or tsunamis) in the sedimentation, diagenesis and karst initiation of tropical shallow water carbonate platforms and atolls

ELSEVIER

Sedimentary Geology 118 (1998) 3–36

The role of high-energy events (hurricanes and=or tsunamis) in the sedimentation, diagenesis and karst initiation of tropical shallow water carbonate platforms and atolls F.G. Bourrouilh-Le Jan * Laboratoire CIBAMAR, CIne´matique de BAssins et MARges, Universite´ de Bordeaux I, Avenue des Faculte´s, 33405 Talence cedex, France Received 10 February 1997; accepted 13 September 1997

Abstract Karst morphology appears early, even during carbonate sediment deposition. Examples from modern to 125-ka-old sub-, inter- and supratidal sediments are given from the Bahamas (Atlantic Ocean) and from Tuamotuan atolls (southeastern Pacific Ocean), with mineralogical and hydrological analyses. Karstification is favoured by the aragonitic composition of bioclasts coming from the shallow marine bio-factory. Lithification by aragonite cements appears as a rim around carbonate deposits and dissolution and non-cementation start at the same time on modern supratidal deposits (Andros micrite or atoll coral rudite) and provoke the formation of a central depression on small or large carbonate platforms. In fact, this early solution of the centre of platforms is closely related to the location of each of the studied examples on hurricane tracks. High-energy events, such as hurricanes and tsunamis, affect sediment transport but hurricanes also affect diagenesis as a result of the enormous volume of freshwater carried and discharged along their paths. This couple, lithification–solution, is localised at sea level and accompanies sea-level fluctuations along the eustatic curve. Because of the precise location of hurricane action all around the Earth, early karstification by aragonite solution, cementation and supratidal carbonate sediment accumulations (high-energy trails) act together on all the platforms and atolls located inside the Tropics (23º270 ) between roughly 5º–10º and 25º on both hemispheres. However, early karstification acts alone on shallow carbonate platforms including atolls along the equatorial belt between 5º–10ºN and 5º–10ºS. These early steps of karstification are linked to the ocean–atmosphere interface due to the bathymetrical position of shallow carbonate platforms, including atolls. They lead to complex karstified emerged platforms, called high carbonate islands, where carbonate diagenesis, together with the development of bauxite- and=or a phosphate-rich cover and phreatic lens, will occur.  1998 Elsevier Science B.V. All rights reserved. Keywords: aragonite; Bahamas; atolls; Pacific Ocean; solution; karst; hurricanes; tsunamis; sea-level fluctuations

1. Introduction Sedimentary and diagenetic studies of modern carbonate depositional models, the Great Bahama Ł Tel.:

C33 556 848823; Fax: C33 556 848877; E-mail: [email protected]

Bank in the Atlantic Ocean and Polynesian atolls in the Pacific Ocean, show that karstic morphologies appear as early as during sediment deposition and also show that they are a consequence of the presence and action of lithification and solution. The latter is itself strictly dependent on the freshwater phreatic lens (Dupuit–Ghyben–Herzberg lens) lo-

0037-0738/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 0 3 - 7

4

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

cated within shallow water carbonate deposits, and is thus dependent on all the climatological parameters of the area (that is to say humid tropical climate, high rainfall and hurricane control). The minerals involved in such processes are aragonite, low magnesian calcite, high magnesian calcite and calcian dolomite. Aragonite is usually considered as originating from the destruction of reef material and of tropical shallow water communities. More precisely the main aragonite supply comes from calcareous green algal thalli (Penicillus, Halimeda, Ripocephalus). Hence, it is considered that 1 nm long and 0.1 nm wide aragonite needles are the main component of shallow water aragonite mud (as nanobioclasts), after Lowenstam and Epstein (1957); Matthews (1966) and Stockman et al. (1967). On tropical carbonate platforms, from 0 to 10 m or 60 m water depth, high magnesian calcite is essentially present in the form of foraminiferal tests and red algal thalli and, less importantly, from synor post-depositional cements (Lighty, 1985; Pierson and Shinn, 1985). The occurrence of this mineral is also related to the presence of cyanobacterial mats in carbonate mangrove zones and=or in supratidal flats or plains. Low magnesian calcite is entirely diagenetic and derives from high magnesian calcite by an early loss of Mg2C (Bourrouilh-Le Jan, 1972; Land, 1973), or by aragonite solution and reprecipitation of CO23 and Ca2C in freshwaters. Dolomite is essentially diagenetic in the supratidal zones of western Andros (Bahamas). Thus, cementation and solution affect sediment which is mainly composed of 90 to 99% aragonite with a very small amount of high Mg calcite. This is because, regarding the two studied examples, sediment is largely derived from the reef biocoenosis which is composed of corals, Halimeda and pelecypods, all producing aragonite. On the other hand, the dilution of seawaters by meteoric waters as observed on the tidal flats of western Andros (Bahamas), in freshwater ponds of the Tuamotu rim atolls and in Clipperton atoll (E Pacific), provokes the solution of aragonite. In conclusion, the two phenomena, i.e. precipitation of aragonite and its solution, are linked: their combined evolution has sedimentological and geomorphological consequences for the initiation and development of specific karstic features and typical reworked deposits.

2. Area description, methods and material studied Both models, Bahamian platforms and Tuamotuan atolls, are located in the tropical belt, characterized by hurricanes which appear around C5ºN or S and then head towards the west, northwest and northeast or towards the south and then southwest and southeast, according to the hemisphere (Cry, 1965; Crutcher and Quayle, 1974). The importance of hurricanes (occurring on average once every 5 years) for the deposition, erosion and diagenesis of the tidal flats as well as the existence of a MidHolocene higher sea level, following an insolation maximum from 8000 to 5000 yr BP (Davis, 1984) have been demonstrated previously (Bourrouilh-Le Jan, 1990, 1993). The general sedimentary model of the Bahamian platforms is now well understood, especially on the tidal flats of western Andros following the classic works of Field (1931); Black (1933), Smith (1940); Newell and Rigby (1957); Shinn et al. (1965, 1969) and Hardie (1977). We will focus on new results obtained from Andros Island studies: western supratidal plain deposits and the eastern karstic Pleistocene coast. It has been previously demonstrated (Bourrouilh-Le Jan, 1978a,b, 1980, 1982a, 1990) that the tidal flats of western Andros act as a ‘sedimentological filter’ during high-energy events (cyclones and hurricanes). The following main sedimentological zones have been recognized: the middle and south zones of the Great Bahama Bank act as the ‘carbonate factory’ and the subtidal talus located all along the southwest coast of Andros constitutes a storage zone for carbonate mud, clearly visible on satellite photographs (bathymetry between 0 and 1.5 m). The south-southwest tidal plain constitutes the supratidal accumulation zone of the mud stored in the subtidal coastal zone and numerous examples of filled estuaries are visible. The north-northwest coast on the other hand is the hurricane flood evacuating zone, easily recognizable by its numerous channels and estuaries. As for the Pacific atolls of the northwest Tuamotu and Clipperton Island, the main influence comes from hurricanes which are responsible for the surface morphology of the atoll rim and the supply of

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

sediment to the lagoon (Bourrouilh-Le Jan, 1984, 1990; Bourrouilh-Le Jan and Talandier, 1985). In addition, we try to demonstrate how karstification starts very early by modelling the interface between atmosphere and ocean, that is the upper surface of the atoll rim or of the shallow water carbonate platform, both modified by the combined action of lithification–solution. The latter action is directly related to hurricane tracks and is displaced vertically with sea-level fluctuations. Chemical conditions affecting the preservation or alteration of the different minerals within sediments are controlled by pore fluid composition (e.g. marine and brackish waters or phreatic lens) and therefore by the climatic conditions in the area (temperatures, rainfall, winds, etc.). The study of lithification–solution therefore requires mineralogical (X-ray diffraction) data and the hydrological study of the sediments and their surrounding waters (pore fluids and emerged water tables: ponds, lakes). The Bahama Bank area, and especially Andros Is., was sampled during three oceanographic field trips sponsored by Elf-Aquitaine and Universite´ Pierre-etMarie-Curie, Paris. The western, eastern and northern zones of Andros were sampled with a hand field corer (entering the sediments to a depth of 2.5 m) (Fig. 1). Water samples and 90 hand cores were obtained from the tidal flats of western Andros and in the Joulter’s Cay area. After X-raying, the cores were successively cut in two, impregnated, and sampled for thin sections, 14 C analysis and X-ray diffractometry. In each case, we tried to compare the petrographical studies with the hydrological studies. Water salinity was calculated from chlorinity: S D 0:03C.1:805 [Cl ]) (Horne, 1970). This salinity–chlorinity is usually given in the literature. To find out whether solution or cementation was occurring, ionic dilution– concentration coefficients were calculated and compared with Bahamian or Pacific oceanic seawaters by the ratio 100 [ion]=[Cl ] of a sample site (station) to 100 [same ion]=[Cl ] from Bahamian or Pacific oceanic seawater. Ions contained in a simple diluted seawater have an ionic dilution–concentration coefficient of one, less than one if this ion is removed from the studied waters (by mineral precipitation) and more than one if chemical solution has occurred.

5

3. Modern karstification of modern deposits, Great Bahama Bank 3.1. Aragonite lithification During sub- to intertidal diagenesis, the diagenesis of aragonite consists of cementation and solution and begins very early with direct precipitation of aragonite from seawater causing sediment lithification, either in bioclastic reef-derived sediments or in carbonate mud environments (Friedman et al., 1974; Dravis, 1979; Sandberg, 1985). 3.1.1. Lithification of carbonate mud in the supratidal zone As soon as fine-grained carbonate deposits are formed in the supratidal zone, they are subjected to a special alternating hydrological regime characterized by hurricane-provoked flooding of marine water which is, however, quickly diluted by freshwater, roughly once every 5 years, forming new tidal channels or reactivating old ones located on the northwest coasts. These marine incursions lead to a period of sedimentary fill which closes the tidal basins, and lowers the hydrostatic level by balance with the upper level of the phreatic lens. The tidal basins then evolve into numerous hyperalkaline, low salinity (3 g=l) lakes. The systematic observations from thin sections, coupled with X-ray mineralogical identification and geochemical analyses show that aragonite precipitation occurs within the sediment at a depth of several tens of centimetres. This occurs at the border of the tidal plain and close to the platform sea of the Great Bahama Bank (Fig. 2). Contemporaneously, dolomitisation occurs on the periphery of the mesohaline to oligohaline lakes without evaporite formation. In the supratidal zone where dry winters alternate with wet summers, high magnesian calcite pisoids are formed. 3.1.2. Lithification of oolitic sand Numerous workers have studied the Bahamian oolitic sands (Loreau, 1970; Hine and Neumann, 1977; Hine, 1977, 1983; Harris, 1977, 1978, 1983; Dravis, 1979; Cros, 1979; Simone, 1981; Carney and Broadman, 1993) which are generally located on the margin of the carbonate platform.

6

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 1. The Great Bahama Bank (A) and the studied areas (B): 1 D tidal plain of western Andros (2500 km2 ), mangrove zone; 2 D Fresh Creek area, with 125-ka Pleistocene outcrops; 3 D Joulter’s Cays, with oolite formation; 4 D submerged Bimini beachrocks. (C) Main statistical direction of hurricane tracks since 1873 centred on the western cape of Andros on an area with a diameter of 100–200 km.

The Joulter’s Cays area is a very shallow area to the north of Andros Island and close to the Tongue of the Ocean. It is completely uncovered at low tide (Fig. 3A and B). Pure oolitic sands occur on the east part as deltas and bars and a wide Thalassia and Syringodium bank with a few tidal channels extends towards the platform. Studies of water chemistry show that the Great Bahama Bank waters, west of Joulter’s Cays and on the Joulter’s Cay area itself, are enriched in Ca2C , HCO3 , SO24 (Fig. 3B). The waters remaining in the area lose these same ions. Precipitation of aragonite occurs. Quiescent ooids which are trapped on the

Thalassia and Syringodium banks are fed on the one hand, but also aragonite cement precipitates in between the grains and causes the beginning of early lithification. 3.1.3. The Holocene transgression of the Great Bahama Bank and the lithification of its rim: the beachrocks of Bimini Is Formation of beachrock has been studied by Ginsburg (1953) at Dry Tortugas (south of Florida). The lithification occurs by means of aragonite needles which precipitate from the seawater held in the beach sand during the ebb tide. This phenomenon

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

7

Fig. 2. Schematic distribution of magnesian carbonates (calcian dolomite and high magnesian calcite) in surface sediments, and of aragonitic lithification inside deep parts of cores. Location on Fig. 1; 1 and 2 D calcian dolomite <5%; 1 D measured; 2 D extrapolated; 3 and 4 D calcian dolomite between 10 and 15%; 3 D measured; 4 D extrapolated; 5 and 6 D high magnesian calcite percentage >10%; 5 D measured; 6 D extrapolated; 7 D aragonite lithification by precipitation of cement in deep zones of cores (mudstone to wackestone facies) located on hurricane trails (thick black lines); 8 D main statistical trends and lengths of hurricane trails from aerial photographs, grouped at 5º intervals. The numerous ponds, lakes and basins have not been drawn for clarity.

is controlled by temperature, beach drainage and the stability of the beach face sediment. Hanor (1978) stated that precipitation can be induced by the loss of CO2 which is necessary to cause supersaturation due to vertical fluid dispersion in the phreatic zone resulting from tidal oscillation. Northwest of Bimini Islands (Fig. 1), on the seaward side of the Great Bahama Bank, Rebikoff

(1972) discovered alignments of roughly rectangular-shaped blocks, 5 and 6 m deep. Two cores were sent to our laboratory for examination. These blocks are ancient beachrocks derived from pre-Holocene sandy shorelines and the age of the two core samples is 27,000 yr BP and 29,000 yr BP (Gifford and Ball, 1971). The facies are packstone with numerous and various bioclasts (pelecypods, echinoids,

8

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 3. Joulter’s Cays, northern Andros Island. (A) Location of samples and isohaline curves in g=l (italic numbers). (B) Variations of the ionic concentration–dilution coefficients along the cross-section A–B. Orthogonal projection of sampling stations on the A–B line. Arithmetic scale.

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

foraminifera) and intraclasts derived from a calcitic yellow caliche crust. Echinoids show syntaxial overgrowths and analyses show that this overgrowth is a low magnesium calcite. Only one generation of aragonite cement occurs around bioclasts in the top of the cores. One core presents a calcite crust (caliche) situated under the beachrock. The succession of sedimentological events was as follows: during the Late Pleistocene emersion, the platform was covered with a caliche formation of marsh or swamp origin (Bourrouilh-Le Jan, 1977), followed by transgression and new sandy shoreline facies which became partly cemented to form beachrock at an unknown time between 29,000 yr BP and the present day, when sea level was around 5 or 6 m lower than today. The Holocene transgression then covered and submerged these beachrocks. 3.2. Solution of aragonite at the present day Solution is frequent in modern carbonate environments and affects not only modern sediments but also the 125 ka Pleistocene calcarenite on which they were laid down. 3.2.1. Modern aragonite solution in the estuaries of the tidal plain of western Andros The tidal plain of western Andros occupies 2500 km2 , and comprises numerous ponds, lakes and tidal channels linked to estuaries and to the sea of the Great Bahama Bank. The thickness of the sediment has been evaluated at 6 m (Gebelein, 1975) and is composed of 90% aragonite carbonate mud (Bourrouilh-Le Jan, 1982a, 1990). The different bodies of water of the Andros mangrove swamps are not a simple result of the dilution of seawater by meteoric waters due to high rainfall. On the contrary, these lake, pond and marsh waters show an enrichment in Ca2C and NaC ions, 9 to 10 times more than in diluted seawater, correlated to the early solution of aragonite and forming the spectacular round ponds of the western Andros tidal plain, lying on the present-day tidal flat sediments (Fig. 4a, Bourrouilh-Le Jan, 1980). The conclusions are summarized in Table 1 and Fig. 2. Table 1 links diagenetic events with hydrology and ecology.

9

3.2.2. Modern solution of Pleistocene aragonite in the estuaries of the east coast of Andros Island All workers agree that traces of meteoric erosion at the surface of the Bahamian Banks can be recognised. Agassiz (1894) was the first to describe a submerged sinkhole (aven in the karstic literature) or blue hole that he measured at a depth of 61.2 m located on the platform and covered with 3 to 4 fathoms of water; Vaughan (1914) measured a second one at 57.6 m. Doran (1955) found a depth of 59.4 m for an aven located in Acklin Island. The Bahamian karst has been described briefly by Newell and Rigby (1957), then by Monty (1967) and by Smart et al. (1987). The British Speleological Association made two expeditions in 1981 and 1982 (Palmer and Williams, 1984; Palmer, 1986) and one in 1985 (Raleigh Expedition). Smart (1984) published a salinity map around creeks and adjacent mangrove swamps. Our water samples from the different estuaries (Fresh Creek, Stadford Creek and Caddle Creek, Bourrouilh-Le Jan, 1980) indicate that ionic dilution–concentration coefficients fluctuate for Ca2C up to 9 and up to 2 for Sr2C and Mg2C , indicating that these ions are not in the same proportion as in seawater and that the solution of Pleistocene aragonite has started, enriching creek waters in these ions. 3.3. Geological and geomorphological consequences In modern intertidal and supratidal shallow carbonate environments (Great Bahama Bank), aragonite lithification and solution provoke the birth of karst and its peculiar topography as well as the appearance of detrital carbonate sedimentation. 3.3.1. Modern karstic morphology 3.3.1.1 Modern alteration of modern sediments of the western Andros mangrove. Numerous stagnant freshwaters attack modern tidal flat deposits (Fig. 4a), or the Mid-Holocene deposits as ‘hummocks’ (hurricane trails) as we have previously shown and defined (Bourrouilh-Le Jan, 1978a,b, 1978c, 1979, 1980, 1981, 1982a), because of their location several tens of km from the coast, the absence of marine deposits (to be more precise, rare and exceptional during hurricanes) and the high rainfall on Andros island.

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

11

Table 1 Summary of geographical location, ecological conditions, hydrologic plus ionic variations and carbonate diagenetic phenomena observed in the sedimentary tidal and ‘supratidal machinery’ of western Andros Sedimentary machinery:

Geographical location, example: Salinities: Ionic solution–concentration coefficient: Ecology:

Sedimentology

Diagenetic interpretation:

Estuary and tidal channel

Tidal basin

Permanent ponds and lakes inside V-shaped hurricane trails with high evaporation rate around them

Wide Opening 33–19 g=l D 1: simple dilution of marine seawater from the Great Bahama Bank Marine fauna and flora confinement

Turner Sound 14–11 g=l >1 for Ca2C , Sr2C , <1 for Mg2C , KC , NaC , >1 for SO24 Mangrove forest (Avicenia and Rhizophora)

North or south Pelican ponds 3–6 g=l ×1 for NaC , Ca2C , KC , HCO3 , SO24

Grey subtidal mud with peneroplids and gastropods. Banks with pellets and smooth stromatolite laminations Dilution of marine water C marine sediment transport

Parts of hurricane trails disappearing in contact with ponds which dissolve deposits can frequently be observed on vertical aerial photographs. The consequence which can be seen on aerial photographs of western Andros is the enlargement of water surfaces in the central part of the tidal plain (Fig. 4a). 3.3.1.2 Modern alteration of the 125 ka deposits. The deposits underlying the Holocene are Pleistocene oolitic packstones which constitute the eastern part of Andros Island. Dated to 125 ka by Broecker and Thurber (1965) this age has been confirmed by Hearty and Kindler (1995) and Neumann

Cushion-like stromatolites Subtidal mud with peneroplids C gast. Banks with mud cracks, mangrove roots, burrowings etc. Aragonite solution C early diagenesis around ponds and lakes: surficial dolomitisation and high Mg calcite with stromatolites and pisoids

Charophytes and belt of vascular plants (Tracheophytes, Juncus and graminaceae) Veil-like stromatolites ‘Lacustrine’ mud with peneroplids. Banks with mud cracks, burrowings teepee structures, thick hurricane laminae with didemnid spicules. Aragonite solution C early diagenesis around ponds and lakes: surficial dolomitisation and high Mg calcite with stromatolites and pisoids

and Hearty (1996). On this young lithified limestone, it is easy to observe the first steps of karstification on the rock of mixed calcite=aragonite mineralogy (Bourrouilh-Le Jan, 1976, 1977). A geomorphological survey of the surface of the Pleistocene calcarenite has shown two topographical features: (1) blue holes, similar to those described outside the island (Benjamin, 1972), and (2) small karstic basins (Fig. 4b–e). The blue holes are circular features and are usually 60 to 100 m in diameter. The small karstic basins are numerous, shallow, a few metres in diameter and give a characteristic honeycomb aspect to the topographic surface when seen

Fig. 4. Examples of morphology due to modern and Pleistocene aragonite solution on Andros Island and New Providence Island (Bahamas). (a) Modern solution of Holocene to Mid-Holocene aragonite deposits of western Andros tidal plain. North of Wide Opening low salinity and undersaturated ponds and lakes are dissolving the hurricane trails shown by arrow (hummocks). (b–h) Modern and previous solution of Upper Pleistocene calcarenite of eastern Andros Island. (b, c) Karstified Pleistocene limestone: small dry and water-filled karstic basins filled with stromatolitic carbonate sediment with desiccation cracks, convex upper surface and high magnesian calcite composition. (d) Charophytes (Chara zeylanica var. zeylanica and var. diaphana, Nitella tenuissima) between two stromatolite cushions inside a small karstic basin. (e) Low altitude aerial view (50 to 100 m) of the upper karstified surface of the Fresh Creek emerged Pleistocene. (f) Area of downstream Pleistocene lithoclast deposits (station 50). (g) Aerial view of a young karst on a young limestone, now drowned by the Holocene transgression: the Upper Pleistocene of New Providence Island (Bahamas). (h) Small karstic remnants of the Upper Pleistocene packstone of eastern Andros, emerging from the wet mangrove (hammer for scale).

12

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

from low altitude (Newell and Rigby, 1957; Bourrouilh-Le Jan, 1974, 1977) (Fig. 4e). The elevation of the surface is close to sea level: C1 to C5 m, in an area with high rainfall which again provokes pond and lake formation. In fact, located on the Pleistocene calcarenite, they represent the visible and aerial part of the Dupuit–Ghyben–Herzberg (DGH) phreatic lens. Pleistocene calcarenite outcrops are attacked by biochemical agents and by estuarine water at the level of the upper surface of the water creek and their joint action leads to aragonite solution. Yellowish small to coarse clasts are produced by the destruction of the caliche crust and oolitic calcarenite (Fig. 4f). A submerged karstic surface also exists, consisting of the same features and submerged by the end-Holocene transgression (Fig. 4g), described also by Rasmussen and Neumann (1988). The modern karstic relief started 125 ka ago and may have had different steps which are difficult to demonstrate in the present topography. Blue holes are avens or sinkholes of this young karst. They have been formed as much by solution as by the collapse of the upper roof of karstic caverns and can be interpreted as the necessary drainage holes on the emerged platform because of the total absence of rivers on the island during the previous drop in sea level. According to the karstic morphology, it is possible to link the genesis of Bahamian blue holes to the genesis of Yucatan ‘cenotes’, that is to say to avens or sinkholes. The water of terrestrial blue holes has a salinity close to that of the waters of the neighbouring creek (17.5 g=l), showing links through conduits, caves and fractures with the platform edge (Smart et al., 1987) and opposite to that of small karstic basins which have low salinity waters (0.7 g=l to 3.8 g=l), only fed by rain and which are populated with stromatolites and Charophytes (Bourrouilh-Le Jan, 1974) (Fig. 4b, 4c and 4d). Out of 120 sites explored on the Bahamian land, 4 blue holes are deeper than 100 m with a maximum of 110 m and were formed during subaerial exposure at the 14,000 yr BP lowstand of sea level. All of them are filled with water and contain stalactites in their deeper parts (Farr and Palmer, 1984). Some of them present complex horizontal cave networks as underwater blue holes communicate with several horizontal cave networks over great distances (1.5

km). Violent tidal currents also traverse these cave networks and blue holes. Each terrestrial blue hole works as a meromictic lake, with a very small upper surface area and a great depth. Following the formula of the Dupuit– Ghyben–Herzberg lens .H D 40h, with H, thickness of the phreatic lens, and h, elevation of the upper level surface of the phreatic lens), this phreatic lens would be 30 m thick and would thin towards the estuaries and the coast, as is normal. The blue holes of southern Andros and their associated galleries show a strong structural control (Palmer and Williams, 1984). Their alignment is parallel to the underwater cliff margin of the Great Bahama Bank along the Tongue of the Ocean. The chemistry and biology of terrestrial blue holes are very complex (Smart, 1984). Dissolution would start at the boundary between the phreatic lens and the underlying brackish water, and would be facilitated by a layer of suspended and decaying sulphur-rich organic matter at this boundary and acting as a plug. The same observation has been made on Clipperton Island (an atoll with a small volcanic remnant) of the east-northeast Pacific Ocean (Bourrouilh-Le Jan et al., 1985a). Back et al. (1984) have shown that, on Andros, active carbonate solution is occurring at the upper limit of the freshwater phreatic zone, like on the Clipperton atoll. Carbonate solution in open air fluctuates at about 0.18 mg cm 2 yr 1 according to the carbonate position. In the upper parts of terrestrial blue holes, this solution rate would be 0.01 mg cm 2 yr 1 (Smart and Whitaker, 1991). In conclusion, the 14,000 yr BP emersion resulted in the total emersion of the Great Bahama Bank and the formation of this young karst on Pleistocene calcarenite, characterized by vertical avens and formed by the rapid solution of aragonite within the 125-kaold deposits (Fig. 5; Goodell and Garman, 1969). The Holocene transgression and the hurricane-influenced climate reinforce the porosity of the Andros limestone, because of high rainfall. Andros limestone is perforated by numerous interconnected vertical pot holes or avens of nearly 100 m in depth and horizontal cave networks, the latter corresponding to stillstands of sea level. There is no polje and a superficial yellow calcitic crust (caliche) locally prevents the surface of

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

13

Fig. 5. Diagram of avens or blue holes located on the carbonate platform of the Great Bahama Bank, in relation to the position of the phreatic lens which follows sea level during glaciations and deglaciations. (A) The phreatic lens is in hydrostatic equilibrium with the paleo-sea level of 14,000 yr BP (exaggerated scale). (B) The phreatic lens has followed the Holocene eustatic rise of sea level and is again in hydrostatic equilibrium with the new modern level, drowning the previous karstic morphology and creating ‘blue holes’ and ‘boiling holes’.

the aragonite C calcite oolitic packstone from undergoing dissolution by freshwater (Bourrouilh-Le Jan, 1977). 3.3.2. Sedimentological consequences The aragonite C calcite Pleistocene packstone is progressively dissolved and the progression of the erosion can be seen in the centre of Andros (Fig. 4b– e and h, Figs. 6 and 7). The higher zones of central Andros can be interpreted as ancient underwater marine dunes of the Upper Pleistocene. These zones are dissected by a karstic surface with small ver-

tical pits that is easily visible through vegetation. The lower zones of Andros are covered with a thin sheet of water, a few centimetres deep, with a profusion of cyanobacteria and charophytes, transformed into a dry savannah during the dry season. In these zones, the 125-ka-old aragonite–calcite packstones have been weathered to leave isolated and small (decimetre sized) stumps of very altered limestone (Fig. 4h). 3.3.2.1 Modern detrital carbonate sedimentation on eastern Andros Island. The sedimentological analy-

14 F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36 Fig. 6. Sample location map for study of modern detrital carbonate sedimentation of low magnesian calcite on the east coast of Andros Island (consequence of karst initiation of the 125-ka Bahamian limestone).

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

15

Table 2 Consequences of the early karstogenesis on the 125-ka Bahamian limestone: core characteristics and mineralogical results of top and bottom core sampling Station

Water Core No. Core depth length (m) (m)

43

0.2

58199

0.5

46

2

58201

0.30

49

1

58202

0.20

50–53

0

58203

1 m reduced to 0.30

58372

3 m reduced to 1.55 to 1.55

36 Middle 1.50 Bight

Sampling Aragonite Quartz Low Mg High Mg Halite Clay/or Total position in (%) (%) calcite calcite non-measured salinity core (cm) (g/l) 5 46 5 24 1 17 1

95 99 33 45 8 20 18

40 41 53 61 31

1 1 x 2

5 1 19 6 6 7 30

7 7 33 12 17

26 5

28 74

23 3

35 12

3 2

11 9

21.0

143

69

9

15

2

9

34.7

sis of the eastern coast deposits of Andros Island shows the presence of modern sediments of low Mg calcite and high Mg calcite rather than the modern aragonitic sediments found on western coasts. These sediments are found along the creeks and in and beside the different ponds and marshes. Cores are 30 to 50 cm long, some of them displaying a high compaction rate when taken out of the water. They contain 7 to 33% aragonite located in distinct layers (tempest or hurricane deposits because they are found 10 km from the marine shorelines), 31 to 61% high Mg calcite and the remainder low Mg calcite. A SEM study shows that the low Mg calcite is made of abraded grains, neither euhedral nor anhedral. Tubules of high Mg calcite are also clearly visible (Fig. 6, Table 2 and Fig. 7). Grain size variations and different lengths of the transport can be found. At station 50 on Fig. 6, the modern karstic erosion produces yellow nodules which represent reworked caliche. These nodules are transported downstream a few km forming a levee on a modern creek beach. So, each material, coarse or fine, is transported over a few km: the caliche giving yellow nodules, and the low Mg calcite of the oolitic Pleistocene packstone giving the 0.50 to 1 m thick mud lining the bottom and the banks of the eastern Andros creeks. Quantitative results are given in Table 2 and Fig. 7. For comparison, Middle Bight core results have been plotted: aragonite reaches 74% at the surface in this marine-related environment. Thus, the layer of mud,

2

11.7 16.1 20.4

0.5 to 1 m thick, inside eastern Andros creeks is derived from Pleistocene limestone. 3.3.2.2 Biogenic carbonate sedimentation inside small karstic basins: modern high Mg calcite and non dolomitic crusts. Waters of various salinities can differentially fill small karstic basins according to their location relative to the marine waters of the creek and the water table of the phreatic lens. Winter is the dry season, and often these small karstic basins are dry and filled with dry carbonate sediments. The upper surface of these sediments is made of a lithified crust, either surficial, or located under a dry anabiotic stromatolitic cushion. According to their distance from the creek water some of these sediments can also be wet and soft. All these sediments, wet or dry, have vertical desiccation cracks, indicating the alternation of wet and dry conditions and so of emersions (Fig. 4b, c and g). The mineralogical data indicate an average of 96 to 100% high Mg calcite (defined by 7 to 13% Mg). The 0 to 4% aragonite is located inside a thin bed of trapped mud at the surface of the algal mat, probably deposited during a high-energy climatic event such as a hurricane and not during an ordinary storm because of the great distance from the shore. Mineral tubes around cyanobacteria are visible using the light microscope as well as using SEM, showing that they are composed of high Mg calcite rhombs.

16

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

17

Fig. 8. Construction of Rangiroa atoll (Tuamotu), one of the largest atolls of the Pacific Ocean, modified from Bourrouilh-Le Jan (1990, 1996), showing the importance of karstic surfaces in the building of the carbonate substrate of an atoll and the importance of high-energy events (hurricanes and tsunamis) for the morphology of the atoll rim, sediment transport and for lagoonward sediment migration.

4. Modern karstification on an atoll: Tuamotu Archipelago (French Polynesia) 4.1. Aragonite lithification The atoll morphology can be considered as the opposite model of the Bahamian platforms. Their dimensions are reduced to only a few km in diameter, up to 80–90 km maximum in length. They are characterized by the presence of a coral-dominated biocoenosis (giving later the biofacies) located

between 0 and 60 m water depth on their periphery or inside the lagoon on pinnacles, producing a carbonate sediment composed of 90% aragonite. As on the Bahamas, the same phenomena of lithification and solution can be observed on atolls and may be amplified due to the local climate because these small platforms are still located on hurricane tracks. Fig. 8 reminds us of the different elements of atoll morphology (shown for the Rangiroa atoll, but typical of nearly all atolls, Bourrouilh-Le Jan, 1990, 1994).

Fig. 7. Consequences of karst initiation on the 125-ka Bahamian limestone. (a–e) SEM facies of detrital carbonate sedimentation with low magnesian calcite from the east coast of Andros, coming from the 125-ka reworked deposits. (f) Comparison with modern marine aragonitic facies (Middle Bight). Depth of core given in cm. (a) Core 58201, Station 46, 5 cm deep. General view of the sediments. (b) Core 58199, Station 43, 4 cm deep. Numerous cyanobacterial tubes. Absence of aragonite needles. (c) Core 58202, Station 49, 2 cm deep. Detail of the sediments. Abraded grains, neither euhedral, nor anhedral. (d) Core 58202, Station 49, 2 cm deep. Few organic debris and abraded low magnesian calcite nanograins. (e) Core 58201, Station 50, 24 cm deep in the core. Detrital carbonate SEM facies. (f) Core 58372, Station 36, 5 cm deep in the core. For comparison, aragonite needles of an open marine environment (Middle Bight).

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

The atoll rim, more or less emergent, is composed of successive sandy islands (or ‘motu’ in Polynesian), alternating with ‘hoa’ where only high tides penetrate the lagoon (Fig. 9a and b). This atoll rim, 200 to 500 m wide, surrounds a lagoon 10 to 40 or 60 m deep filled with marine water. The atoll rim can be dissected by deep boat channels or passes (‘ava’ in Polynesian). The ‘motu’, or sandy islands, are themselves regularly spaced with ‘hoa’, previously defined. They are interpreted as the sedimentary results of high-energy meteorological or seismically generated events (hurricanes or tsunami) which provoke an upward surge of seawater of nearly 10 m, accompanied by wave trains over a period of several hours (Bourrouilh-Le Jan and Talandier, 1985). Each sandy island is also encircled on its seaward side by high-positioned beachrocks often fossilising mid-Holocene in-place corals (dated to 4000 yr BP) as demonstrated in Mataı¨va and Tikehau (Bourrouilh-Le Jan, 1990, 1993, 1996) (Fig. 9c). In northwest Tuamotu, karstic remnants of the previous and underlying carbonate platform pierce up through modern deposits. Called ‘feo’ by the Polynesian people, they are karstic, usually dolomitic peaks, from a few metres (Mataı¨va atoll) to several tens of metres high (Tikehau atoll) and dated as Early Miocene (Bourrouilh-Le Jan and Hottinger, 1988) (Figs. 8 and 9d). The effects of early diagenesis on the atoll are numerous and are the direct consequences of (1) the intertidal location of the atoll rim, (2) the geographical location of the atoll on hurricane tracks and, consequently, the high rainfall, and (3) its geological setting on the Pacific Plate, on all the tsunami tracks

19

coming from Peru, Chile, Tonga or Japan, Kamchatka or the Kuriles Islands, etc. (Fig. 8). 4.1.1. Modern beachrocks on Rangiroa atoll and their pore waters (northwest Tuamotu) Modern beachrocks can be found in situ on French Polynesian atolls by digging to a depth of 60 cm on modern beaches (personal observation made in 1974 during an intercyclonic period). In the course of the year 1983, five hurricanes passed over the Tuamotu Archipelago and swept the beach zone clear of loose sediment and disturbed the process of lithification inside the beach although this is on its way to being restored within the new beach. There is a close relationship between beachrock cementation and the chemistry of the surrounding water. On Rangiroa atoll, pore water analyses have shown that ions are not in the same proportions as in seawater. Ionic dilution–concentration coefficients show that beachrock pore waters are enriched in Ca2C , and very much depleted in HCO3 compared to surrounding oceanic water. KC , NaC and SO24 were slightly depleted with respect to normal seawater. These evaluations reflect the complexity of diagenetic phenomena which seem to involve, in a combined action, aragonite solution and precipitation: aragonitic bioclast solution and cementation by aragonite. 4.1.2. Hurricanes and lithification: a process of exposure to high evaporation rate Geological studies of the effects of the passage on the atoll of numerous hurricanes in 1983 reveal

Fig. 9. Solution and cementation processes linked to high-energy events (hurricanes, cyclones and tsunamis) on the rim of the atolls of the northwest Tuamotu archipelago (SE Pacific). (a) Successive sandy islands (or ‘motu’ in Polynesian), alternating with ‘hoa’ where only high tides penetrate the lagoon, Rangiroa atoll, north fac¸ade. (b) Idem, south fac¸ade. (c) Each sandy island is encircled on its seaward side by high-positioned beachrocks often fossilising mid-Holocene in-place corals (dated here to 4000 yr BP), seen here on the north coast of Mataı¨va atoll. (d) Karstic remnants of the previous and underlying carbonate platform pierce up through modern deposits. Called ‘feo’ by the Polynesian people, they are karstic, usually dolomitic peaks, from a few metres to several tens of metres high and are Early Miocene in age (Bourrouilh-Le Jan and Hottinger, 1988), seen here on Tikehau atoll. (e) During high-energy events, new ‘hoa’ are formed and they are only active during tsunamis or hurricane storm-surges; aerial photograph of the east-northeast fac¸ade of Mataı¨va atoll, after the passage of five hurricanes during the year 1983: ‘hurricane hoa’. (f) Effects of five hurricane passages during the year 1983 on the Tikehau atoll: sweeping of the beach and lagoonward migration of the sandy island (motu) with the destruction of coconut plantations. (g) Called ‘dalle conglome´ratique’, i.e. ‘coral-rubble paving’, this is the result of two processes: high-energy events (hurricanes and tsunamis) and post-depositional lithification. Lithification is related to its newly exposed (shown by its white colour) post-hurricane position due to hurricane sediment-denudation action, Mataı¨va atoll. (h) Early solution of coral sediments inside brackish ponds of Tikehau atoll. There is a huge shallow pond where the sandy island (‘motu’) used to be, filled with brackish water, surrounded by a semicircle of higher beachrocks and filled with stromatolites. In these brackish ponds, aragonite is dissolved and there is no cementation; the ponds are the emerged part of the DGH freshwater lens contained inside the atoll rim.

20

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 10. Lithification and effects of the 1983 hurricanes on northwest Tuamotu. (A) Lagoonward migration of the Avatoru sandy island (‘motu’) after the 1983 hurricanes, which caused the sweeping of sediment from the upper part of the sandy island and the exposure of its lower coral rubble pavement (‘dalle corallienne’) to the atmosphere. The lithification of the coral rubble is thus enhanced. (B) Stratigraphic position of the coral rubble pavement, covering the Mid-Holocene dated corals of Mataı¨va. Under the combined effects of seawater impregnation, runoff of meteoric waters, solar heating and evaporation, this lower part, made of coarse coral rubble, is slowly lithified.

several different points. Besides the usual hurricanerelated observations such as a general drop of the top of the beach due to the removal of sands, new ‘hoa’ (passage of marine waters only during high tide and towards the lagoon) related to the passage of hurricane floods are formed (Fig. 9e) and the latter are only active during tsunamis or hurricane stormsurges (those which result in a rise of sea level, 10 m above average, Bourrouilh-Le Jan and Talandier, 1985; Bourrouilh-Le Jan et al., 1985b). Other effects include the build-up of sedimentary levees, the transport of sedimentary materials ranging in size from blocks to sand, mud and organic matter (as vegetal clasts). Facies distributions are also strongly modified with the destruction of cyanobacterial mats and the formation of intraclasts, the transport of marine shells such as small Tridacna to coconut plantations. Moreover, it was possible to observe the lagoonward

movements of the sandy islands (‘motu’, Fig. 9f). In northwest Tuamotu, the lagoonward movement of sediment bodies and landforms was estimated to have amounted to between 4 and 30 m horizontally in 1983 only (Fig. 10A) when 5 hurricanes passed over northwest Tuamotu. This landward movement of sandy islands does not affect the position of shorelines at low tide located on or just under the algal ridge. As soon as the coarse coral rubble at the base of a sandy island is exposed on the oceanic side of an atoll and as it is located in the supratidal zone, it is exposed to solar warming and night-time cooling. Daytime warming causes the evaporation of porewaters and provokes the lithification of the coarse and loose basal deposits of the sandy island, transforming them into a massive coral conglomerate, well known in other denuded parts of the atoll but

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

usually misinterpreted (Fig. 9g). Called ‘dalle conglome´ratique’, i.e. ‘coral-rubble paving’, it is the result of two processes: high-energy events (hurricanes and tsunamis) and post-depositional lithification because of its newly exposed post-hurricane position due to sediment-denudation action of hurricanes. In Mataı¨va atoll for example (Fig. 10B), a rapid evaluation has shown that 25 cyclonic periods are necessary to cause the sandy islands to move 100 m towards the internal lagoon. This 100 m distance is frequent in some atolls between the top of the algal ridge and the margins of the sandy depositional island. Despite falling sea level and because of hurricane-forced sediment movement, there is a lagoonward movement of sediments and sandy islands. We also have to consider that the topographical deposition conditions on the atoll rim are specific and that beach facies and sandy islands are in fact deposited on a narrow 200-m-wide platform (the atoll rim) in between the very deep ocean and the deep lagoon (on the big atolls). The only trace of falling sea level, apart from 14 C dates, are the lithified parts of the beach left behind, i.e. successive lines of old beachrocks (10 lines can be counted on Tikehau’s north coast). 4.2. Modern aragonite solution on the atoll 4.2.1. At the atoll rim The early solution of coral sediments has been observed precisely inside brackish ponds of Tikehau

21

atoll (Fig. 9h) and can be seen on other atolls (Rangiroa, northwest Tuamotu). The transverse cross-section of the Tikehau atoll rim through the Teavatia or Matiti sandy islands (northwest of the atoll) shows that in the place of the sandy island (‘motu’) exists a huge shallow pond, filled with brackish water (Fig. 11), surrounded by a semicircle of higher beachrocks and where orange cyanobacteria are abundant. The cyanobacteria belong to the Oscillatoria group (Defarge, 1983; Defarge and Trichet, 1984, 1985). The brackish pond is situated on the atoll rim in place of a sandy island. Its water table is in hydrostatic equilibrium with the sea level and the phreatic lens located inside the atoll rim (DGH lens). The orange cyanobacteria in the ponds form mats which cover a black carbonate mud which both contain only Soritid foraminifera (Fig. 12), and these forams clearly show the early diagenetic phenomena characterized by aragonite solution and the preservation of low Mg calcite (originally high Mg calcite) and so the early loss of Mg2C in solution. These ponds are usually protected from the close marine flooding by the presence of old beachrocks which represent the only remains of previous sandy islands. Aerial photographs show that the inland part of the sandy island has been excavated by high-energy hydrodynamic events (in this case, the 1906 hurricane track which passed over Tikehau) and aided by the absence of lithification in this part because of the small freshwater lens which exists under each sandy island (Fig. 13).

Fig. 11. Early diagenesis with solution of aragonitic coral material in the place of a previous sandy island (‘motu’) on Tikehau atoll (northwest Tuamotu, French Polynesia): a freshwater pond is forming in relation with the phreatic lens located in the atoll rim.

22

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 12. Detail of the cyanobacterial mat called ‘copara’: an accumulation of cyanobacteria and chlorophytes in the freshwater ponds of the atoll rim of Tikehau atoll (northwest Tuamotu, French Polynesia).

4.2.2. In the lagoon: solution and modern biodegradation of all carbonates; the meromictic lake of the almost-atoll Clipperton The hydrology of the Clipperton lagoon is remarkable due to several specific characteristics: a surface salinity of 4 g=l, 6 g=l at a depth of 12 m deep and 33 g=l below (Niaussat et al., 1970; Niaussat, 1978; Bourrouilh-Le Jan et al., 1985a). In addition, it exhibits a high concentration of O2 falling rapidly with depth, a high concentration of H2 S until toxic and lethal values are reached (100 mg=l below 15 m depth) and a high pH of surface waters falling rapidly with depth from and beyond 15 m (Fig. 14). However, the restricted characteristic of this meromictic lake cannot by itself explain the presence of some particular ions in the water. On the other hand, all of the physico-chemical parameters of this restricted enclosed lagoon are responsible for aragonite solution, apatite and double carbonate formation (magnesite, MgCO3 , and kutnahorite CaMg=Mn(CO3 )2 ). The presence of these three minerals has been confirmed by two different laboratories; details of this discovery in a Pacific lagoon in Bourrouilh-Le Jan et al. (1985a). Along the lagoon shorelines, erosion has already been noted by Sachet (1962). A succession of steps surrounds the surficial freshwater of the lagoon and a cliff of 0.80 to 1 m in height can exist. Moreover, tilted broken blocks coming from the indurated coral rim fall into the lagoon waters. As a consequence, the coral rudite of the atoll rim is dissolved, the results of which can be observed at various scales. We have studied the solution processes by means of a field

survey on the island, examination of hand samples with a light microscope and by using the SEM. Island scale. Comparison of vertical aerial photographs dating from 1935 with modern ones and direct aerial observation, shows clearly that the lagoon shoreline has retreated towards the ocean. On land, this retreat is clearly indicated by the presence of the previously described erosional cliff (0.80 to 1 m high). Sample scale. Like Sachet (1962); Obermuller (1959) observed that the coral blocks which had fallen into the lagoon were nearly completely dissolved and that sometimes only their moulds remain in phosphate indurated mud. SEM scale. Marine molluscs from the lagoon bottom are of course dead, and include Pinna, Codakia, Lithophaga (Niaussat, 1978). The shells have no more colour; they are very friable (chalky) and have sometimes lost some part of the valves. SEM studies show the presence of micro-cavities which could be attributed to the borings of micro-organisms such as algae, bacteria or fungae (Fig. 15). Boring microorganisms disorganise the shells and more precisely the nacreous layer with microplate curvation and disorganisation and the fragmentation of the other aragonitic needle layers (Bourrouilh-Le Jan et al., 1985a). 4.3. Geological and geomorphological consequences 4.3.1. High carbonate islands of the Pacific: alteration of Miocene to Pliocene deposits In the Pacific Ocean, three types of islands are found: atolls or low carbonate islands (like Bikini,

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

23

Fig. 13. Karst initiation on a modern atoll: results of the combined action between ‘high-energy sedimentation and carbonate solution’ on the coral rim of an atoll (Tuamotu). (A) Cross-section of the atoll rim characterized by a ‘motu’ (sandy island), an additional and repetitive hurricane deposit. (B) Cross-section of the atoll rim where motu are replaced by freshwater ponds but are still surrounded by mid-Holocene beachrocks: the coarse and unlithified coral rubble has been swept off by hurricane passages and the DGH phreatic lens located inside the emerged part of the atoll rim provokes the presence of a freshwater pond where cyanobacterial mats develop. An intense solution of coral bioclast aragonite occurs which forms a modern karstic solution surface under each pond.

24

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 14. Clipperton, almost-atoll (east-northeast Pacific), an example of an atoll located 5ºN, outside the hurricane belt but receiving high rainfall which has transformed the marine lagoon into a meromictic lake with stratified waters. This lake is characterized by low oxygenation and high H2 S (lethal value) from 5 m and an organic decay layer at a depth of 20 m. Contrary to other atolls (for example, northwest Tuamotu atolls), the lagoonal shorelines of Clipperton are being dissolved by the freshwater surface layer of lagoon waters. Solution of the bottom marine aragonite sediments previously formed when the lagoon was marine also occurs (Bourrouilh-Le Jan et al., 1985a).

Rangiroa, Mururoa), high volcanic islands (like Tahiti, Hawaii) and high carbonate islands with high vertical cliffs, dropping directly into the sea (like Rennell, Nauru, Mare´, Makatea, Fig. 16). The latter are quite unknown and have been incorrectly called ‘uplifted atolls’ for a long time (Darwin, 1842; Dana, 1843). As a matter of fact, they are carbonate platforms from a few km to several tens of km long, from C10 m to C200 m high. The geomorphological features have given to these islands the name of ‘uplifted or raised atoll’. They present a high rim, thought to represent ancient reefs by previous authors, surrounding an inner flat hollow thought to be an ‘ancient lagoon’. The erosional interpretation has been given by MacNeil (1954); Purdy (1974) and by Bourrouilh-Le Jan (1975–1977). From outcrops as old as 24 Ma, these platforms have recorded the global eustatic sea-level variations and movements of the Pacific

plate (Bourrouilh-Le Jan and Hottinger, 1988). As a result, the inner depression is the result not of differential coral growth between the lagoon and reef, but rather of biochemical erosion which has attacked the inner sediments of a carbonate platform, during the progressive tectonic or eustatic emersion of the island. As seen from the Quaternary isotopic curve, first published by Emiliani (1978), more than 30 lowstands of sea level have occurred over the last 700 ka and, during each of them, atoll platforms were emerged about 100 m above sea level, according to the paleo-level reached at sea-level highor lowstands, and, in consequence, karst processes played an important part in modelling the modern atoll shape, reinforced by coral growth during high sea-level stands. On carbonate platforms of the central and southwest Pacific, the observations are consistent with those made in the Bahamas, namely that karstic avens

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

25

Fig. 15. Solution and biologic degradation by micro-organisms (bacteria, cyanobacteria, fungus, etc.) of Holocene-dated pelecypod shells, lying dead on the bottom of the meromictic lake of Clipperton atoll (depth between 5 and 10 m) (Bourrouilh-Le Jan et al., 1985a). (a–d) Progressive dissociation of upper prismatic layers and lower nacreous layers of a Lithophaga hanckoki valve. (e, f) The final stages.

26

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 16. Synthetic cross-section of Mare´ Island, a high carbonate island of the Loyalty Archipelago (New Caledonia) showing the fitting of the different karstic morphologies, emerged or submerged and the karstic surfaces separating deposits dated from Middle Miocene to Upper Pleistocene. Mineralogy and Sr content of carbonates have been indicated to show the importance of karstification through the diagenetic processes (dolomitisation and calcitisation) at the same time as the formation of bauxite- and phosphate-rich soils (from Bourrouilh-Le Jan, 1989, 1990).

or pot-holes of the platform formed during the 125 ka and 205 ka sea-level highstands have now been submerged by the Holocene transgression. In Ouve´a (Loyalty Islands, New Caledonia), three deep sinkholes, or avens, exist, filled up with marine water. South of Ouve´a, in Lifou and Mare´ (Fig. 16), two important high carbonate islands, avens open at C20 to C50 m and their deepest parts reach the phreatic lens (ancient freshwater resource of the island during dryness) (Koch, 1958; Thomas and Charrier, 1988) (Fig. 16). These high carbonate islands show the same

diagenetic diagram as the Bahamian platforms with the dissolution of pre-existing aragonite sediments in the centre and cementation of reef deposits on the periphery of the platform, these two phenomena occurring with the slow emergence of the platform. 4.3.2. Sedimentological consequences: reworked platform deposits Fissure fills are a characteristic of karstified carbonate platforms (Smart et al., 1987) and also of Pacific Ocean high carbonate islands where different

Fig. 17. Examples of karstification, fissures and fissure fillings from different carbonate platforms of the Pacific, called high carbonate islands. Outcrops, polished sections and micropetrography. (a, b) Nauru, Central Pacific. Freshwater deposits with ostracods lying on a karstic solution surface of Lower Miocene dolomitic wackestones. (b) Phosphatised coral and caliche gravels inside a karstic fissure with geopetal figures. Nauru is a high carbonate island well known for its phosphate cover. (c, d) Makatea, Tuamotu, SE Pacific. (c) Geopetal fissure fillings at the base of the Lower Miocene platform. (d) Speleothems dissolved (arrow) by ostracod-rich and brackish deposits and then covered by phosphatised oolites. Makatea is a high carbonate island with a phosphate cover excavated until 1966. (e) Minami Daito Jima, Japan. Mn- and Fe- rich fissure fillings coming from the upper Al- and P-rich soils of the carbonate platform. (f) Tikehau atoll (northwest Tuamotu). Mid-Holocene beach deposits inside the Lower Miocene karstic tower (‘feo’) of the atoll. (g) Mare´, Loyalty Is. (New Caledonia, southwest Pacific). Mn-rich carbonate sediments inside fissures, coming from an overlying bauxite- and phosphate- rich soil. (h) Rennell, Solomon Is., southwest Pacific. Micrite (with detrital dolomitic rhombs plus rotaliids and ostracods) filling fissures within the dolomitic Middle Miocene, high carbonate island with a bauxite- and phosphate-rich soil. For detailed studies, see Bourrouilh-Le Jan, 1996.

28

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

fills can be found (Bourrouilh-Le Jan, 1982b, 1990, 1996): reworked dolomitic crystals (e.g. Makatea, Tuamotu), Mn coloured carbonate mud (e.g. Rennell, Salomon Is.) or phosphate deposits (e.g. Nauru and Makatea) (Fig. 17). These facts have been established by looking at vertical sections and by systematic sampling, every 1 m or every 5 m, by the presence of a karstic solution surface between different carbonate facies or by the study of different types of cementation and fissure filling. 5. Discussion of karst initiation: lithification and solution processes 5.1. Aragonite lithification In the subtidal and intertidal zones of tropical and shallow water carbonate platforms, lithification occurs by the precipitation of aragonite within carbonate mud or within oolitic sands as pore filling cement. The cement is composed of aragonite needles which form radially around aragonitic particles (Dravis, 1979). At this very early stage, no cement is formed around high magnesian calcite bioclasts (e.g. foraminifera), showing the interaction between the nucleus and the cement in a known fluid (Bourrouilh-Le Jan, 1977, Table 2 and Plate IV). In the tidal and supratidal zones, on high-energy coastlines, mid-Holocene beachrocks are cemented with aragonite which can be tracked with the ionic concentration in pore water, but, at the same time, a combined action already dissolves aragonite bioclasts. Between the Tropics, because of the presence of hurricanes, lithification is accelerated on the atoll rim because of the sandy island denudation and the lithification of coral conglomerates is accelerated by strong evaporation rates and warming–cooling cycles (Fig. 10). These temperature alternations might influence carbonate supersaturation and CO2 pressure and degassing (Hanor, 1978). Still in a tropical zone but now in a muddy environment, besides early dolomitisation which happens in relation to mesohaline and oligohaline lakes, an early aragonitic lithification occurs inside aragonitic mud at the boundary between freshwater and marine water.

5.2. Chemical solution and biological degradation of carbonates In Clipperton atoll (east Pacific), bioclastic aragonite disappears from the lagoon sediments because of the chemistry of lagoon waters, where the lagoon has become a meromictic lake with stratified anoxic waters (like in Bahamian blue holes) and because of borings and decaying by micro-organisms. At the same time, magnesite, kutnahorite, dolomite (?) and apatite appear in the intertidal sediments of the newly meromictic lake (Fig. 13). In other models, the solution of modern as well as Pleistocene aragonite can occur on tidal flats (e.g., at Fresh Creek, east of Andros, Atlantic Ocean, Fig. 7 and Table 2) or within coarse coral rudite deposits of atoll rims (e.g., Tuamotu, Pacific Ocean, Fig. 13). In the extreme model of Clipperton, all the biogenic carbonate minerals can be dissolved because a meromictic lake occurs in place of a marine lagoon. The consequences of early aragonite solution are geomorphologic features with vertical solution pits, with ceiling collapse structures called blue holes, cenotes or avens according to the country. Because of the total absence of rivers, and the high porosity of the rocks, these vertical pits act as the drainagetubes for the phreatic balance along the up-and-down movements of the phreatic lens during the Quaternary for the recent studied models of carbonate tropical platforms (Fig. 5). 5.3. Modern asymmetrical sedimentation We have shown asymmetry in the distribution of sediments around Andros Island on the Great Bahama Bank because of the absence of marine aragonite sediment input on the east coast of Andros Island. The main sedimentological and hydrological features are summarised on Fig. 18 with the principal points: (1) large marine sedimentary input on the western and southern side, versus no or hardly any marine sedimentary input on the eastern side; (2) solution versus lithification; (3) marine biogenic aragonite on the western side, versus detrital low Mg calcite from meteoritic diagenesis (125 ka, Pleistocene) on the eastern side; (4) marine biogenic aragonite on the western side, versus freshwater stromatolitic high Mg calcite on the eastern side.

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

29

Fig. 18. Beginning of karstification on a modern carbonate platform: example of Andros Island, Great Bahama Bank. Diagram of sedimentary and diagenetic (non-dolomitic) phenomena on Andros Island showing the combined action of solution and lithification on a carbonate platform at the interface between the atmosphere and the ocean. Map (top) and cross-section (bottom). It can be observed that subtidal and supratidal aragonite lithification occurrs on the rim of the platform and aragonite solution occurs in the centre of the Holocene supratidal deposits and 125-ka-old emerged Pleistocene. These early karstic phenomena act at the same time as detrital carbonate sedimentation and stromatolitic high magnesian calcite sedimentation.

30

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

5.4. Shallow water carbonate platforms, atolls, hurricanes and tsunamis: construction of high-energy sedimentary bodies In two different sedimentary environments, we have demonstrated the building up of high-energy sedimentary bodies, called ‘hurricane and=or tsunami trails’ (Fig. 19). The first described is in the supratidal muddy environment of the western Andros mangrove where the main statistical trail direction coincides with the main direction of hurricane tracks and happens statistically once every 5 years, considering the area, 100 and 200 km in diameter, centred on Casuarina Point (Fig. 2). The second model concerns the reef-derived materials, from giant blocks to sand and mud, building up the sandy islands (motu), spread repeatedly all along the rim of the atoll (Fig. 8). In both cases of high-energy build-ups, the sediments are transported from the subtidal environment of the carbonate factory (the Great Bahama Bank for the first and the atoll outer reef for the second) to the supratidal zone where carbonate diagenesis starts (see below). 5.5. Solution=lithification Finally, in the platform model or on oceanic carbonate islands, modern sedimentary environments such as the Great Bahama Bank (Newell et al., 1959; Milliman, 1974) are asymmetrical because of the interference of hurricanes with shallow water carbonate sedimentation. This action occurs at the interface between the ocean and the atmosphere. As a result, asymmetrical aragonite diagenesis exists, characterized by solution and lithification because of the great volume of freshwater poured on these shallow environments under the control of hurricanes. This asymmetrical diagenesis is related to the contact between fluid zones and results in differential erosion of an annular type (Fig. 18). On Pacific atoll rims (Fig. 13), early diagenesis on the northwest Tuamotuan atoll rim can be detected in several environments and in several ways: (1) Early lithification by aragonite cement within beach deposits forms an oblique succession of numerous beachrocks because of the slow fall of sea level to the modern position: lithification accompanies the

up-and-down movement of the sea-level curve. (2) Early lithification on the pavement of the reef flat occurs because of the removal of its coarse coral material by high-energy events such as hurricanes and tsunamis. (3) By aragonite solution within the sedimentary material on the hurricane trail (‘motu’, sandy island) because of its location in the supratidal and ‘continental’ zone and the presence of the freshwater DGH lens. These sedimentary and diagenetic processes are all contemporaneous. They have been acting for the last 4000 years on the atoll rim. They are the direct consequence of the low emergence of the atoll rim since the mid-Holocene. If these solution and lithification processes affect a whole carbonate platform (Fig. 18) such as the Great Bahama Bank, the lithification processes act near sea level, at the boundary between sea and land, freshwater and seawater, but always within the marine domain. This zone of lithification has tracked the Holocene sea level. At the same time, the centre of the platform is subjected to very early solution. The final result is early lithification along the rim of carbonate platforms and solution in the centre of shallow water carbonate platforms (Fig. 18). These two early diagenetic phenomena occur at sea level and if this sea level varies under the combined effects of eustasy and tectonics, then lithification and solution will follow sea-level variations (Fig. 5). 5.6. Karstification and diagenesis on high carbonate islands Fissure fillings are traces of the constant reworking affecting the walls of karstic or tectonic fissures and the surface of the platforms. Formed during highstands of sea level or emerged due to tectonics, the high carbonate islands have recorded the up-anddown movements of sea level as well as those of the phreatic lens leading to dolomitisation, bauxite concentration or phosphate formation (Figs. 16 and 17; Bourrouilh-Le Jan, 1982b, 1989). Thus, modern atolls and high carbonate islands illustrate and are different steps of the evolution of Tertiary platforms at different periods of their evolution and with different speeds of formation.

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

31

Fig. 19. Comparison between two high-energy sedimentary bodies. (A) In tropical Atlantic zones and carbonate mangrove facies, the hummocks or hurricane trails are 0.5 to 1 km wide and several km long. (B) In tropical Pacific zones and in coral reef facies, the Polynesian ‘motu’ or sandy island. There is a parallel between the two constructions: the subtidal source of biogenic carbonate, trapping the supratidal zone and the elongate shape of the sedimentary body.

6. Conclusions For oolitic sands in the Bahamas, in the intertidal zone the water chemistry on the Joulter’s Cays area

shows low concentrations of Ca2C and HCO3 . Aragonite feeds oolites and lithifies their deposits located as rims around carbonate platforms (Dravis, 1979; Cros, 1979).

32

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

On the tidal plain of the western Andros mangrove, a mineralogical zonation develops around each lake and around each hurricane trail (hummocks). The high magnesian calcite is linked to the presence of pisolites and stromatolites (cushionlike stromatolites adjacent to brackish channels or estuaries and veil-like algal mats around the hyperalkaline and low salinity lakes). These stromatolites are enriched in elements like Pb, Cu, Cr and Ba (Bourrouilh-Le Jan, 1990). Lithification occurs deep inside the sediment on the periphery of the tidal plain (Fig. 2). For the tidal plain of western Andros, there is at the same time aragonite solution in the tidal basin zone (e.g., Turner Sound) and early diagenesis on the rim of each body of water with dolomitisation and high Mg calcite appearance. Diagenetic events, hydrology and ecology are all linked to each other. For the eastern side of Andros, a brief survey of the different blue holes or avens shows that they become deeper towards the margin of the platform. The bottom of the aven is related to the upper level of a previous phreatic lens which is a convex surface linked to sea level on each side (Fig. 5). This phreatic lens followed sea-level variations and thus its surface is now drowning previous karstic avens (Fig. 5), i.e. the water table moves upward and downward with the rising and falling of sea level. In the Fresh Creek area, aragonite of marine origin decreases from the creek mouth landward until it disappears totally (Fig. 6): (1) the percentage of aragonite in sediment is always low, compared to a marine environment even in the bights (Middle Bight, salinity 34.4 g=l); (2) with changing environment from marine to brackish areas, high Mg calcite increases first, then disappears totally in the creek sediments; (3) 2% of quartz recorded at station 50 in the creek sediments (Table 2) can be related to siliceous diatoms; (4) the most striking fact is the constant increase from marine to marsh and pond environments of low Mg calcite, reaching 99% in the sediment. SEM studies show the presence of heterogenous low Mg calcite mud composed of calcite grains with a rounded shape, with various grain sizes from 15 to 0.1 nm (Fig. 7), indicating the reworking phase undergone by Pleistocene calcite. Thus, on the 125 ka carbonate packstone, when marine sediment input is low, early karstogenesis

forms not only a typical young karstic surface but also modern purely detrital calcitic deposits, derived from and overlying the Upper Pleistocene deposits. In the few small karstic basins of eastern Pleistocene Andros, the sediments which are forming are essentially of cyanobacterial origin and are composed only of high Mg calcite without any trace of dolomite or aragonite except for thin hurricane-derived beds of aragonite trapped between two cyanobacterial growth zones. On Pacific atolls, beachrock cementation can also be tracked by ion concentrations in pore waters. In northwest Tuamotu, the sandy island (‘motu’) has undergone slow migration towards the lagoon, revealing its bare basement formed by a coarse poorly lithified coral accumulation which is slowly being lithified by the effects of atmospheric agents (solar warming, seawater saturation, rain circulation and freshwater lens evaporation). In other parts of the atoll, strong tempests, hurricanes or tsunamis have cleared the upper surface of the atoll rim of the unconsolidated material (‘motu’) which covers it. Only the hard midHolocene beachrock is preserved as a half-crownlike ridge from 1 to 1.5 m above the brackish water of the ponds (Figs. 11 and 12). This freshwater does not allow the precipitation of aragonite and on the contrary, tends to dissolve it, so that coarse coral material stays mobile and uncemented (Fig. 13). In equatorial atolls, CaCO3 is disappearing from all the sediments through chemical and biological means and by the stratified lagoon waters: by the solution of bioclastic aragonite and by biological nibbling of aragonite with micro-cavities, formation of microtubules, resulting in a great brittleness of bioclasts and loss of colour. We have demonstrated that at the same time, this aragonite solution occurs parallel with the nucleation and formation of magnesite and kutnahorite [Ca Mn=Mg (CO3 )2 ], dolomite and apatite (Fig. 14). Because of their location inside the tropical belt where hurricanes are active, and because of their shallow bathymetry, carbonate platforms and atolls reveal: (1) the transport action of high-energy events (Fig. 18) towards their supratidal zone of sedimentary material formed in the shallow subtidal zone of the ocean, and (2) the chemical action on the same material through meteoric waters (Figs. 18 and 19).

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Fig. 20. Hurricane tracks (from Crutcher and Quayle, 1974) for both hemispheres and the position of modern shallow-water carbonate platforms inside the tropics where major shallow-water carbonates are currently forming; reef biocoenosis limited by 18ºC winter isotherms in the Northern Hemisphere (A), and in the Southern Hemisphere (B). Between 5ºN and 5ºS, hurricanes have little or no influence but there is heavy rainfall, thus karstification starts and acts alone and deep lagoons are formed by solution (e.g., Clipperton atoll). North of 5ºN and south of 5ºS, hurricanes frequently pass (one every 5 year in both cited examples, Bahamas (Bourrouilh-Le Jan, 1978a) and French Polynesia (Bourrouilh-Le Jan and Talandier, 1985) and with rain for the phreatic lens and diagenesis, the lagoons of small atolls are filled with sediments removed from the carbonate factory into the lagoon which becomes a repository and a trap for Holocene sediments (e.g., Mataı¨va, northwest Tuamotu).

33

34

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Despite their location inside the Tropics, hurricanes have no action on a narrow equatorial zone between 5º–10ºN and 5º–10ºS (Fig. 20). In this zone, the transport of shallow water carbonate sediments from the reef tract factory to the lagoon is not occurring as in other atolls located in higher latitudes (both hemispheres). Intense equatorial rain falls remain which can modify the lagoon waters as in the example of the Clipperton atoll. In conclusion, according to their latitudinal location (Fig. 20), shallow carbonate platforms will be well fed with sediment from the shallow water carbonate bio-factories if they are roughly between 10ºN or S and 30ºN or S. But if they lie in the equatorial belt between 5º–10ºN and 5º–10ºS, they are not well fed and receive, on the contrary, meteoric waters in great quantities so that meromictic lakes and aragonite solution can start a strong karstification. All these modern phenomena allow us to understand the geological evolution of high carbonate islands. Dating from the Early Miocene, these shallow carbonate platforms have undergone strong karstification together with carbonate diagenesis (dolomitisation and calcitisation) and mineral concentrations (bauxite and phosphates). Acknowledgements We thank Elf Aquitaine, TOTAL, Fondation Singer-Polignac, Fondation Cousteau, Muse´um National d’Histoire Naturelle de Paris and Ecole Pratique des Hautes Etudes, for helping and assisting us during these studies. I am grateful to Pr. I.J. Fairchild, Dr. A.K. Satterley and Dr. L. Korpas for editing the paper and greatly improving the manuscript by careful reading and discussions. References Agassiz, A., 1894. A reconnaissance of the Bahamas and of the elevated reefs of Cuba in the steam yacht ‘Wild Duck’, January to April 1893. Bull. Museum Comparative Zoo¨logy, Harvard College, XXVI, 1, 203 pp. Back, W., Hanshaw, B.B., Van Driel, J.N., 1984. Role of groundwater in shaping the eastern coastline of the Yucatan Peninsula, Mexico. In: La Fleur, R.G. (Ed.), Groundwater as a Geomorphic Agent. Allen and Unwin, London, pp. 282–293. Benjamin, G.J., 1972. Diving in the blue holes of the Bahamas. Nat. Geogr. Mag. 138, 347–363. Black, M., 1933. The precipitation of calcium carbonate. Geol.

Mag. 52, 455–466. Bourrouilh-Le Jan, F.G., 1972. Diagene`se re´cifale: calcitisation et dolomitisation. Leur re´partition horizontale dans un atoll souleve´. Ile Lifou. Territoire de la Nouvelle Cale´donie. Cah. ORSTOM, Se´r. Ge´ol., IV 2, 121–148. Bourrouilh-Le Jan, F.G., 1974. Donne´es ge´omorphologiques sur la re´gion de Fresh Creek, Ile Andros, Bahama. Mar. Geol. 16, 213–235. Bourrouilh-Le Jan, F.G., 1975–1977. Ge´omorphologie de quelques atolls dits ‘souleve´s’ du Pacifique W et SW. Origine et e´volution des formes re´cifales actuelles. Me´m. B.R.G.M. 89, 419–439. Bourrouilh-Le Jan, F.G., 1976. Se´dimentologie et pe´trographie se´dimentaire du Ple´istoce`ne bahamien: re´gion de Fresh Creek, Ile Andros, Bahama. In: Causse, R. (Ed.), VIIe Conf. Ge´ol. des Caraı¨bes. Serv. de l’Ind. et des Mines Re´gions Antilles, Guyane Franc¸aise, pp. 285–292. Bourrouilh-Le Jan, F.G., 1977. Les particules du Ple´istoce`ne bahamien et de sa crouˆte: se´dimentologie et diagene`se. Sediment. Geol. 17, 247–284. Bourrouilh-Le Jan, F.G., 1978a. Roˆle des ouragans et des cyclones tropicaux sur la se´dimentation carbonate´e: la plaine d’estran de l’Ouest d’Andros (Bahama). Interfe´rences de la climatologie, de l’hydrologie et de la diagene`se. C.R. Acad. Sci. Paris, Se´r. D 287, 907–910. Bourrouilh-Le Jan, F.G., 1978b. Hydrologie des nappes d’eau superficielles de l’ıˆle Andros, Bahama. Rapport avec la diagene`se. Rapport Interne Elf–R.E., mars 1978, 35 pp. Bourrouilh-Le Jan, F.G., 1978c. Bahamian tidal Flats hydrology, first results. NW Andros. Relationship with dolomitization. Sixth Meeting of Carbonate Sedimentologists, Liverpool, Abstr., p. 7. Bourrouilh-Le Jan, F.G., 1979. Hurricanes and rainfalls. Key for dolomitization in the tidal flats of western Andros, Bahamas. Bull. Am. Assoc. Pet. Geol. 63, 422–423. Bourrouilh-Le Jan, F.G., 1980. Hydrologie des nappes d’eau superficielles de l’ıˆle Andros, Bahama. Dolomitisation et diagene`se de plaine d’estran en climat tropical humide. Bull. Cent. Rech. Explor. Prod. Elf Aquitaine, Pau 4 (2), 661–707. Bourrouilh-Le Jan, F.G., 1981. Hurricane supratidal mud bodies on the tidal flats of western Andros. Geol. Soc. Am., Penrose Conf., Controls of Carbonate Platform Evolution, Abstr., p. 4. Bourrouilh-Le Jan, F.G., 1982a. Ge´ome´trie et mine´ralogie des corps se´dimentaires dans une mangrove carbonate´e sous l’influence des ouragans, ˆıle Andros, Bahamas. Me´m. Soc. Ge´ol. Fr., N.S. 144, 77–92. Bourrouilh-Le Jan, F.G., 1982b. Stages of dolomitization process. Geochemistry and sedimentology of an uplifted atoll in the Sw Pacific and of its bauxitic cover: Rennell, Solomon Islands. In: Inst. Sci. Terre Dijon (Ed.), Livre Jub. Prof. Lucas, Me´m. Ge´ol. Univ. Dijon, pp. 3–20. Bourrouilh-Le Jan, F.G., 1984. Re´cifs quaternaires souleve´s du Pacifique: ge´ochronologie, origine et e´volution des formes re´cifales actuelles et se´dimentologie des facie`s re´cifaux. In: Geister, J., Herb, R. (Ed.), Ge´ologie et Pale´oe´cologie des Re´cifs. Inst. Ge´ol., Univ. Berne, pp. 4-1=4-23. Bourrouilh-Le Jan, F.G., 1989. The Oceanic karsts: modern bauxite and phosphate ore deposits on the high carbonate islands (so-called ‘uplifted atolls’) of the Pacific Ocean. In: Bosak, P., Ford, D.C., Glazek, J., Horacek, I. (Eds.), Pale-

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36 okarst. A Systematic and Regional Review. Elsevier, Amsterdam and Academia, Praha, pp. 443–471. Bourrouilh-Le Jan, F.G., 1990. Diagene`se des carbonates de plates-formes, re´cifs et mangroves en Atlantique et Pacifique. Controˆle de la diagene`se par les variations thermoglacio-eustatique d’e´mersion–submersion. Aragonite, calcite, dolomite. The`se, Doct. Etat, Univ. P.-et-M. Curie, Paris VI, 216 pp., avec 2 documents annexes. Bourrouilh-Le Jan, F.G., 1993. A Mid-Holocene sea-level stand, higher than now, on the Bahamian platforms and the setting up of a hurricane regime on the North-American SE coast. UNESCO–IUGS, Climates of the Past, CIFEG, Occ. Publ. 1993=25, p. 18. Bourrouilh-Le Jan, F.G., 1994. Les re´cifs coralliens: indicateurs de l’environnement et des pale´oenvironnements. In: Maire, R. Pomel, S., Salomon, J.-N. (Eds.), Enregistreurs et Indicateurs de l’E´volution de l’Environnement en Zone Tropicale. Presses Univ. Bordeaux, pp. 275–297. Bourrouilh-Le Jan, F.G., 1996. Plates-formes carbonate´es et atolls du Centre et Sud Pacifique. Stratigraphie, se´dimentologie, mine´ralogie et ge´ochimie. Diagene`ses et e´mersions: aragonite, calcite, dolomite, bauxite et phosphate. Doc. B.R.G.M. 249, 365 pp. Bourrouilh-Le Jan, F.G., Hottinger, L., 1988. Occurrence of rhodolites in the tropical Pacific, a consequence of Mid-Miocene paleo-oceanographic change. Sediment. Geol. 60, 355–357. Bourrouilh-Le Jan, F.G., Talandier, J., 1985. Se´dimentation et fracturation de haute e´nergie en milieu re´cifal: tsunamis, ouragans et cyclones et leurs effets sur la se´dimentologie de la ge´omorphologie d’un atoll: motu et hoa, a` Rangiroa, Tuamotu, SE Pacifique. Mar. Geol. 67, 263–333. Bourrouilh-Le Jan, F.G., Carsin, J.L., Niaussat, P.-M., Thommeret, Y., 1985a. Se´dimentation phosphate´e actuelle dans le lagon confine´ de Clipperton, ENE Pacifique. Datations, se´dimentologie et ge´ochimie. Sci. Ge´ol. Strasbourg, Me´m. 77, 108–124. Bourrouilh-Le Jan, F.G., Talandier, J., Salvat, B., 1985b. Early diagenesis from 6,000 years ago and the geomorphology of atoll rims in the Tuamotu. Proc. 5th Int. Coral Reef Congr., Tahiti 3, 235–240. Broecker, W.S., Thurber, D.L., 1965. Uranium series dating of corals and oolites from Bahama and Florida Key limestones. Science 149, 55–57. Carney, C., Broadman, M.R., 1993. Trends of sedimentary microfabrics of ooid tidal channels and deltas. In: Rezak, R., Lavoie, D.L., (Eds.), Carbonate Microfabrics. Springer-Verlag, New York, pp. 29–39. Cros, P.G., 1979. Gene`se d’oolithes et de grapestones, plateforme des Bahamas (Joulters Cays, Grand Banc). Bull. Cent. Rech. Explor. Prod. Elf Aquitaine 3, 63–139. Crutcher, H.L., Quayle, R.G., 1974. Mariners World-wide Climatic Guide to Tropical Storms at Sea. Direction of Commander, Naval Weather Service Command, Navair 50-1C-61, USA, 61, US Government Printing Office, Washington D.C. Cry, G.W., 1965. Tropical cyclones of the North Atlantic Ocean tracks and frequencies of hurricanes and tropical storms, 1871–1963. U.S. Weather Bur. Tech. Pap. 55, 148 pp. Dana, J.D., 1843. On the area of subsidence in the Pacific, as indicated by the distribution of Corals Islands. Am. J. Sci. Arts 45, 131–145.

35

Darwin, Ch., 1842. The Structure and Distribution of Coral Reefs. Smith Elder, London, 214 pp. Davis, O.K., 1984. Multiple thermal maxima during the Holocene. Science 225, 617–619. Defarge, C., 1983. Contribution a` l’e´tude ge´ochimique et pe´trologique des formations protostromatolithiques de Polyne´sie. Application a` la connaissance des me´canismes de la pre´cipitation des carbonates de calcium au sein des matie`res organiques se´dimentaires. The`se doct. spe´cialite´, Universite´ d’Orle´ans, 155 pp. Defarge, C., Trichet, J., 1984. Microstructure de se´diments d’origine cyanobacte´rienne au sein desquels pre´cipitent des carbonates de calcium. Application a` la compre´hension des me´canismes de la biomine´ralisation. C.R. Acad. Sci. Paris, Se´r. II 299, 711–716. Defarge, C., Trichet, J., 1985. Premie`res donne´es sur la bioge´ochimie des de´poˆts de Kopara de l’atoll de Rangiroa. Proc. 5th Int. Coral Reef Congr., Tahiti 3, 365–370. Doran Jr., E., 1955. Land forms of the southeast Bahamas. Univ. Texas Publ. 5509, 1–38. Dravis, J., 1979. Rapid and widespread generation of Recent oolitic hardgrounds on a high energy Bahamian platforms, Eleuthera Bank, Bahamas. J. Sediment. Petrol. 49, 195–208. Emiliani, C., 1978. The cause of Ice Ages. Earth Planet. Sci. Lett. 37, 349–352. Farr, M., Palmer, R., 1984. The blue holes: description and structure. Cave Sci., Trans. Br. Cave Res. Assoc. 11 (1), 9–22. Field, R.M., 1931. Geology of the Bahamas. Bull. Geol. Soc. Am. 42, 759–784. Friedman, G.M., Amiel, A.J., Schneidermann, N., 1974. Submarine cementation from reefs: example from the Red Sea. J. Sediment. Petrol. 44, 816–825. Gebelein, C.D., 1975. Holocene sedimentation and stratigraphy, southwest Andros Island, Bahamas. 9th Int. Sedimentol. Congr., Nice, The`me 5, 1, pp. 193–198. Gifford, J.A., Ball, M., 1971. Investigation of submerged beachrock deposits off Bimini, Bahamas. Nat. Geogr. Soc. Res. Rep. 12, 21–38. Ginsburg, R.N., 1953. Beach rock in South Florida. J. Sediment. Petrol. 39, 49–69. Goodell, H.G., Garman, R.K., 1969. Carbonate chemistry of Superior Deep Test Well, Andros Island, Bahamas. Am. Assoc. Pet. Geol. Bull. 53, 513–536. Hanor, J.S., 1978. Precipitation of beachrock cements: mixing of marine and meteoric waters vs. CO2 degassing. J. Sediment. Petrol. 48, 489–501. Hardie, L.A., 1977. Sedimentation on the modern carbonate tidal flats of northwest Andros Island, Bahamas. Johns Hopkins Univ., Stud. Geol. 22, 202 pp. Harris, P.M., 1977. Sedimentology of the Joulters Cays Ooid Sand Shoal, Great Bahama Bank. Ph.D. Thesis, Coral Gables, Fla., 451 pp. Harris, P.M., 1978. Holocene marine cemented sands, Joulters Ooid Shoal, Bahamas. Trans. Gulf Coast Assoc. Geol. Soc. 28, 175–183. Harris, P.M., 1983. The Joulters Ooid Shoal, Great Bahama Bank. In: Peryt, T. (Ed.), Coated Grains. Elsevier, Amsterdam, pp. 132–141. Hearty, P.J., Kindler, P., 1995. Sea-level highstand chronology from stable carbonate platforms (Bermuda and the Bahamas). J. Coastal Res. 11, 675–689.

36

F.G. Bourrouilh-Le Jan / Sedimentary Geology 118 (1998) 3–36

Hine, A.C., 1977. Lily Bank, Bahamas: history of an active oolite sand shoal. J. Sediment. Petrol. 47, 1554–1581. Hine, A.C., 1983. Relict sand bodies and bedforms of the northern Bahamas: evidence of extensive early Holocene sand transport. In: Peryt, T. (Ed.), Coated Grains. Elsevier, Amsterdam, pp. 116–131. Hine, A.C., Neumann, A.C., 1977. Shallow carbonate bank margin growth and structure, Little Bahama Bank, Bahamas. Am. Assoc. Pet. Geol. Bull. 61, 376–406. Horne, R.A., 1970. Marine Chemistry. The Structure of Water and the Chemistry of the Hydrosphere. Wiley-Interscience, New York, 568 pp. Koch, P., 1958. Hydroge´ologie des ˆıles Loyaute´. Bull. Ge´ol. Nouvelle Cale´donie, Noume´a 1, 135–185. Land, L.S., 1973. Contemporaneous dolomitization of Middle Pleistocene reefs by meteoric water, north Jamaica. Bull. Mar. Sci. 23, 64–92. Lighty, R.G., 1985. Preservation of internal reef porosity and diagenetic sealing of submerged early Holocene barrier reef, southeast Florida shelf. In: Schneidermann, N., Harris, P.M. (Eds.), Carbonate Cements. Soc. Econ. Paleontol. Mineral., Spec. Publ. 36, 123–151. Loreau, J.P., 1970. Ultrastructure de la phase carbonate´e des oolithes marines actuelles. C.R. Acad. Sci. Paris, Se´r. D 271, 816–819. Lowenstam, H.A., Epstein, S., 1957. On the origin of sedimentary aragonite needles of the Great Bahama Bank. J. Geol. 65, 364–375. MacNeil, F.S., 1954. The shape of atolls: an inheritance from subaerial erosion forms. Am. J. Sci. 252, 402–427. Matthews, R.K., 1966. Genesis of recent lime mud in southern British Honduras. J. Sediment. Petrol. 36, 428–454. Milliman, J.D., 1974. Marine Carbonates. Springer-Verlag, Berlin, 375 pp. Monty, C.L.V., 1967. Distribution and structure of recent stromatolitic algal mats, eastern Andros Island, Bahamas. Ann. Soc. Geol. Belg. 90 (B3), 55–100. Neumann, A.C., Hearty, P.J., 1996. Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology. Geology 24, 775–778. Newell, N.D., Rigby, J.K., 1957. Geological studies on the Great Bahama Bank. In: Le Blanc, R.J., Breeding, J.G. (Eds.), Regional Aspects of Carbonate Deposition. Soc. Econ. Paleontol. Mineral., Spec. Publ. 5, 15–72. Newell, N.D., Imbrie, J., Purdy, E.G., Thurber L., 1959. Organism communities and bottom facies, Great Bahama Bank. Bull. Am. Mus. Nat. Hist., New York, pp. 177–228. Niaussat, P.-M., 1978. Le lagon et l’atoll de Clipperton. Trav. Me´m. Acad. Sci. Outre-Mer, C.R.S.S.A., Paris, 189 pp. Niaussat, P.-M., Ehrhardt, J.-P., Piozin, J.-F., 1970. Etude hydrologique et hydrobiologique du lagon de Clipperton. Ann. Hydrogr. Paris 4, 1–35. Obermuller, A.G., 1959. Contribution a` l’e´tude ge´ologique et mine´rale de l’ıˆle de Clipperton en Polyne´sie franc¸aise. In: Recherches Ge´ologiques et Mine´ralogiques en Polyne´sie Franc¸aise. Inspection des Mines et de la Ge´ologie, Paris, pp. 47–60. Palmer, R.J., 1986. Hydrology and speleogenesis beneath Andros

Island. Cave Sci., Trans. Br. Cave Res. Assoc. 13, 7–12. Palmer, R.J., Williams, D., 1984. Cave development under Andros Island, Bahamas. Cave Sci., Trans. Br. Cave Res. Assoc. 11, 50–52. Pierson, B.J., Shinn, E.A., 1985. Cement distribution and carbonate mineral stabilization in Pleistocene limestones of Hogsty Reef, Bahamas. In: Schneidermann, N., Harris, P.M. (Eds.), Carbonate Cements. Soc. Econ. Paleontol. Mineral., Spec. Publ. 36, 153–168. Purdy, E.G., 1974. Reef configurations: cause and effects. In: Laporte, L.F. (Ed.), Reefs in Time and space. Soc. Econ. Paleontol. Mineral., Spec. Publ. 18, 9–76. Rasmussen, K.A., Neumann, A.C., 1988. Holocene Overprints of Pleistocene Paleokarst: Bight of Abaco, Bahamas. In: James, N.P., Choquette, P.W. (Eds.), Paleokarst. Springer-Verlag, New York, pp. 132–148. Rebikoff, D., 1972. Precision underwater photomosaic technique for archaeological mapping. Interim experiment on the Bimini ‘cyclopean’ complex. Int. J. Naut. Archaeol. Underwater Expl. 1, 184–186. Sachet, M.-H., 1962. Monographie physique et biologique de l’ıˆle de Clipperton. Ann. Inst. Oce´anogr. 40, 1–107. Sandberg, Ph., 1985. Aragonite cements and their occurence in ancient limestones. In: Schneidermann, N., Harris, P.M. (Eds.), Carbonate cements. Soc. Econ. Paleontol. Mineral., Spec. Publ. 36, 33–57. Shinn, E.A., Ginsburg, R.N., Lloyd, R.M., 1965. Recent supratidal dolomite from Andros Island, Bahamas. In: Pray, L.C., Murray, R.C. (Eds.), Dolomitization and Limestone Diagenesis: A Symposium. Soc. Econ. Paleontol. Mineral., Spec. Publ. 13, 112–123. Shinn, E.A., Lloyd, R.M., Ginsburg, R.N., 1969. Anatomy of a modern carbonate tidal flat, Ros Island, Bahamas. J. Sediment. Petrol. 39, 1202–1228. Simone, L., 1981. Ooids: a review. Earth Sci. Rev. 16, 319–355. Smart, C.C., 1984. The hydrology of the inland Blue Holes, Andros Island. Cave Sci., Trans. Br. Cave Res. Assoc. 11, 23–29. Smart, P.L., Whitaker, F.F., 1991. Distribution of dissolution erosion rates in exposed carbonates: an example from the Bahamas. In: Bosselin, A., Braudner, R., Flu¨ger, E. et al. (Eds.), Dolomieu Conference on Carbonate Platforms and Dolomitization. Tour. Off. Ortisei=S. Ulrich Publ., Abstr., p. 248. Smart, P.L., Palmer, R.J., Whitaker, F., Wright, V.P., 1987. Neptunian dikes and fissure fills: an overview and account of some modern examples. In: James, N.P., Choquette, P.W. (Eds.), Paleokarst. Springer-Verlag, New York, pp. 149–163. Smith, C.L., 1940. The Great Bahama Bank, I. General hydrographical and chemical features. J. Mar. Res. 3, 147–189. Stockman, K.W., Ginsburg, R.N., Shinn, E.A., 1967. The production of lime mud by algae in South Florida. J. Sediment. Petrol. 37, 633–648. Thomas, C., Charrier, J.C., 1988. Spe´le´ologie canaque. Me´m. Ined., Fe´d. Fr. Spe´le´ologie, Lyon, 53 pp. Vaughan, T.W., 1914. Preliminary remarks on the geology of the Bahamas with special reference to the origin of the Bahaman and Floridan oolite. Carnegie Inst. Publ. 182, 47–54.