up to 8 m deep. The lagoon floor is densely covered by Thalassia and Halimeda. Sponges (Spheeciospongia sp.) are also common; corals (branched Porites sp.) and Manicina areolata occur occasionally. Coral patch reefs are rare and the Halimedadominated sediments have a dark brown stain due to elevated amounts of humic acids from decaying organic matter (Gischler and Lomando, 1999). In the summer of 1939, Smith (1941) recorded extreme temperature and salinity values within the Turneffe main lagoon with water temperatures exceeding 31 jC and salinities reaching 70x . Collectively, these observations support the inference of restricted Turneffe lagoon circulation. Systematic data on temperature and conductivity were recently obtained from two data loggers (R. Brancker, model XL210) which were deployed on each of the three platform lagoon floors from December 2000 – December 2001. According to these data, water temperature ranges from 23 –31 jC in Glovers and Lighthouse Reefs and from 22 –32 jC in Turneffe Islands. Salinity fluctuates from 38.6 – 42.0x in Glovers Reef, from 37.7– 41.6xin Lighthouse Reef, and from 34.2– 42.5xin Turneffe Islands lagoons.
3. Methods A total of 31 cores were taken from the Belize platform lagoonal areas in March and April 2000 with a Lanesky et al. (1979)-type vibracorer using 6 m long aluminum pipe (diameter 7.5 cm) with core
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catchers (Fig. 1, Table 1). Cores were taken at sea level and in shallow water depths using snorkel and dive mask, and in greater water depths of up to 15 m using scuba. Lengths of cores ranged from 0.4 to 5.63 m and penetration usually stopped when Pleistocene limestone bedrock was reached. At a few locations small fragments of Pleistocene coral or limestone were recovered, but at others the bedrock was not reached. After drilling, the core tubes were marked at the sediment surface to record compaction effects, and the tops sealed with plastic caps. In shallow depths, cores were retrieved using aluminum clamps with handles and a car jack. Where scuba diving was necessary the clamps were used together Table 1 Water depth, core recovery and compaction of vibracores Core
Water depth (m)
Recovery (m)
True thickness (m)
Compaction (%)
L1 L2 L3 L4 L5 L6 G1 G2 G3 G4 G5 G6 G7 G8 G9 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17
6.6 8.1 3.0 9.0 3.6 5.7 9.0 9.6 12.0 12.0 13.5 13.8 11.7 14.7 15.3 0 0 0.9 0.5 3.6 0 3.6 2.1 3.3 3.3 4.5 2.4 0.9 0.3 4.8 1.8 0
2.40 1.94 1.70 2.50 3.75 2.46 5.63 3.36 1.40 3.14 4.95 2.48 3.47 3.08 1.80 1.86 1.84 – 2.24 1.88 3.30 5.35 0.70 1.47 1.87 2.25 0.40 0.50 2.32 1.08 0.35 1.92
2.75 2.25 1.95 2.85 3.80 2.70 5.70 4.20 1.50 3.30 5.70 3.30 3.90 3.75 1.95 4.50 4.50 – 4.80 2.10 5.70 5.70 0.90 1.80 2.10 2.40 0.60 1.20 3.75 1.80 2.40 2.40
12.7 13.8 12.8 12.3 1.3 8.9 1.2 20.0 6.7 4.8 13.2 24.8 11.0 17.9 7.7 58.7 59.1 – 53.3 10.5 42.1 6.1 22.2 18.3 11.0 6.3 33.3 58.3 38.1 40.0 85.4 20.0
Core T3 was lost. Core G4 was drilled at same spot as G3 because G3 hit rock early.
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with lift bags that were filled with air from the scuba tanks. On the boat, the upper, water-filled part of the aluminum pipe was cut off, and any compaction of the sediment was noted. Core pipes were marked with waterproof pens, sealed with plastic caps and tape, and later cut into 1– 1.5 m long sections for transport. In the laboratory, cores were cut open with a portable saw and the sediment split in half using metal wire or knifes. Cores were logged, photographed and, from one core half, 5-cm long samples were taken for sedimentologic (texture, composition), mineralogic (XRD), and chronologic (C14-dating) analyses. The other half of the core was sealed and kept for reference. Samples were washed in a dilute solution of hydrogen peroxide and later dried. Dry samples were split with a sample splitter. One part
was used for XRD analysis, and relative abundances of carbonate minerals were determined using the method of Milliman (1974, pp. 21 –29). The other part of the sample was sieved into the common grades >2, 2 –1, 1 – 0.5, 0.5– 0.25, 0.25 –0.125, 0.125– 0.063, and < 0.063 mm. Relative amounts of the mud ( < 0.063 mm) and coarse fractions (>2 mm) were used to assign depositional texture, in addition to the visual description of texture during core logging (following Dunham, 1962). Fractions >0.125 mm were impregnated with resin and thin sections were made. Constituent grain composition of the samples was determined by counting 200 grains per thin section. The carbonate portion of soil samples from the bases of five cores was dissolved using hydrochloric acid and the insoluble residue analyzed by XRD. Dried peat and soil samples from the bases of
Fig. 2. Vibracore logs from Lighthouse Reef (corrected for compaction).
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five cores were combusted for 3 h at 600 jC. By subtracting ash weight from dry weight, the relative abundance of organic matter was determined. Throughout the text conventional ages (uncorrected for ocean reservoir effects) are used when referring to C14 dates. Dating was performed by Beta Analytic, Miami, FL, USA.
tremely high compaction in the cores from the leeward mangrove rim (cores T1 –T4, T6, T16) and from the northern lagoon (T12 –T14), where sediment is rich in pore water and therefore has a ‘‘soupy’’ texture. The general facies succession begins with a basal soil overlying Pleistocene limestone, and is followed by mangrove peat, and carbonate sediments, as described below.
4. Results
4.1. Pleistocene limestone bedrock
A total of 71.5 m of core was recovered (Figs. 2– 5; Table 1) in which compaction averages 18% and is lowest in Glovers and Lighthouse cores (10 – 12%) and highest in Turneffe (average 33%). The relatively high value for Turneffe results from the ex-
Small pieces of buff-colored, well-cemented Pleistocene limestone were recovered at the base of a number of cores. Thin sections reveal indications of meteoric diagenesis such as abundant blocky calcite cement and coral, mollusk, and Halimeda fragments
Fig. 3. Vibracore logs from Glovers Reef (corrected for compaction). See Fig. 2 for legend. Note that Core G3 has same vertical succession as core G4.
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Fig. 4. Vibracore logs from Turneffe Islands (corrected for compaction). See Fig. 2 for legend. Note that cores T1 and T2 have same vertical succession. Core T3 was lost. Core T17 (not shown) is composed entirely of peat.
that are partly recrystallized to calcite. At the leeward margin of Turneffe Islands (cores T1 and T2) fragments of corals such as Diploria strigosa, M. areolata, and Porites astreoides could be identified. The Porites fragment still had 80% aragonite preserved. Unidentified mollusk shells were also found in fragments of bedrock (e.g., core T5).
ples have up to 30% of low magnesium calcite. Organic matter makes up 56 – 67% of three analyzed samples. No macrofauna was found in the soils. Firmground burrows filled with the overlying carbonate sediment are common. The thickness of soils was highest within Turneffe Islands where it may reach as much as 1 m (Fig. 4).
4.2. Soil
4.3. Peat
Overlying the bedrock, centimeter thick soils are developed which consist of dark brown to greenish mudstones with plant and root remains. Mineralogically, the mudstones consist of carbonate, quartz, clay minerals, and organic matter, with the quartz and clay minerals probably being of eolian origin. Although the carbonate is dominated by aragonite, some sam-
The soil-generated mudstones are overlain by reddish brown to brown peat which is composed mostly of long slender fibers and which releases clear to teacolored water upon compression. Usually the prop roots of red mangroves (Rhizophora) and the pneumatophores of black mangroves (Avicennia) are still identifiable in the core. Organic matter makes up
E. Gischler / Sedimentary Geology 159 (2003) 113–132 Fig. 5. Selected photographs from vibracores; scale in cm. (a) Base of core T9 showing weathered buff Pleistocene limestone, brown soil, black peat with firmground burrows and, brown Halimeda wackestone. (b) Base of core T4 peat firmground with open burrows infilled by overlying Halimeda packstone. (c) Core T4, about 1.5 m above base showing Halimeda packstone with four specimens of C. orbicularis. (d) Core T5, about 0.5 m below top showing Halimeda packstone with corals Porites porites and M. areolata. (e) Base of core L6 showing intensively burrowed peat with burrows infilled by overlying skeletal packstone. (f) Base of core L1 with basal shell bed (largely L. laevigatum) and overlying skeletal packstone. (g) Base of core G5 showing brown to greenish soil, burrowed mangrove peat, skeletal wackestone with shells of L. laevigatum and Anadara sp. (h) Core G4, about 0.6 m below top showing skeletal wackestone with shells of Ostrea sp., Anadara sp., and L. laevigatum. 121
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Table 2 Textural data of carbonate sediments from vibracores based on sieving Sample
>2
2–1
1 – 0.5
0.5 – 0.25
0.25 – 0.125
0.125 – 0.063
< 0.063
L1-100/105 L1-210/215 L2-90/95 L2-180/185 L3-85/90 L4-40/45 L4-125/130 L4-240/245 L5-55/60 L5-250/255 L5-340/345 L6-00/05 L6-120/125 L6-205/210 G1-75/80 G1-300/305 G1 525/530 G2-30/35 G2-165/170 G2-300/305 G4-70/75 G4-240/245 G5-70/75 G5-235/240 G5-420/425 G5-475/480 G6-120/125 G6-225/230 G7-120/125 G7-285/290 G8-255/260 G9-00/05 G9-100/105 T1-75/80 T1-165/170 T4-85/90 T4-165/170 T5-70/75 T6-165/170 T6-325/330 T7-60/65 T7-235/240 T7-505/510 T8-65/70 T9-00/05 T9-60/65 T9-120/125 T14-90/95 T15-100/105
6.8 13.0 13.9 11.5 16.5 9.2 25.6 34.0 9.6 6.6 11.0 0.2 11.0 14.3 6.2 6.6 12.3 1.0 2.9 6.2 17.5 29.6 0.2 2.5 2.8 1.0 4.8 12.1 3.1 8.7 1.2 24.3 18.0 4.5 38.1 2.9 30.4 7.5 0.2 22.7 37.8 18.6 8.5 27.7 16.5 2.7 4.3 4.1 16.7
7.0 4.4 5.5 6.2 12.5 4.8 9.2 10.1 7.5 5.3 10.7 0.7 9.3 8.1 4.8 3.7 5.5 1.2 5.2 6.8 12.8 6.4 0.8 6.4 1.4 2.4 5.5 6.2 6.2 7.3 1.5 12.5 9.9 7.7 16.7 0.9 7.8 5.3 1.7 7.8 19.9 11.2 2.7 3.4 5.9 1.8 6.4 4.7 4.2
9.4 6.6 6.1 8.0 12.6 5.9 6.5 7.4 9.4 5.7 10.2 4.8 8.2 11.6 13.7 4.6 3.9 8.0 10.7 12.2 10.6 5.3 3.5 5.4 5.2 3.9 8.5 9.9 8.3 13.9 8.2 14.0 19.4 6.7 13.5 1.2 5.8 4.9 3.4 7.5 14.3 8.2 1.6 2.7 4.7 1.3 18.1 8.5 18.1
14.9 10.2 7.9 8.0 15.5 8.0 5.4 6.4 14.1 13.2 13.2 9.0 9.2 12.5 15.9 18.9 15.7 11.7 8.7 14.2 10.4 13.3 5.1 4.2 8.5 18.6 10.4 11.6 9.5 12.2 7.2 13.7 14.7 17.9 12.8 10.8 20.8 28.0 14.0 15.5 10.7 19.1 31.3 16.2 20.5 2.7 20.2 43.1 13.9
30.9 22.4 12.9 12.6 24.7 17.6 12.7 9.0 26.1 22.0 16.8 27.3 21.3 19.6 18.3 23.7 18.2 17.2 14.8 17.4 31.8 20.1 9.8 10.7 13.2 24.9 13.2 15.7 17.8 17.5 10.6 12.3 13.9 19.8 10.9 26.3 13.9 28.8 25.7 14.9 7.4 20.5 32.8 16.2 28.4 41.6 20.2 24.7 25.0
20.7 24.6 32.3 23.7 11.5 33.4 19.4 17.8 18.3 30.8 14.4 39.9 27.5 22.1 27.7 28.0 36.5 29.9 32.8 31.5 10.3 14.0 48.3 17.4 45.8 19.6 40.8 23.4 35.4 25.3 26.9 13.5 13.7 21.4 3.9 33.0 14.4 15.2 39.1 17.7 4.8 9.3 16.7 16.2 15.0 37.6 12.8 8.8 11.1
10.3 18.8 21.4 30.0 6.8 21.1 21.1 15.4 15.0 16.4 23.7 18.1 13.5 11.9 13.3 14.4 7.9 31.0 24.9 11.7 6.5 11.3 32.2 53.4 23.1 29.5 16.8 21.1 19.7 15.0 44.4 9.8 10.4 21.8 4.1 24.8 6.8 10.3 15.7 14.0 5.1 13.3 6.4 17.6 9.1 12.4 18.1 6.0 11.1
Numbers after core name identify depth of 5-cm interval sampled in compacted core. Grain sizes are in mm; abundances are in %.
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Table 3 Compositional data from carbonate sediments based on point-counting of thin sections (fractions > 0.125 mm) Sample
HA
CO
RA
MO
FO
PE
MI
UK
FINES
L1-100/105 L1-210/215 L2-90/95 L2-180/185 L3-85/90 L4-40/45 L4-125/130 L4-240/245 L5-55/60 L5-250/255 L5-340/345 L6-00/05 L6-120/125 L6-205/210 G1-75/80 G1-300/305 G1 525/530 G2-30/35 G2-165/170 G2-300/305 G4-70/75 G4-240/245 G5-70/75 G5-235/240 G5-420/425 G6-120/125 G6-225/230 G7-120/125 G7-285/290 G8-255/260 G9-00/05 G9-100/105 T1-75/80 T1-165/170 T4-85/90 T4-165/170 T5-70/75 T6-165/170 T6-325/330 T7-60/65 T7-235/240 T7-505/510 T9-00/05 T9-60/65 T14-90/95 T15-100/105
20.0 18.7 2.5 7.9 27.8 2.5 0 5.7 18.7 21.1 37.4 13.9 5.5 14.2 13.6 6.9 44.8 9.8 6.3 19.9 23.6 32.0 3.8 0.3 8.9 6.4 13.9 11.0 20.1 7.2 28.0 38.0 31.5 37.7 32.5 72.9 62.2 35.3 41.3 86.0 63.9 64.6 67.2 42.1 42.6 56.9
0.7 0.8 0.7 0.7 0.4 0.2 0.6 0.3 0.7 0.5 0.3 1.1 0.7 1.3 0.3 0 0 0.2 0.2 0 1.2 0.4 0.6 0.9 0 2.1 1.4 0.2 0 0.3 0.8 0 0.9 0.5 0.2 0 0.4 0.5 0 0 1.6 0 0 0 0 0
1.0 0.6 0 0 2.0 0 0 0 0.3 0 0 1.1 1.0 0.3 0 0 0 0.2 0 0 0 0.4 0.3 0 0.2 0.2 0.3 0.4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
26.2 27.7 16.7 17.1 34.3 23.2 46.4 42.1 26.7 13.2 12.4 10.7 40.4 44.0 41.6 23.6 2.1 16.8 19.2 24.4 30.2 27.1 6.0 21.3 16.3 22.9 34.7 16.8 25.7 13.8 39.1 28.8 12.8 43.2 1.5 4.3 6.3 1.8 23.8 2.3 5.8 7.7 3.8 7.1 37.9 12.9
7.6 2.3 7.6 7.2 5.7 9.3 4.5 6.0 4.0 8.2 6.5 6.7 3.5 1.7 0.9 3.7 0.2 4.7 6.8 7.7 9.1 1.9 3.9 2.6 2.2 4.2 1.9 6.3 6.9 2.9 3.1 3.4 7.7 5.5 1.7 0 3.4 3.4 1.7 0.9 1.6 3.5 0.8 0 1.3 0.4
5.9 1.4 16.0 6.5 6.5 5.0 2.7 3.7 10.3 3.7 2.5 2.7 9.3 1.0 1.2 20.4 0 2.9 5.9 1.7 15.7 10.2 1.0 1.0 2.2 2.8 0.8 6.1 3.0 3.2 0.4 4.6 0.9 0.5 2.7 1.2 0 2.0 0 0 3.5 0 0 0 0 1.3
2.1 1.4 0.7 2.1 0.8 1.1 0 5.3 1.7 0.3 0 1.7 0.7 2.0 0.3 2.6 0 1.6 0.6 0 2.1 2.3 1.4 0.4 0.2 0.6 0.6 0 0.3 0 1.5 0 0.3 1.4 0.4 0 0 0.5 0 0.5 1.2 1.2 2.7 0.4 0 2.7
5.5 3.7 2.1 4.9 4.1 4.1 5.4 3.7 4.3 5.8 2.8 4.2 7.9 1.7 1.2 0.3 0.5 2.9 3.2 3.1 0.8 1.1 2.5 2.6 1.2 3.2 1.9 3.6 3.0 1.4 3.8 1.1 2.8 3.2 3.2 0.4 2.2 2.0 1.4 0.5 0 0 1.5 0.4 3.4 3.6
31.0 43.4 53.7 53.7 18.3 54.5 40.5 33.2 33.3 47.2 38.1 58.0 31.0 34.0 41.0 42.4 52.3 60.9 57.7 43.2 17.3 24.8 80.5 70.8 68.9 57.6 44.5 55.1 40.3 71.3 23.3 24.1 43.2 8.0 57.8 21.2 25.5 54.8 31.7 9.9 22.6 23.1 24.1 50.0 14.8 22.2
Numbers after core name identify depth of 5-cm interval sampled in compacted core. HA = Halimeda, CO = coral, RA = red algae, MO = mollusk, FO = foraminifera, PE = peloidal, MI = miscellaneous, UK = unknown. Miscellaneous includes echinoderm fragments, sponge and tunicate spicules, and ostracods. FINES = fraction < 0.125 mm, data obtained from sieving.
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between 57% and 72% of sample weight. No macrofauna was found in the peats, however, burrows filled in with overlying carbonate sediment are common. Peat thicknesses in lagoons range from 0.3 to 0.5 m but can be thicker within the mangrove rims of Turneffe Islands where peats as thick as 2 m were recovered (Figs. 2 – 4). 4.4. Carbonate sediment Mineralogically, the carbonate sediments consist largely of aragonite and high magnesium calcite. Average aragonite content is 85%, high magnesium calcite, 13.5%, and low magnesium calcite 1.5%. Texturally, packstones predominate in Lighthouse Reef samples, but both packstone and wackestones are found in Glovers Reef and Turneffe Islands cores (Figs. 2 –4, Table 2). In a number of cores (G1, G4, T1, T2, T6, L4), an upward transition from packstones to wackestones (‘‘fining-upward’’) can be seen. Conversely, other cores from Turneffe Islands (T7, T8, T9, T13) show upward transitions from
wackestone to packstone (‘‘coarsening-upward’’). Three facies can be distinguished: shell beds (mollusk rudstones), Halimeda packstones and wackestones, and mollusk packstones and wackestones (Tables 2 and 3). 4.4.1. Shell beds (mollusk rudstones) In all platforms, shell beds consisting of disarticulated infaunal bivalves are a common feature at the base and in the lower sections of the carbonate sediment successions (cores L1, L4, G5, G6, T4, T6, T13). Contacts with the underlying peat facies are sharp (cores G5, T4). Most common mollusks include Laevicardium laevigatum and Chione sp. in Glovers and Lighthouse Reefs, and Codakia orbicularis in Turneffe Islands. Whereas Laevicardium and Chione prefer open and also sandy substrates, C. orbicularis is indicative of sea grass beds (Abbott, 1974). As a consequence, the shell bed concentrations likely represent the reworking of shallow sandy and grasscovered lagoonal substrates by storms and/or bioturbation (e.g., Suchanek, 1983; Wanless et al., 1988).
Table 4 Standard radiocarbon ages (14) and accelerator mass spectrometry ages (3) from vibracores Sample
Depth below Material SL (m)
Measured age (year BP)
13
C/12C Conventional age Corrected age Calibrated calendar age, BETA, (year BP) (year BP) 2-sigma range sample no.
L4-base L5-base L6-base T4-base T5-basey T6-base
11.9 7.4 8.4 5.3 5.7 5.7
shell peat peat peat soil peat
4120 F 60 6950 F 70 6660 F 90 5610 F 60 7310 F 40 4620 F 70
0.0 25.0 25.0 25.0 26.2 25.0
4530 F 60 6950 F 70 6660 F 90 5610 F 60 7290 F 40 4620 F 70
4130 F 60 6550 F 70 6260 F 90 5210 F 60 7290 F 90 4220 F 70
T7-base T12-base T9-basey G4-basey G5-base
9.3 3.3 5.1 15.3 19.2
peat peat soil sediment soil
6010 F 80 2690 F 80 4900 F 40 7570 F 40 9950 F 220
25.0 25.0 23.9 23.6 25.0
6010 F 80 2690 F 80 4920 F 40 7590 F 40 9950 F 220
5610 F 80 2290 F 80 4920 F 40 7190 F 40 9950 F 220
G5-460/465* G5-380/385* G5-240/245* G5-120/125* G7-base G9-base
18.1 17.3 15.9 14.7 15.6 17.3
peat sediment sediment sediment peat peat
7890 F 70 4540 F 70 2480 F 70 1670 F 60 7660 F 90 8450 F 130
25.0 0.0 0.0 0.0 25.0 25.0
7890 F 70 4960 F 80 2890 F 70 2080 F 70 7660 F 90 8450 F 130
7490 F 70 4560 F 80 2490 F 70 1680 F 70 7260 F 90 8050 F 130
BC 2900 – 2590 BC 5990 – 5710 BC 5720 – 5470 BC 4550 – 4340 BC 6230 – 6050 BC 3620 – 3580, BC 3530 – 3270, BC 3240 – 3110 BC 5070 – 4710 BC 1000 – 780 BC 3780 – 3540 BC 6470 – 6400 BC 10640 – 10550, BC 10420 – 8780 BC 7050 – 6580 BC 3520 – 3090 BC 820 – 500 AD 130 – 450 BC 6660 – 6380 BC 7730 – 7170
148944 148945 148946 148947 148948 149148
148950 148953 148951 148940 148941 153962 153965 153964 153963 148942 148943
Dating by Beta Analytic, Miami, USA. Conventional ages are corrected for d13C/12C. Corrected ages: ocean reservoir correction of 400 years applied for marine samples. Calibrations based on INTCAL 98 database; reservoir correction applied; 2-sigma range has 95% probability. * Depth of 5-cm sample interval in compacted core. y AMS dating.
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4.4.2. Halimeda packstones and wackestones This facies predominates in the Turneffe Islands Holocene successions. It also occurs at the bases of several cores from Glovers and Lighthouse Reefs (G1, G4, L5). The abundance of Halimeda segments ranges
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from 30% to more than 80% with an average value of 48%. Mollusks and foraminifera are common; however, their abundances are mostly below 10%. The most common mollusk is the lucinid bivalve C. orbicularis, which is known to dwell in sea grass beds
Fig. 6. Belize atoll sea level data from rotary cores and vibracores plotted on Holocene sea level curve of western Atlantic (Lighty et al., 1982). Data points not indexed by P ( = peat) or S ( = soil) are from corals or carbonate sediment.
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(Abbott, 1974). The most common benthic foraminifera include Archaias angulatus, Quinqueloculina sp., Triloculina sp., and Cribroelphidium sp. In the Turneffe cores, small corals such as branched Porites sp. and M. areolata are found occasionally. The Halimedarich facies is interpreted to represent a restricted lagoon environment with abundant sea grass beds as seen within the Turneffe Islands platform today. 4.4.3. Mollusk packstones and wackestones In Glovers and Lighthouse Reefs, the Holocene successions are dominated by mollusk packstones and wackestones. The abundance of mollusk shell fragments ranges from 6% to 46% with an average value of 25%. Typical mollusks in Glovers and Lighthouse Reefs cores include the bivalves L. laevigatum, Glycimeris sp., Arca sp., Anadara sp., Tellina sp., Chione sp., and Gouldia sp., and the gastropods Strombus sp., Astraea phoebia, and Cerithium sp. While Laevicardium, Glycimeris, and Chione prefer rather open, sandy, or shallow bottoms, A. phoebia is indicative of sea grass beds (Abbott, 1974), and Gouldia is commonly found in rather protected lagoon areas (Kuhn, 1984). Fragments of Halimeda are common and abundances range from 0% to 38% (average of 14%). The abundance of Halimeda fragments usually decreases upcore. A. angulatus, Quinqueloculina sp., Triloculina sp., and Cribroelphidium sp. are the most common benthic foraminifera. Small branched Porites corals and peloids (largely nonskeletal indurated fecal pellets) are found occasionally. Rare indurated burrow linings are indicative of Callianassa (cores G5, G6). The mollusk facies is interpreted as being deposited in lagoons with open circulation as seen in Glovers and Lighthouse Reefs today. Deposits of rather open, sandy-silty bottoms as well as sea grass beds are included. 4.5. Ages, sedimentation rates The oldest ages in individual cores were obtained from soils (Table 4, Figs. 6 and 7). Among these, the oldest soil date extends the base of core G5 back to
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the beginning of the Holocene. Basal peat ages in the Glovers Reef lagoon range from 7500 to 8500 years BP, in Lighthouse Reef from 7000 to 6500 years BP, and in the Turneffe Islands lagoon area from 7000 to 2700 years BP (Table 3). In Turneffe Islands, the Holocene sediment accumulation rate ranges from 0.22 to 1.23 m/ky with an average of 0.82 m/ky. In Lighthouse Reef, accumulation rates are between 0.41 and 0.63 m/ky with an average value of 0.53 m/ky. In Glovers Reef, the accumulation rates range from 0.23 to 0.67 m/ky with an average of 0.46 m/ky. The average rate for all three platforms is 0.6 m/ky.
5. Discussion 5.1. Holocene flooding scenario During the late Pleistocene, Glovers Reef, Lighthouse Reef, and Turneffe Islands were dish-shaped limestone islands, that were probably fringed by coral reefs (Fig. 8). As the rising Holocene sea level approached the level of the bedrock surface, rising groundwater enhanced soil development. Subsequently, marine waters breached the peripheral bedrock rim (perhaps through relic tidal channels in the elevated Pleistocene reef margin) and entered the central depressions, allowing mangroves to colonize the inner parts of the islands. In Glovers Reef, this happened between 8500 and 7500 years BP, in Lighthouse Reef between 7000 and 6500 years BP, and in Turneffe Islands between 7000 and 2700 years BP. As sea level continued to rise, mangrove-rimmed lagoons with extensive Halimeda growth formed, comparable to the present situation in Turneffe Islands or, for example, in the Marquesas Keys of south Florida (Hudson, 1985). At even higher sea levels, mangrove areas diminished in size and reefs colonized or retrograded onto the peripheral rim of the Pleistocene islands. In Glovers, this stage of peripheral reef development started around 7500 years BP, in Lighthouse at about 6500 years BP, and in Turneffe Islands at around 4800 years BP
Fig. 7. Cross sections through the Belize atolls based on both vibracore data from lagoons and rotary core data from reefs (Gischler and Hudson, 1998; Gischler and Lomando, 2000). Pleistocene faulting is used to explain significant differences in bedrock elevation in Lighthouse Reef and Turneffe Islands. Faults are interpreted to underlie the NNE-striking Middle Reef patch reef trend in Lighthouse Reef and the NNW-striking island trend in the main lagoon of Turneffe Islands.
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reflecting the different bedrock elevation (Gischler and Hudson, 1998; Gischler and Lomando, 2000). Open marine conditions eventually developed in the lagoons of Glovers Reef, Lighthouse Reef, and the northernmost part of Turneffe Islands. In the main lagoonal part of Turneffe Islands, open marine conditions did not develop due to the high bedrock elevation and the protected position of Turneffe behind another platform. Flooding of the Turneffe platform at relatively low rates of sea level rise as well as protection and lack of open marine conditions has led to extensive mangrove development. Late Quaternary bedrock – soil – peat – carbonate facies successions found in the platform lagoon cores are quite similar to those found in other Belize areas such as Ambergris Cay (Ebanks, 1975), and the coastal plain (High, 1975), or further afield, in the North Lagoon of Bermuda (Kuhn, 1984). However, several studies have found differences in this general succession, particularly the development of transitional phases between terrestrial and marine. In the Bahamian Bight of Abaco for example, Rasmussen (1989) identified terrestrial, limnic, brackish, hypersaline, and marine phases. Marine conditions were established only after 5500 years BP when a shallow sill (3.7 m below present sea level) was eventually breached by the rising Holocene sea. Similarly, in the North Sound of Grand Cayman, MacKinnon and Jones (2001) found freshwater to brackish deposits below marine mangrove peats and marine carbonate sediments. Marine circulation did not develop until quite late in the Holocene due to a bedrock sill (4 m below present sea level) that prevented exchange with fully marine waters before 3800 –2500 years BP.
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No brackish or hypersaline carbonates, transitional between terrestrial and marine phases, have been found in this study. A possible explanation for this absence may be that the Belize platforms were first flooded earlier in the Holocene (8500 – 6000 years BP) as compared to the Bight of Abaco (Bahamas) or North Sound (Grand Cayman). Consequently, the Belize platforms were flooded during the more rapid rise of Holocene sea level, which prevented the establishment of long-lasting freshwater lakes or closed lagoon environments. Rates of sea level rise during initial flooding were around 7.0 m/ky for Glovers Reef, 4.1 m/ky for Lighthouse Reef, and 2.8 m/ky for Turneffe Islands (see Fig. 6). In addition, the absence of the shallow sills in the Belize platforms lessened the chance for restricted circulation. The depth of Pleistocene bedrock beneath marginal reefs is up to 12 m in Glovers Reef, up to 8 m in Lighthouse Reef and up to 6 m below present sea level in Turneffe Islands (Gischler and Hudson, 1998; Gischler and Lomando, 2000). These depths are probably exceeded beneath the modern 5 –8 m deep tidal channels in the Belize platform margins as they are likely to be located over even deeper Pleistocene tidal channels. 5.2. Comparison with other data and significance A comparison of the limited data on Holocene atoll and isolated platform lagoon development (Table 5) reveals no statistically significant trends, or linkages between parameters such as lagoon size and Holocene sediment thickness or sedimentation rates. There are poor correlations between accommodation space (here estimated from simple multiplication of lagoon area with maximum depth to Pleistocene bedrock) and
Fig. 8. Schematic sketches showing Holocene development of Belize atolls. Data on peripheral reef development from Gischler and Hudson (1998) and Gischler and Lomando (2000); (a) 8000 years BP, sea level ca. 15 m below present level. Turneffe Islands and Lighthouse Reef were emergent limestone islands; interior part of Glovers Reef characterized by mangrove environment; transgression of marine water, probably through the precursors of modern tidal channels; Pleistocene patch reefs probably still emergent as small cays in lagoon (not shown); (b) 7000 years BP, sea level about 10 m below present level. Parts of Turneffe Islands and Lighthouse Reef flooded and mangrove-dominated; Glovers Reef with mangrove rims; Halimeda-dominated early carbonate sedimentation; small Pleistocene areas probably still emergent at margin and in lagoon (not shown); (c) 6000 years BP, sea level about 7 m below present level. Mangrove-dominated parts of Turneffe Islands increase in area; western parts of Lighthouse Reef flooded and mangrove-covered; eastern lagoon mantled by Halimeda-dominated carbonates; Glovers Reef carbonate sedimentation no longer Halimeda-dominated; maybe some mangrove areas left; Pleistocene patch reefs in lagoon submerged; (d) 5000 years BP, sea level about 5 m below present level. In Turneffe Islands, large areas in the east are still emergent, but a mangrove-encircled lagoon develops with Halimeda-dominated sediment; Glovers and Lighthouse Reefs are characterized by carbonate sedimentation; western parts of Lighthouse possibly with larger mangrove areas and Halimeda-rich sediment; (e) 4000 years BP, sea level about 3.5 m below present level. Glovers and Lighthouse Reefs almost as at present time; Turneffe Islands’ main lagoon largely developed; seaward tidal channels open; parts of the north still emergent; carbonate sedimentation initiated on northernmost part, probably still surrounded by mangroves. (f) Present situation.
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Table 5 Areas, depths, Holocene sediment thicknesses and sedimentation rates of some isolated platform and atoll lagoonal settings Location
Lagoon Maximum Maximum Average area depth (m) Holocene sediment (km2) sediment rate (m/ky) (m)
(1) Glovers Reef 194 (2) Lighthouse Reef 112 (3) Turneffe Islands 225 (4) One Tree Reef 10 (5) Bermuda 290 (6) Hogsty 23 (7) Enewetak 980 (8) Mururoa 130 (9) Mayotte 1500 (10) North Sound 90 (11) Abaco Bight 2750
18 8 8 10 20 8 62 55 80 6 7
6–7 6–7 6–7 9 – 10 2–3 3 10 20 6 3 2
0.46* 0.53* 0.82* 1.25* 0.32 unknown 1.25* unknown 0.35* 0.85 0.21 – 0.36 (average 0.24)
Data sources: (1 – 3) this study; (4) Marshall and Davies (1982); (5) Kuhn (1984, p. 100, Fig. 14); (6) Pierson and Shinn (1985); (7) Buddemeier and Oberdorfer (1997); (8) Buigues (1997); (9) Zinke (2000, Fig. 3.4); (10) MacKinnon and Jones (2001); (11) Rasmussen (1989, Fig. 14). * Calculated from sediment thickness and time of Holocene flooding.
maximum Holocene sediment thickness (r = 0.04), and between accommodation space and average sedimentation rate (r = 0.11). Apparently, the rate of filling of lagoons in modern isolated platforms and atolls is not simply a function of the size of accommodation space but controlled by a number of factors that may vary among individual localities. These factors include carbonate production within the lagoon, size and carbonate production rates of the windward margin that includes lagoonward sediment transport, mud export over leeward margins, as well as individual shape and elevation of antecedent topography, with the latter being responsible for the timing of Holocene inundation. Differences in depth and relief of the underlying Pleistocene limestone among the Belize platforms are interpreted to reflect differential subsidence along the underlying fault blocks that has been accentuated by north to south differences in meteoric dissolution (Gischler et al., 2000). The data in Table 5 also reveal that for many Holocene lagoons it will take tens to hundreds of thousands of years to be completely filled (e.g., Turneffe 10 ky, Lighthouse 15 ky, Glovers 40 ky,
Enewetak 50 ky, Bermuda 60 ky, Mayotte 230 ky; note that these estimates are based on average Holocene sedimentation rates and maximum lagoon depths, ignoring sediment export). In the light of the Quaternary record of eustatic sea level change and the lengths of interglacial high sea level cycles, it is unlikely that the platform and atoll lagoons will be completely filled during the Holocene. Similar conclusions were drawn by Boss and Rasmussen (1995) for the northern part of Great Bahama Bank. They showed that there is no significant correlation between sediment (cycle) thickness and depth to the Pleistocene – Holocene boundary (accommodation space), and that lagoonal sedimentation rates are not sufficient to completely fill available accommodation space during the Holocene. Cycle thickness can therefore not be used indiscriminately as a record of eustatic sea level change. The findings of the present study support this conclusion and lend further evidence to the observation that ‘‘empty buckets’’ (sensu Schlager, 1993) are a common feature among Holocene isolated carbonate platforms and atolls. Understanding why these Holocene buckets fail to fill, however, is still unclear. Apart from stressing apparent differences in amplitudes of eustatic sea level fluctuations, Read (1985) and Koerschner and Read (1989) made the point that fossil cyclic platforms often lack rims whereas Quaternary platforms usually have raised rims. Based on this observation it can be speculated that the significantly higher growth potential of the reefal rims around modern platforms (Schlager, 1981) acts in concert with high-amplitude sea level changes to produce unfilled accommodation space in Quaternary platforms.
6. Summary and conclusions Holocene successions in three isolated carbonate platforms of Belize include basal soils above Pleistocene limestone bedrock, followed by mangrove peats, and marine lagoonal carbonate sediments. Early restricted carbonate sedimentation within the platforms is characterized by abundant Halimeda. Due to differences in depth of Pleistocene bedrock, Glovers, Lighthouse, and Turneffe atolls were flooded at successively later stages during the Hol-
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ocene transgression between 8500 and 6000 years BP. In contrast to Glovers and Lighthouse Reefs, which have developed open lagoon circulation, Turneffe Islands is characterized by restricted circulation with abundant mangroves and Halimeda-dominated lagoon sediments. Contributory factors to this restricted circulation are the fact that this platform was flooded relatively late during the Holocene as compared to the other two platforms, and that Turneffe Islands is located in a protected, low-energy position in the lee of Lighthouse Reef. After flooding, lagoon sediment accumulation rates averaged 0.6 m/ky, but the rate of sedimentation lagged behind sea level rise, increasing accommodation space to the present maximum lagoon depths. Other data available on Holocene lagoonal development supports these findings and indicate that, regardless of differences in facies successions and flooding scenarios, unfilled accommodation space is a common feature in carbonate platforms and atolls that develop during periods of glacio-eustacy. This implies that cycle thickness—as a record of eustasy—does not necessarily equal accommodation space in fossil carbonate platforms.
Acknowledgements This research was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, Project Gi 222/4). I am very grateful to Harold Hudson (N.O.A.A., Key Largo), Malo Jackson, Patrick Silverback (Dangriga), Wolfgang Oschmann, Kai Spresny, and Gabi Meyer (J.W. Goethe Universita¨t, Frankfurt/Main) for their assistance during drilling operations and transport of equipment and core samples. Stefanie Scheidt (Frankfurt/Main) helped with textural and compositional analyses; Rainer Petschick (Frankfurt/Main) performed XRD analyses. The support of the Fisheries Department in Belize City, especially by James Azueta, is gratefully acknowledged. I thank Edward Purdy (Weybridge, UK) for his thoughtful comments on an earlier version of the manuscript. The constructive reviews by Paul Blanchon (National University of Mexico) and Christopher Perry (Manchester Metropolitan University) improved this paper and are gratefully acknowledged.
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Purdy, E.G., Pusey III, W.C., Wantland, K.F., 1975. Continental shelf of Belize—regional shelf attributes. In: Wantland, K.F., Pusey III, W.C. (Eds.), Belize Shelf—Carbonate Sediments, Clastic Sediments, and Ecology. Am. Assoc. Petrol. Geol. Studies in Geology, vol. 2. American Association of Petroleum Geologists, Tulsa, pp. 1 – 52. Rasmussen, K.A., 1989. Unconformities and transgressive carbonate sequences within a karstic platform interior; Bight of Abaco, Bahamas. Unpublished PhD thesis, University of North Carolina at Chapel Hill. 142 pp. Read, J.F., 1985. Carbonate platform models. Am. Assoc. Pet. Geol. Bull. 69, 1 – 21. Read, J.F., Goldhammer, R.K., 1988. Use of Fischer plots to define third-order sea-level curves in Ordovician peritidal cyclic carbonates, Appalachians. Geology 16, 895 – 899. Schlager, W., 1981. The paradox of drowned reefs and carbonate platforms. Geol. Soc. Amer. Bull. 92, 197 – 211. Schlager, W., 1993. Accommodation and supply—a dual control on stratigraphic sequences. Sediment. Geol. 86, 111 – 136. Smith, F.G.W., 1941. Sponge disease in British Honduras and its transmission by water currents. Ecology 22, 415 – 421. Stoddart, D.R., 1962. Three Caribbean atolls: Turneffe Islands, Lighthouse Reef, and Glovers Reef, British Honduras. Atoll Res. Bull. 87, 1 – 151. Suchanek, T.H., 1983. Control of seagrass communities and sediment distribution by Callianassa (Crustacea, Thalassinidea) bioturbation. J. Mar. Res. 41, 281 – 298. Wallace, R.J., Schafersman, S.D., 1977. Patch – reef ecology and sedimentology of Glovers Reef Atoll, Belize. In: Frost, S.H., Weiss, M.P., Sanders, J.B. (Eds.), Reefs and Related Carbonates—Ecology and Sedimentology. Am. Assoc. Petrol. Geol. Studies in Geology, vol. 4. American Association of Petroleum Geologists, Tulsa, pp. 37 – 52. Wanless, H.R., Tedesco, L.P., Tyrrell, K.M., 1988. Production of subtidal tubular and surficial tempestites by Hurricane Kate, Caicos Platform, British West Indies. J. Sediment. Petrol. 58, 739 – 750. Zinke, J., 2000. Development of the lagoon of Mayotte during the Holocene (Comoro Archipelago, SW Indian Ocean). Unpublished PhD thesis, Universita¨t Kiel. 204 pp.
Holocene lagoonal development in the isolated carbonate platforms off Belize Eberhard Gischler * Geologisch-Pala¨ontologisches Institut, J.W. Goethe-Universita¨t, Senckenberganlage 32-34, 60054 Frankfurt am Main, Germany
Abstract Thirty-one vibracores were taken in interior lagoons of Glovers Reef, Lighthouse Reef, and Turneffe Islands—three isolated carbonate platforms offshore Belize, Central America. Holocene facies successions overlying the Pleistocene limestone bedrock begin with soils, followed by mangrove peats, and marine carbonate sediments of lagoonal origin. The soils formed on top of subaerially exposed Pleistocene limestone before the Holocene transgression. Mangrove peats developed during initial flooding of the platforms (Glovers ca. 8.5 ky, Lighthouse ca. 7 ky, Turneffe ca. 6 ky BP). As water depths increased, reefs colonized platform margins, lagoonal circulation improved thereby promoting carbonate production. The basal lagoonal carbonate sediments are characterized by shell beds and/or Halimeda packstones – grainstones. Mollusk-dominated wackestones and packstones follow upsection in Glovers and Lighthouse Reefs. At present, open circulation prevails in Glovers and Lighthouse lagoons. In contrast, organic-rich Halimeda wackestones and packstones dominate the Turneffe Islands Holocene succession. The main lagoon area of Turneffe is enclosed by mangroves, and restricted circulation prevails. Factors that explain the differences in geomorphology, circulation, and facies are variations in depth of antecedent topography and degree of exposure to waves and currents. The thickness of Holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls. Holocene sedimentation rates average 0.6 m/ky, with highest rates in Turneffe (0.82 m/ky), followed by Lighthouse (0.53 m/ ky), and Glovers (0.46 m/ky). Like in many other isolated carbonate platforms and atolls, lagoon floor sedimentation did not keep pace with rising sea level, leading to unfilled accommodation space. At present, Glovers has an 18 m deep lagoon, while Lighthouse and the main Turneffe lagoon are 8 m deep. It is unlikely that the lagoons will be completely filled during the Holocene sea level highstand cycle. This observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Carbonate platform; Atoll lagoon; Coral reef; Holocene; Belize; Carbonate
1. Introduction Meter-scale shallowing-upward cycles of lagoonal and peritidal sequences in the interior of Paleozoic and Mesozoic carbonate platforms have been used as
* Tel.: +49-69-798-25136; fax: +49-69-798-22958. E-mail address: [email protected] (E. Gischler).
records of high-frequency (104 – 105 year) eustatic sea level fluctuations (e.g., Fischer, 1964; Goldhammer et al., 1987; Read and Goldhammer, 1988). In these studies, cycle thickness is assumed to roughly represent accommodation space and, after correcting for tectonic subsidence, cycle thickness is used as a proxy for sea level change. Goldhammer et al. (1990) acknowledged, however, that high-frequency, meterscale shallowing-upward cycles may well be missing
0037-0738/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0037-0738(03)00098-8
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in settings of high-frequency and high-amplitude (>120 m) glacio-eustacy such as the Pleistocene where large differences in relative amplitudes of different orders of sea level fluctuations (eccentricity: 100 ky, precession: 20 ky) exist. Whereas the 100-ky cycle highstands submerged Pleistocene platforms and also recorded some of the 20-ky cycles, the 100-ky cycle lowstands kept the platforms exposed so that two or more superimposed 20-ky cycles were missed on the platform, according to these authors. Similarly, Read (1985) and Koerschner and Read (1989) also noted the differences between the abundance of Paleozoic high frequency, peritidal platform cycles and their scarcity in Quaternary carbonate platforms. They concluded that modern platforms are poor analogs for most ancient cyclic platform sequences because the large amplitude of eustatic sea level fluctuations produced accommodation space, which could not be subsequently filled (Koerschner and Read, 1989, p. 685). Indeed, several studies on the Holocene development of modern isolated carbonate platform and atoll lagoons have shown that unfilled accommodation space is a common feature (e.g., Purdy and Winterer, 2001). In the north lagoon of the Bermuda platform, for example, Kuhn (1984) discovered Holocene successions of soils, peats, restricted marine and eventually open marine sediments. He found that sedimentation rates generally lagged rates of Holocene sea level rise. Rasmussen (1989) reported a transition form terrestrial, brackish, hypersaline, and marine phases in the Holocene successions of the Bight of Abaco, Bahamas. Based on average sedimentation rates, it would take another 30,000 years to completely fill this lagoon. Boss and Rasmussen (1995) showed that Holocene cycle thickness and accommodation space across the Great Bahama Bank are uncorrelated. They concluded that, due to low sedimentation rates and the limited duration of the interglacial sea level highstand, unfilled Holocene accommodation space will probably result. MacKinnon and Jones (2001) found a transition from fresh to brackish to marine sediments in the North Sound of Grand Cayman and noted an upward increase in accommodation space through time. In the Indian Ocean, an investigation of a mixed siliciclastic-carbonate lagoon around the island of Mayotte by Zinke (2000), found strong lateral variability of facies and
accumulation rates depending on terrigenous or carbonate predominance. In all locations studied, however, lagoonal sedimentation rates lagged behind sea level rise, creating unfilled accommodation space. These detailed studies of Holocene lagoon development support the contention that unfilled accommodation space is common to modern platforms and atolls. Even so, there are considerable differences with respect to Holocene facies successions and interpreted flooding scenarios. Controlling factors include topography of the Pleistocene bedrock surface, rates of Holocene sea level rise, exposure to waves and currents, size of platform interior lagoons, carbonate productivity in lagoons and surrounding marginal reefs, and sediment export. This study further examines the processes which control the development of modern platform lagoons by investigating Holocene facies sequences in three interior lagoons of the carbonate platforms off Belize. These three localities are well suited to study lagoon development as (1) there is significant morphological and environmental variation among the three platform interior lagoons, and (2) a framework of reference for this study had been provided previously by rotary core drilling on marginal areas and reefs of the platforms (Gischler and Hudson, 1998; Gischler and Lomando, 2000).
2. Setting The isolated carbonate platforms of Glovers Reef, Lighthouse Reef, and Turneffe Islands are located east of the Belize Barrier Reef (Fig. 1). The structural basement of the platforms and the barrier reef itself are situated on NNE-trending ridges interpreted as tilted normal fault blocks along the passive margin (Dillon and Vedder, 1973). Exploration wells on Turneffe Islands and Glovers Reef have recovered 1030 m of largely Tertiary reef and shallow water limestone, underlain by late Cretaceous to early Tertiary clastics and metavolcanics (Dillon and Vedder, 1973; Gischler and Lomando, 2000). Belize lies in the trade wind belt, and consequently winds from the E and NE predominate for most of the year. Accordingly, the mean wave approach is from ENE (75j). In the winter, winds from the N to NW are common. Average air temperatures range from 24 jC
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in the winter to 27 jC in the summer. Precipitation rates on the mainland amount to 150 –200 cm/year in the low north and may exceed 400 cm/year in the mountainous area of the Maya Mountains (Purdy, 1974; Purdy et al., 1975). Water temperatures range from 27 (winter) to 29 jC (summer). The tide range is 30 cm. The passing Caribbean currents produces N – S flowing countercurrents both on the Belize shelf
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(Purdy, 1974; Purdy et al., 1975), and within the open lagoon of Glovers Reef (Wallace and Schafersman, 1977). All three platforms are surrounded by deep water and have breakwater reefs encircling lagoons (Stoddart, 1962). The interior lagoons of the three platforms are significantly different with regard to geomorphology, water circulation, and sediment distribution
Fig. 1. (a) Map of Belize with locations of the investigated atolls Glovers Reef, Lighthouse Reef, and Turneffe Islands. Fault blocks after Dillon and Vedder (1973). (b) Map of the Belize atolls with locations of 31 vibracores. Fourteen rotary core hole locations from Gischler and Hudson (1998) and Gischler and Lomando (2000).
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Fig. 1 (continued).
(Gischler, 1994; Gischler and Lomando, 1999). Glovers Reef covers 260 km2 and has a lagoon, which is up to 18 m deep and contains f 860 patch reefs that
are, for the most part, randomly distributed. There are five small marginal sand – shingle cays. Lighthouse Reef covers 200 km2 with a lagoon that has an 8 m
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deep eastern part and a shallow 3 m deep western part, divided by a linear trend of patch reefs (‘‘Middle Reef’’). Two large mangrove – sand cays are developed. Lagoon floors in Glovers and Lighthouse Reefs are covered by patches of seagrass (Thalassia testudinum, Syringodium filiforme) and calcareous algae (Penicillus sp., Halimeda sp., Udotea sp., Caulerpa sp.). Sediment facies include mollusk –foram – Halimeda facies in the deep lagoon and mixed peloidalskeletal facies in the shallow parts (Gischler, 1994; Gischler and Lomando, 1999). The existence of abundant coral patch reefs and clear waters in the lagoons of these atolls supports the contention that open circulation prevails. Turneffe Islands has an area of 525 km2 and 25% of the platform is covered by mangroves. These mangroves encircle a lagoon, which is up to 8 m deep. The lagoon floor is densely covered by Thalassia and Halimeda. Sponges (Spheeciospongia sp.) are also common; corals (branched Porites sp.) and Manicina areolata occur occasionally. Coral patch reefs are rare and the Halimedadominated sediments have a dark brown stain due to elevated amounts of humic acids from decaying organic matter (Gischler and Lomando, 1999). In the summer of 1939, Smith (1941) recorded extreme temperature and salinity values within the Turneffe main lagoon with water temperatures exceeding 31 jC and salinities reaching 70x . Collectively, these observations support the inference of restricted Turneffe lagoon circulation. Systematic data on temperature and conductivity were recently obtained from two data loggers (R. Brancker, model XL210) which were deployed on each of the three platform lagoon floors from December 2000 – December 2001. According to these data, water temperature ranges from 23 –31 jC in Glovers and Lighthouse Reefs and from 22 –32 jC in Turneffe Islands. Salinity fluctuates from 38.6 – 42.0x in Glovers Reef, from 37.7– 41.6xin Lighthouse Reef, and from 34.2– 42.5xin Turneffe Islands lagoons.
3. Methods A total of 31 cores were taken from the Belize platform lagoonal areas in March and April 2000 with a Lanesky et al. (1979)-type vibracorer using 6 m long aluminum pipe (diameter 7.5 cm) with core
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catchers (Fig. 1, Table 1). Cores were taken at sea level and in shallow water depths using snorkel and dive mask, and in greater water depths of up to 15 m using scuba. Lengths of cores ranged from 0.4 to 5.63 m and penetration usually stopped when Pleistocene limestone bedrock was reached. At a few locations small fragments of Pleistocene coral or limestone were recovered, but at others the bedrock was not reached. After drilling, the core tubes were marked at the sediment surface to record compaction effects, and the tops sealed with plastic caps. In shallow depths, cores were retrieved using aluminum clamps with handles and a car jack. Where scuba diving was necessary the clamps were used together Table 1 Water depth, core recovery and compaction of vibracores Core
Water depth (m)
Recovery (m)
True thickness (m)
Compaction (%)
L1 L2 L3 L4 L5 L6 G1 G2 G3 G4 G5 G6 G7 G8 G9 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 T15 T16 T17
6.6 8.1 3.0 9.0 3.6 5.7 9.0 9.6 12.0 12.0 13.5 13.8 11.7 14.7 15.3 0 0 0.9 0.5 3.6 0 3.6 2.1 3.3 3.3 4.5 2.4 0.9 0.3 4.8 1.8 0
2.40 1.94 1.70 2.50 3.75 2.46 5.63 3.36 1.40 3.14 4.95 2.48 3.47 3.08 1.80 1.86 1.84 – 2.24 1.88 3.30 5.35 0.70 1.47 1.87 2.25 0.40 0.50 2.32 1.08 0.35 1.92
2.75 2.25 1.95 2.85 3.80 2.70 5.70 4.20 1.50 3.30 5.70 3.30 3.90 3.75 1.95 4.50 4.50 – 4.80 2.10 5.70 5.70 0.90 1.80 2.10 2.40 0.60 1.20 3.75 1.80 2.40 2.40
12.7 13.8 12.8 12.3 1.3 8.9 1.2 20.0 6.7 4.8 13.2 24.8 11.0 17.9 7.7 58.7 59.1 – 53.3 10.5 42.1 6.1 22.2 18.3 11.0 6.3 33.3 58.3 38.1 40.0 85.4 20.0
Core T3 was lost. Core G4 was drilled at same spot as G3 because G3 hit rock early.
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with lift bags that were filled with air from the scuba tanks. On the boat, the upper, water-filled part of the aluminum pipe was cut off, and any compaction of the sediment was noted. Core pipes were marked with waterproof pens, sealed with plastic caps and tape, and later cut into 1– 1.5 m long sections for transport. In the laboratory, cores were cut open with a portable saw and the sediment split in half using metal wire or knifes. Cores were logged, photographed and, from one core half, 5-cm long samples were taken for sedimentologic (texture, composition), mineralogic (XRD), and chronologic (C14-dating) analyses. The other half of the core was sealed and kept for reference. Samples were washed in a dilute solution of hydrogen peroxide and later dried. Dry samples were split with a sample splitter. One part
was used for XRD analysis, and relative abundances of carbonate minerals were determined using the method of Milliman (1974, pp. 21 –29). The other part of the sample was sieved into the common grades >2, 2 –1, 1 – 0.5, 0.5– 0.25, 0.25 –0.125, 0.125– 0.063, and < 0.063 mm. Relative amounts of the mud ( < 0.063 mm) and coarse fractions (>2 mm) were used to assign depositional texture, in addition to the visual description of texture during core logging (following Dunham, 1962). Fractions >0.125 mm were impregnated with resin and thin sections were made. Constituent grain composition of the samples was determined by counting 200 grains per thin section. The carbonate portion of soil samples from the bases of five cores was dissolved using hydrochloric acid and the insoluble residue analyzed by XRD. Dried peat and soil samples from the bases of
Fig. 2. Vibracore logs from Lighthouse Reef (corrected for compaction).
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five cores were combusted for 3 h at 600 jC. By subtracting ash weight from dry weight, the relative abundance of organic matter was determined. Throughout the text conventional ages (uncorrected for ocean reservoir effects) are used when referring to C14 dates. Dating was performed by Beta Analytic, Miami, FL, USA.
tremely high compaction in the cores from the leeward mangrove rim (cores T1 –T4, T6, T16) and from the northern lagoon (T12 –T14), where sediment is rich in pore water and therefore has a ‘‘soupy’’ texture. The general facies succession begins with a basal soil overlying Pleistocene limestone, and is followed by mangrove peat, and carbonate sediments, as described below.
4. Results
4.1. Pleistocene limestone bedrock
A total of 71.5 m of core was recovered (Figs. 2– 5; Table 1) in which compaction averages 18% and is lowest in Glovers and Lighthouse cores (10 – 12%) and highest in Turneffe (average 33%). The relatively high value for Turneffe results from the ex-
Small pieces of buff-colored, well-cemented Pleistocene limestone were recovered at the base of a number of cores. Thin sections reveal indications of meteoric diagenesis such as abundant blocky calcite cement and coral, mollusk, and Halimeda fragments
Fig. 3. Vibracore logs from Glovers Reef (corrected for compaction). See Fig. 2 for legend. Note that Core G3 has same vertical succession as core G4.
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Fig. 4. Vibracore logs from Turneffe Islands (corrected for compaction). See Fig. 2 for legend. Note that cores T1 and T2 have same vertical succession. Core T3 was lost. Core T17 (not shown) is composed entirely of peat.
that are partly recrystallized to calcite. At the leeward margin of Turneffe Islands (cores T1 and T2) fragments of corals such as Diploria strigosa, M. areolata, and Porites astreoides could be identified. The Porites fragment still had 80% aragonite preserved. Unidentified mollusk shells were also found in fragments of bedrock (e.g., core T5).
ples have up to 30% of low magnesium calcite. Organic matter makes up 56 – 67% of three analyzed samples. No macrofauna was found in the soils. Firmground burrows filled with the overlying carbonate sediment are common. The thickness of soils was highest within Turneffe Islands where it may reach as much as 1 m (Fig. 4).
4.2. Soil
4.3. Peat
Overlying the bedrock, centimeter thick soils are developed which consist of dark brown to greenish mudstones with plant and root remains. Mineralogically, the mudstones consist of carbonate, quartz, clay minerals, and organic matter, with the quartz and clay minerals probably being of eolian origin. Although the carbonate is dominated by aragonite, some sam-
The soil-generated mudstones are overlain by reddish brown to brown peat which is composed mostly of long slender fibers and which releases clear to teacolored water upon compression. Usually the prop roots of red mangroves (Rhizophora) and the pneumatophores of black mangroves (Avicennia) are still identifiable in the core. Organic matter makes up
E. Gischler / Sedimentary Geology 159 (2003) 113–132 Fig. 5. Selected photographs from vibracores; scale in cm. (a) Base of core T9 showing weathered buff Pleistocene limestone, brown soil, black peat with firmground burrows and, brown Halimeda wackestone. (b) Base of core T4 peat firmground with open burrows infilled by overlying Halimeda packstone. (c) Core T4, about 1.5 m above base showing Halimeda packstone with four specimens of C. orbicularis. (d) Core T5, about 0.5 m below top showing Halimeda packstone with corals Porites porites and M. areolata. (e) Base of core L6 showing intensively burrowed peat with burrows infilled by overlying skeletal packstone. (f) Base of core L1 with basal shell bed (largely L. laevigatum) and overlying skeletal packstone. (g) Base of core G5 showing brown to greenish soil, burrowed mangrove peat, skeletal wackestone with shells of L. laevigatum and Anadara sp. (h) Core G4, about 0.6 m below top showing skeletal wackestone with shells of Ostrea sp., Anadara sp., and L. laevigatum. 121
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Table 2 Textural data of carbonate sediments from vibracores based on sieving Sample
>2
2–1
1 – 0.5
0.5 – 0.25
0.25 – 0.125
0.125 – 0.063
< 0.063
L1-100/105 L1-210/215 L2-90/95 L2-180/185 L3-85/90 L4-40/45 L4-125/130 L4-240/245 L5-55/60 L5-250/255 L5-340/345 L6-00/05 L6-120/125 L6-205/210 G1-75/80 G1-300/305 G1 525/530 G2-30/35 G2-165/170 G2-300/305 G4-70/75 G4-240/245 G5-70/75 G5-235/240 G5-420/425 G5-475/480 G6-120/125 G6-225/230 G7-120/125 G7-285/290 G8-255/260 G9-00/05 G9-100/105 T1-75/80 T1-165/170 T4-85/90 T4-165/170 T5-70/75 T6-165/170 T6-325/330 T7-60/65 T7-235/240 T7-505/510 T8-65/70 T9-00/05 T9-60/65 T9-120/125 T14-90/95 T15-100/105
6.8 13.0 13.9 11.5 16.5 9.2 25.6 34.0 9.6 6.6 11.0 0.2 11.0 14.3 6.2 6.6 12.3 1.0 2.9 6.2 17.5 29.6 0.2 2.5 2.8 1.0 4.8 12.1 3.1 8.7 1.2 24.3 18.0 4.5 38.1 2.9 30.4 7.5 0.2 22.7 37.8 18.6 8.5 27.7 16.5 2.7 4.3 4.1 16.7
7.0 4.4 5.5 6.2 12.5 4.8 9.2 10.1 7.5 5.3 10.7 0.7 9.3 8.1 4.8 3.7 5.5 1.2 5.2 6.8 12.8 6.4 0.8 6.4 1.4 2.4 5.5 6.2 6.2 7.3 1.5 12.5 9.9 7.7 16.7 0.9 7.8 5.3 1.7 7.8 19.9 11.2 2.7 3.4 5.9 1.8 6.4 4.7 4.2
9.4 6.6 6.1 8.0 12.6 5.9 6.5 7.4 9.4 5.7 10.2 4.8 8.2 11.6 13.7 4.6 3.9 8.0 10.7 12.2 10.6 5.3 3.5 5.4 5.2 3.9 8.5 9.9 8.3 13.9 8.2 14.0 19.4 6.7 13.5 1.2 5.8 4.9 3.4 7.5 14.3 8.2 1.6 2.7 4.7 1.3 18.1 8.5 18.1
14.9 10.2 7.9 8.0 15.5 8.0 5.4 6.4 14.1 13.2 13.2 9.0 9.2 12.5 15.9 18.9 15.7 11.7 8.7 14.2 10.4 13.3 5.1 4.2 8.5 18.6 10.4 11.6 9.5 12.2 7.2 13.7 14.7 17.9 12.8 10.8 20.8 28.0 14.0 15.5 10.7 19.1 31.3 16.2 20.5 2.7 20.2 43.1 13.9
30.9 22.4 12.9 12.6 24.7 17.6 12.7 9.0 26.1 22.0 16.8 27.3 21.3 19.6 18.3 23.7 18.2 17.2 14.8 17.4 31.8 20.1 9.8 10.7 13.2 24.9 13.2 15.7 17.8 17.5 10.6 12.3 13.9 19.8 10.9 26.3 13.9 28.8 25.7 14.9 7.4 20.5 32.8 16.2 28.4 41.6 20.2 24.7 25.0
20.7 24.6 32.3 23.7 11.5 33.4 19.4 17.8 18.3 30.8 14.4 39.9 27.5 22.1 27.7 28.0 36.5 29.9 32.8 31.5 10.3 14.0 48.3 17.4 45.8 19.6 40.8 23.4 35.4 25.3 26.9 13.5 13.7 21.4 3.9 33.0 14.4 15.2 39.1 17.7 4.8 9.3 16.7 16.2 15.0 37.6 12.8 8.8 11.1
10.3 18.8 21.4 30.0 6.8 21.1 21.1 15.4 15.0 16.4 23.7 18.1 13.5 11.9 13.3 14.4 7.9 31.0 24.9 11.7 6.5 11.3 32.2 53.4 23.1 29.5 16.8 21.1 19.7 15.0 44.4 9.8 10.4 21.8 4.1 24.8 6.8 10.3 15.7 14.0 5.1 13.3 6.4 17.6 9.1 12.4 18.1 6.0 11.1
Numbers after core name identify depth of 5-cm interval sampled in compacted core. Grain sizes are in mm; abundances are in %.
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Table 3 Compositional data from carbonate sediments based on point-counting of thin sections (fractions > 0.125 mm) Sample
HA
CO
RA
MO
FO
PE
MI
UK
FINES
L1-100/105 L1-210/215 L2-90/95 L2-180/185 L3-85/90 L4-40/45 L4-125/130 L4-240/245 L5-55/60 L5-250/255 L5-340/345 L6-00/05 L6-120/125 L6-205/210 G1-75/80 G1-300/305 G1 525/530 G2-30/35 G2-165/170 G2-300/305 G4-70/75 G4-240/245 G5-70/75 G5-235/240 G5-420/425 G6-120/125 G6-225/230 G7-120/125 G7-285/290 G8-255/260 G9-00/05 G9-100/105 T1-75/80 T1-165/170 T4-85/90 T4-165/170 T5-70/75 T6-165/170 T6-325/330 T7-60/65 T7-235/240 T7-505/510 T9-00/05 T9-60/65 T14-90/95 T15-100/105
20.0 18.7 2.5 7.9 27.8 2.5 0 5.7 18.7 21.1 37.4 13.9 5.5 14.2 13.6 6.9 44.8 9.8 6.3 19.9 23.6 32.0 3.8 0.3 8.9 6.4 13.9 11.0 20.1 7.2 28.0 38.0 31.5 37.7 32.5 72.9 62.2 35.3 41.3 86.0 63.9 64.6 67.2 42.1 42.6 56.9
0.7 0.8 0.7 0.7 0.4 0.2 0.6 0.3 0.7 0.5 0.3 1.1 0.7 1.3 0.3 0 0 0.2 0.2 0 1.2 0.4 0.6 0.9 0 2.1 1.4 0.2 0 0.3 0.8 0 0.9 0.5 0.2 0 0.4 0.5 0 0 1.6 0 0 0 0 0
1.0 0.6 0 0 2.0 0 0 0 0.3 0 0 1.1 1.0 0.3 0 0 0 0.2 0 0 0 0.4 0.3 0 0.2 0.2 0.3 0.4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
26.2 27.7 16.7 17.1 34.3 23.2 46.4 42.1 26.7 13.2 12.4 10.7 40.4 44.0 41.6 23.6 2.1 16.8 19.2 24.4 30.2 27.1 6.0 21.3 16.3 22.9 34.7 16.8 25.7 13.8 39.1 28.8 12.8 43.2 1.5 4.3 6.3 1.8 23.8 2.3 5.8 7.7 3.8 7.1 37.9 12.9
7.6 2.3 7.6 7.2 5.7 9.3 4.5 6.0 4.0 8.2 6.5 6.7 3.5 1.7 0.9 3.7 0.2 4.7 6.8 7.7 9.1 1.9 3.9 2.6 2.2 4.2 1.9 6.3 6.9 2.9 3.1 3.4 7.7 5.5 1.7 0 3.4 3.4 1.7 0.9 1.6 3.5 0.8 0 1.3 0.4
5.9 1.4 16.0 6.5 6.5 5.0 2.7 3.7 10.3 3.7 2.5 2.7 9.3 1.0 1.2 20.4 0 2.9 5.9 1.7 15.7 10.2 1.0 1.0 2.2 2.8 0.8 6.1 3.0 3.2 0.4 4.6 0.9 0.5 2.7 1.2 0 2.0 0 0 3.5 0 0 0 0 1.3
2.1 1.4 0.7 2.1 0.8 1.1 0 5.3 1.7 0.3 0 1.7 0.7 2.0 0.3 2.6 0 1.6 0.6 0 2.1 2.3 1.4 0.4 0.2 0.6 0.6 0 0.3 0 1.5 0 0.3 1.4 0.4 0 0 0.5 0 0.5 1.2 1.2 2.7 0.4 0 2.7
5.5 3.7 2.1 4.9 4.1 4.1 5.4 3.7 4.3 5.8 2.8 4.2 7.9 1.7 1.2 0.3 0.5 2.9 3.2 3.1 0.8 1.1 2.5 2.6 1.2 3.2 1.9 3.6 3.0 1.4 3.8 1.1 2.8 3.2 3.2 0.4 2.2 2.0 1.4 0.5 0 0 1.5 0.4 3.4 3.6
31.0 43.4 53.7 53.7 18.3 54.5 40.5 33.2 33.3 47.2 38.1 58.0 31.0 34.0 41.0 42.4 52.3 60.9 57.7 43.2 17.3 24.8 80.5 70.8 68.9 57.6 44.5 55.1 40.3 71.3 23.3 24.1 43.2 8.0 57.8 21.2 25.5 54.8 31.7 9.9 22.6 23.1 24.1 50.0 14.8 22.2
Numbers after core name identify depth of 5-cm interval sampled in compacted core. HA = Halimeda, CO = coral, RA = red algae, MO = mollusk, FO = foraminifera, PE = peloidal, MI = miscellaneous, UK = unknown. Miscellaneous includes echinoderm fragments, sponge and tunicate spicules, and ostracods. FINES = fraction < 0.125 mm, data obtained from sieving.
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between 57% and 72% of sample weight. No macrofauna was found in the peats, however, burrows filled in with overlying carbonate sediment are common. Peat thicknesses in lagoons range from 0.3 to 0.5 m but can be thicker within the mangrove rims of Turneffe Islands where peats as thick as 2 m were recovered (Figs. 2 – 4). 4.4. Carbonate sediment Mineralogically, the carbonate sediments consist largely of aragonite and high magnesium calcite. Average aragonite content is 85%, high magnesium calcite, 13.5%, and low magnesium calcite 1.5%. Texturally, packstones predominate in Lighthouse Reef samples, but both packstone and wackestones are found in Glovers Reef and Turneffe Islands cores (Figs. 2 –4, Table 2). In a number of cores (G1, G4, T1, T2, T6, L4), an upward transition from packstones to wackestones (‘‘fining-upward’’) can be seen. Conversely, other cores from Turneffe Islands (T7, T8, T9, T13) show upward transitions from
wackestone to packstone (‘‘coarsening-upward’’). Three facies can be distinguished: shell beds (mollusk rudstones), Halimeda packstones and wackestones, and mollusk packstones and wackestones (Tables 2 and 3). 4.4.1. Shell beds (mollusk rudstones) In all platforms, shell beds consisting of disarticulated infaunal bivalves are a common feature at the base and in the lower sections of the carbonate sediment successions (cores L1, L4, G5, G6, T4, T6, T13). Contacts with the underlying peat facies are sharp (cores G5, T4). Most common mollusks include Laevicardium laevigatum and Chione sp. in Glovers and Lighthouse Reefs, and Codakia orbicularis in Turneffe Islands. Whereas Laevicardium and Chione prefer open and also sandy substrates, C. orbicularis is indicative of sea grass beds (Abbott, 1974). As a consequence, the shell bed concentrations likely represent the reworking of shallow sandy and grasscovered lagoonal substrates by storms and/or bioturbation (e.g., Suchanek, 1983; Wanless et al., 1988).
Table 4 Standard radiocarbon ages (14) and accelerator mass spectrometry ages (3) from vibracores Sample
Depth below Material SL (m)
Measured age (year BP)
13
C/12C Conventional age Corrected age Calibrated calendar age, BETA, (year BP) (year BP) 2-sigma range sample no.
L4-base L5-base L6-base T4-base T5-basey T6-base
11.9 7.4 8.4 5.3 5.7 5.7
shell peat peat peat soil peat
4120 F 60 6950 F 70 6660 F 90 5610 F 60 7310 F 40 4620 F 70
0.0 25.0 25.0 25.0 26.2 25.0
4530 F 60 6950 F 70 6660 F 90 5610 F 60 7290 F 40 4620 F 70
4130 F 60 6550 F 70 6260 F 90 5210 F 60 7290 F 90 4220 F 70
T7-base T12-base T9-basey G4-basey G5-base
9.3 3.3 5.1 15.3 19.2
peat peat soil sediment soil
6010 F 80 2690 F 80 4900 F 40 7570 F 40 9950 F 220
25.0 25.0 23.9 23.6 25.0
6010 F 80 2690 F 80 4920 F 40 7590 F 40 9950 F 220
5610 F 80 2290 F 80 4920 F 40 7190 F 40 9950 F 220
G5-460/465* G5-380/385* G5-240/245* G5-120/125* G7-base G9-base
18.1 17.3 15.9 14.7 15.6 17.3
peat sediment sediment sediment peat peat
7890 F 70 4540 F 70 2480 F 70 1670 F 60 7660 F 90 8450 F 130
25.0 0.0 0.0 0.0 25.0 25.0
7890 F 70 4960 F 80 2890 F 70 2080 F 70 7660 F 90 8450 F 130
7490 F 70 4560 F 80 2490 F 70 1680 F 70 7260 F 90 8050 F 130
BC 2900 – 2590 BC 5990 – 5710 BC 5720 – 5470 BC 4550 – 4340 BC 6230 – 6050 BC 3620 – 3580, BC 3530 – 3270, BC 3240 – 3110 BC 5070 – 4710 BC 1000 – 780 BC 3780 – 3540 BC 6470 – 6400 BC 10640 – 10550, BC 10420 – 8780 BC 7050 – 6580 BC 3520 – 3090 BC 820 – 500 AD 130 – 450 BC 6660 – 6380 BC 7730 – 7170
148944 148945 148946 148947 148948 149148
148950 148953 148951 148940 148941 153962 153965 153964 153963 148942 148943
Dating by Beta Analytic, Miami, USA. Conventional ages are corrected for d13C/12C. Corrected ages: ocean reservoir correction of 400 years applied for marine samples. Calibrations based on INTCAL 98 database; reservoir correction applied; 2-sigma range has 95% probability. * Depth of 5-cm sample interval in compacted core. y AMS dating.
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4.4.2. Halimeda packstones and wackestones This facies predominates in the Turneffe Islands Holocene successions. It also occurs at the bases of several cores from Glovers and Lighthouse Reefs (G1, G4, L5). The abundance of Halimeda segments ranges
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from 30% to more than 80% with an average value of 48%. Mollusks and foraminifera are common; however, their abundances are mostly below 10%. The most common mollusk is the lucinid bivalve C. orbicularis, which is known to dwell in sea grass beds
Fig. 6. Belize atoll sea level data from rotary cores and vibracores plotted on Holocene sea level curve of western Atlantic (Lighty et al., 1982). Data points not indexed by P ( = peat) or S ( = soil) are from corals or carbonate sediment.
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(Abbott, 1974). The most common benthic foraminifera include Archaias angulatus, Quinqueloculina sp., Triloculina sp., and Cribroelphidium sp. In the Turneffe cores, small corals such as branched Porites sp. and M. areolata are found occasionally. The Halimedarich facies is interpreted to represent a restricted lagoon environment with abundant sea grass beds as seen within the Turneffe Islands platform today. 4.4.3. Mollusk packstones and wackestones In Glovers and Lighthouse Reefs, the Holocene successions are dominated by mollusk packstones and wackestones. The abundance of mollusk shell fragments ranges from 6% to 46% with an average value of 25%. Typical mollusks in Glovers and Lighthouse Reefs cores include the bivalves L. laevigatum, Glycimeris sp., Arca sp., Anadara sp., Tellina sp., Chione sp., and Gouldia sp., and the gastropods Strombus sp., Astraea phoebia, and Cerithium sp. While Laevicardium, Glycimeris, and Chione prefer rather open, sandy, or shallow bottoms, A. phoebia is indicative of sea grass beds (Abbott, 1974), and Gouldia is commonly found in rather protected lagoon areas (Kuhn, 1984). Fragments of Halimeda are common and abundances range from 0% to 38% (average of 14%). The abundance of Halimeda fragments usually decreases upcore. A. angulatus, Quinqueloculina sp., Triloculina sp., and Cribroelphidium sp. are the most common benthic foraminifera. Small branched Porites corals and peloids (largely nonskeletal indurated fecal pellets) are found occasionally. Rare indurated burrow linings are indicative of Callianassa (cores G5, G6). The mollusk facies is interpreted as being deposited in lagoons with open circulation as seen in Glovers and Lighthouse Reefs today. Deposits of rather open, sandy-silty bottoms as well as sea grass beds are included. 4.5. Ages, sedimentation rates The oldest ages in individual cores were obtained from soils (Table 4, Figs. 6 and 7). Among these, the oldest soil date extends the base of core G5 back to
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the beginning of the Holocene. Basal peat ages in the Glovers Reef lagoon range from 7500 to 8500 years BP, in Lighthouse Reef from 7000 to 6500 years BP, and in the Turneffe Islands lagoon area from 7000 to 2700 years BP (Table 3). In Turneffe Islands, the Holocene sediment accumulation rate ranges from 0.22 to 1.23 m/ky with an average of 0.82 m/ky. In Lighthouse Reef, accumulation rates are between 0.41 and 0.63 m/ky with an average value of 0.53 m/ky. In Glovers Reef, the accumulation rates range from 0.23 to 0.67 m/ky with an average of 0.46 m/ky. The average rate for all three platforms is 0.6 m/ky.
5. Discussion 5.1. Holocene flooding scenario During the late Pleistocene, Glovers Reef, Lighthouse Reef, and Turneffe Islands were dish-shaped limestone islands, that were probably fringed by coral reefs (Fig. 8). As the rising Holocene sea level approached the level of the bedrock surface, rising groundwater enhanced soil development. Subsequently, marine waters breached the peripheral bedrock rim (perhaps through relic tidal channels in the elevated Pleistocene reef margin) and entered the central depressions, allowing mangroves to colonize the inner parts of the islands. In Glovers Reef, this happened between 8500 and 7500 years BP, in Lighthouse Reef between 7000 and 6500 years BP, and in Turneffe Islands between 7000 and 2700 years BP. As sea level continued to rise, mangrove-rimmed lagoons with extensive Halimeda growth formed, comparable to the present situation in Turneffe Islands or, for example, in the Marquesas Keys of south Florida (Hudson, 1985). At even higher sea levels, mangrove areas diminished in size and reefs colonized or retrograded onto the peripheral rim of the Pleistocene islands. In Glovers, this stage of peripheral reef development started around 7500 years BP, in Lighthouse at about 6500 years BP, and in Turneffe Islands at around 4800 years BP
Fig. 7. Cross sections through the Belize atolls based on both vibracore data from lagoons and rotary core data from reefs (Gischler and Hudson, 1998; Gischler and Lomando, 2000). Pleistocene faulting is used to explain significant differences in bedrock elevation in Lighthouse Reef and Turneffe Islands. Faults are interpreted to underlie the NNE-striking Middle Reef patch reef trend in Lighthouse Reef and the NNW-striking island trend in the main lagoon of Turneffe Islands.
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reflecting the different bedrock elevation (Gischler and Hudson, 1998; Gischler and Lomando, 2000). Open marine conditions eventually developed in the lagoons of Glovers Reef, Lighthouse Reef, and the northernmost part of Turneffe Islands. In the main lagoonal part of Turneffe Islands, open marine conditions did not develop due to the high bedrock elevation and the protected position of Turneffe behind another platform. Flooding of the Turneffe platform at relatively low rates of sea level rise as well as protection and lack of open marine conditions has led to extensive mangrove development. Late Quaternary bedrock – soil – peat – carbonate facies successions found in the platform lagoon cores are quite similar to those found in other Belize areas such as Ambergris Cay (Ebanks, 1975), and the coastal plain (High, 1975), or further afield, in the North Lagoon of Bermuda (Kuhn, 1984). However, several studies have found differences in this general succession, particularly the development of transitional phases between terrestrial and marine. In the Bahamian Bight of Abaco for example, Rasmussen (1989) identified terrestrial, limnic, brackish, hypersaline, and marine phases. Marine conditions were established only after 5500 years BP when a shallow sill (3.7 m below present sea level) was eventually breached by the rising Holocene sea. Similarly, in the North Sound of Grand Cayman, MacKinnon and Jones (2001) found freshwater to brackish deposits below marine mangrove peats and marine carbonate sediments. Marine circulation did not develop until quite late in the Holocene due to a bedrock sill (4 m below present sea level) that prevented exchange with fully marine waters before 3800 –2500 years BP.
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No brackish or hypersaline carbonates, transitional between terrestrial and marine phases, have been found in this study. A possible explanation for this absence may be that the Belize platforms were first flooded earlier in the Holocene (8500 – 6000 years BP) as compared to the Bight of Abaco (Bahamas) or North Sound (Grand Cayman). Consequently, the Belize platforms were flooded during the more rapid rise of Holocene sea level, which prevented the establishment of long-lasting freshwater lakes or closed lagoon environments. Rates of sea level rise during initial flooding were around 7.0 m/ky for Glovers Reef, 4.1 m/ky for Lighthouse Reef, and 2.8 m/ky for Turneffe Islands (see Fig. 6). In addition, the absence of the shallow sills in the Belize platforms lessened the chance for restricted circulation. The depth of Pleistocene bedrock beneath marginal reefs is up to 12 m in Glovers Reef, up to 8 m in Lighthouse Reef and up to 6 m below present sea level in Turneffe Islands (Gischler and Hudson, 1998; Gischler and Lomando, 2000). These depths are probably exceeded beneath the modern 5 –8 m deep tidal channels in the Belize platform margins as they are likely to be located over even deeper Pleistocene tidal channels. 5.2. Comparison with other data and significance A comparison of the limited data on Holocene atoll and isolated platform lagoon development (Table 5) reveals no statistically significant trends, or linkages between parameters such as lagoon size and Holocene sediment thickness or sedimentation rates. There are poor correlations between accommodation space (here estimated from simple multiplication of lagoon area with maximum depth to Pleistocene bedrock) and
Fig. 8. Schematic sketches showing Holocene development of Belize atolls. Data on peripheral reef development from Gischler and Hudson (1998) and Gischler and Lomando (2000); (a) 8000 years BP, sea level ca. 15 m below present level. Turneffe Islands and Lighthouse Reef were emergent limestone islands; interior part of Glovers Reef characterized by mangrove environment; transgression of marine water, probably through the precursors of modern tidal channels; Pleistocene patch reefs probably still emergent as small cays in lagoon (not shown); (b) 7000 years BP, sea level about 10 m below present level. Parts of Turneffe Islands and Lighthouse Reef flooded and mangrove-dominated; Glovers Reef with mangrove rims; Halimeda-dominated early carbonate sedimentation; small Pleistocene areas probably still emergent at margin and in lagoon (not shown); (c) 6000 years BP, sea level about 7 m below present level. Mangrove-dominated parts of Turneffe Islands increase in area; western parts of Lighthouse Reef flooded and mangrove-covered; eastern lagoon mantled by Halimeda-dominated carbonates; Glovers Reef carbonate sedimentation no longer Halimeda-dominated; maybe some mangrove areas left; Pleistocene patch reefs in lagoon submerged; (d) 5000 years BP, sea level about 5 m below present level. In Turneffe Islands, large areas in the east are still emergent, but a mangrove-encircled lagoon develops with Halimeda-dominated sediment; Glovers and Lighthouse Reefs are characterized by carbonate sedimentation; western parts of Lighthouse possibly with larger mangrove areas and Halimeda-rich sediment; (e) 4000 years BP, sea level about 3.5 m below present level. Glovers and Lighthouse Reefs almost as at present time; Turneffe Islands’ main lagoon largely developed; seaward tidal channels open; parts of the north still emergent; carbonate sedimentation initiated on northernmost part, probably still surrounded by mangroves. (f) Present situation.
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Table 5 Areas, depths, Holocene sediment thicknesses and sedimentation rates of some isolated platform and atoll lagoonal settings Location
Lagoon Maximum Maximum Average area depth (m) Holocene sediment (km2) sediment rate (m/ky) (m)
(1) Glovers Reef 194 (2) Lighthouse Reef 112 (3) Turneffe Islands 225 (4) One Tree Reef 10 (5) Bermuda 290 (6) Hogsty 23 (7) Enewetak 980 (8) Mururoa 130 (9) Mayotte 1500 (10) North Sound 90 (11) Abaco Bight 2750
18 8 8 10 20 8 62 55 80 6 7
6–7 6–7 6–7 9 – 10 2–3 3 10 20 6 3 2
0.46* 0.53* 0.82* 1.25* 0.32 unknown 1.25* unknown 0.35* 0.85 0.21 – 0.36 (average 0.24)
Data sources: (1 – 3) this study; (4) Marshall and Davies (1982); (5) Kuhn (1984, p. 100, Fig. 14); (6) Pierson and Shinn (1985); (7) Buddemeier and Oberdorfer (1997); (8) Buigues (1997); (9) Zinke (2000, Fig. 3.4); (10) MacKinnon and Jones (2001); (11) Rasmussen (1989, Fig. 14). * Calculated from sediment thickness and time of Holocene flooding.
maximum Holocene sediment thickness (r = 0.04), and between accommodation space and average sedimentation rate (r = 0.11). Apparently, the rate of filling of lagoons in modern isolated platforms and atolls is not simply a function of the size of accommodation space but controlled by a number of factors that may vary among individual localities. These factors include carbonate production within the lagoon, size and carbonate production rates of the windward margin that includes lagoonward sediment transport, mud export over leeward margins, as well as individual shape and elevation of antecedent topography, with the latter being responsible for the timing of Holocene inundation. Differences in depth and relief of the underlying Pleistocene limestone among the Belize platforms are interpreted to reflect differential subsidence along the underlying fault blocks that has been accentuated by north to south differences in meteoric dissolution (Gischler et al., 2000). The data in Table 5 also reveal that for many Holocene lagoons it will take tens to hundreds of thousands of years to be completely filled (e.g., Turneffe 10 ky, Lighthouse 15 ky, Glovers 40 ky,
Enewetak 50 ky, Bermuda 60 ky, Mayotte 230 ky; note that these estimates are based on average Holocene sedimentation rates and maximum lagoon depths, ignoring sediment export). In the light of the Quaternary record of eustatic sea level change and the lengths of interglacial high sea level cycles, it is unlikely that the platform and atoll lagoons will be completely filled during the Holocene. Similar conclusions were drawn by Boss and Rasmussen (1995) for the northern part of Great Bahama Bank. They showed that there is no significant correlation between sediment (cycle) thickness and depth to the Pleistocene – Holocene boundary (accommodation space), and that lagoonal sedimentation rates are not sufficient to completely fill available accommodation space during the Holocene. Cycle thickness can therefore not be used indiscriminately as a record of eustatic sea level change. The findings of the present study support this conclusion and lend further evidence to the observation that ‘‘empty buckets’’ (sensu Schlager, 1993) are a common feature among Holocene isolated carbonate platforms and atolls. Understanding why these Holocene buckets fail to fill, however, is still unclear. Apart from stressing apparent differences in amplitudes of eustatic sea level fluctuations, Read (1985) and Koerschner and Read (1989) made the point that fossil cyclic platforms often lack rims whereas Quaternary platforms usually have raised rims. Based on this observation it can be speculated that the significantly higher growth potential of the reefal rims around modern platforms (Schlager, 1981) acts in concert with high-amplitude sea level changes to produce unfilled accommodation space in Quaternary platforms.
6. Summary and conclusions Holocene successions in three isolated carbonate platforms of Belize include basal soils above Pleistocene limestone bedrock, followed by mangrove peats, and marine lagoonal carbonate sediments. Early restricted carbonate sedimentation within the platforms is characterized by abundant Halimeda. Due to differences in depth of Pleistocene bedrock, Glovers, Lighthouse, and Turneffe atolls were flooded at successively later stages during the Hol-
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ocene transgression between 8500 and 6000 years BP. In contrast to Glovers and Lighthouse Reefs, which have developed open lagoon circulation, Turneffe Islands is characterized by restricted circulation with abundant mangroves and Halimeda-dominated lagoon sediments. Contributory factors to this restricted circulation are the fact that this platform was flooded relatively late during the Holocene as compared to the other two platforms, and that Turneffe Islands is located in a protected, low-energy position in the lee of Lighthouse Reef. After flooding, lagoon sediment accumulation rates averaged 0.6 m/ky, but the rate of sedimentation lagged behind sea level rise, increasing accommodation space to the present maximum lagoon depths. Other data available on Holocene lagoonal development supports these findings and indicate that, regardless of differences in facies successions and flooding scenarios, unfilled accommodation space is a common feature in carbonate platforms and atolls that develop during periods of glacio-eustacy. This implies that cycle thickness—as a record of eustasy—does not necessarily equal accommodation space in fossil carbonate platforms.
Acknowledgements This research was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation, Project Gi 222/4). I am very grateful to Harold Hudson (N.O.A.A., Key Largo), Malo Jackson, Patrick Silverback (Dangriga), Wolfgang Oschmann, Kai Spresny, and Gabi Meyer (J.W. Goethe Universita¨t, Frankfurt/Main) for their assistance during drilling operations and transport of equipment and core samples. Stefanie Scheidt (Frankfurt/Main) helped with textural and compositional analyses; Rainer Petschick (Frankfurt/Main) performed XRD analyses. The support of the Fisheries Department in Belize City, especially by James Azueta, is gratefully acknowledged. I thank Edward Purdy (Weybridge, UK) for his thoughtful comments on an earlier version of the manuscript. The constructive reviews by Paul Blanchon (National University of Mexico) and Christopher Perry (Manchester Metropolitan University) improved this paper and are gratefully acknowledged.
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