Int. Assoc. Sedimentol. Spec. Publ. (2009) 41, 29–46 A re-evaluation of facies on Great Bahama Bank I: new facies maps of western Great Bahama Bank JOHN J.G. REIJMER*†1, PETER K. SWART2, THORSTEN BAUCH*3, ROBERT OTTO2, LARS REUNING*4, SVEN ROTH*5 and SUSANNE ZECHEL*6 *IFM-GEOMAR, Leibniz-Institut für Meereswissenschaften, Dienstgebäude Ostufer, Wischhofstr. 1-3, D-24148 Kiel, Germany †Centre de Sédimentologie-Paléontologie, Laboratoire (EA 4229) ‘Géologie des Systèmes et des Réservoirs Carbonatés’, Université de Provence (Aix-Marseille I), 3, place Victor Hugo, Case 67, F-13331 Marseille Cédex 3, France ABSTRACT A re-evaluation of the sediment distribution patterns on western Great Bahama Bank shows a facies distribution with two end-members. Coarse-grained sediments in the north, west and south of the bank surround a mud-dominated realm located on the western leeward side of Andros Island. This facies distribution is comparable to earlier maps, but shows considerably more detail and a complex distribution from grainstones to mud-rich wackestones. As in other carbonate platforms, sediment distribution appears to be infl uenced by (1) tidal currents, (2) prevailing wind direction, and (3) the interaction of the rate of Holocene sea-level rise with the pre-existing Pleistocene topography. The grain-size distribution very precisely refl ects current-infl uenced and protected areas on the platform. The correlation between the distribution of pellets and the 63–125 µm grain-size fraction most probably refl ects the predominantly biological origin of this grain size. Aragonite dominates the mineralogy on the platform, low- magnesium calcite and high-magnesium calcite occurs in higher quantities only in a few environments on the platform. Keywords Facies distribution, grain-size, mineralogy, Great Bahama Bank, Holocene. INTRODUCTION spatial distribution of sediments on Great Bahama Bank (GBB) is based on the work of Illing (1954), The Great Bahama Bank (Fig. 1a) has served as Ginsburg et al. (1958), Newell et al. (1959) and an inspiration for geologists in understanding Purdy (1963a,b). Valuable as these early descrip- modern processes of carbonate sedimenta- tions are, it is the aim of this study to re-evaluate tion since the pioneering work of Field (1931) the distribution of the surface sediments on GBB and Illing (1954). Most of our knowledge of the using samples collected from precisely located points (Fig. 1b; established using GPS) and to 1Present address: VU University, Faculty of Earth and quantify not only the types of sediments pres- Life Sciences (FALW), De Boelelaan 1085, 1081 HV ent, but also the mineralogy, grain-size distribu- Amsterdam, The Netherlands (Corresponding author: tion and skeletal and non-skeletal content. This E-mail: [email protected]). paper together with a companion paper (Swart 2Marine Geology and Geophysics, Rosenstiel School of et al., 2009) takes a new look at the surface Marine and Atmospheric Science, 4600 Rickenbacker facies and geochemistry of sediments from Great Causeway, Miami, FL 33149, USA. Bahama Bank. 3 Present address: Schlumberger Oilfi eld Services D&M, Various maps show sedimentary facies on Hamrasletta 15, 4056 Tananger, Norway. GBB or in the Bahamas in general (Ginsburg 4Present address: RWTH Aachen University, Wüllnerstr. 2, et al., 1958; Newell et al., 1959; Purdy, 1963a,b; D-52062 Aachen, Germany. 5Present address: Nummerical Rocks, Stiklestadveien 1, Ball, 1967; Enos, 1974). In all these studies sedi- 7041 Trondheim, Norway. ments were classifi ed using distinct facies types 6Present address: TU Bergakademie Freiberg, Institut für with comparable subdivisions. All maps show Geologie und Paläontologie, Bernhardtvon-Cotta Straße 2, that skeletal sediments occur mainly on the 09596 Freiberg, Germany. margin of the bank whereas non-skeletal grains © 2009 International Association of Sedimentologists and published for them by Blackwell Publishing Ltd 29 Swart_C003.indd 29 1/7/2009 1:32:02 PM Swart_C003.indd 30 30 J.J.G. Reijmer (a) (b) 82oW81o 80o 79o 78o 77o 76oW 27oN 79oW78oW77 oW et al. Atlantic Ocean N N Florida NW Providence 26oN 26o Channel Bimini Florida Tongue 25o Great Andros Exuma Sound 25oN of of Island Santaren Bahama the o Straits 24 Cay Sal Bank Ocean Channel Bank 24oN Depth (m) 23o 0 Old BHM 500 Cuba Channel 1000 50 100 km o 0 0 100 km 22 N 1500 23oN Fig. 1. (a) Location map of Great Bahama Bank with main topographic names. Old BHM Channel: Old Bahama Channel. (b) Location map samples (yellow crosses). Satellite image of Great Bahama Bank shows the shallow-water bank in light blue to green colours. Deep-water basins show up in dark blue colours. Morphology of the islands accentuated in yellow. Large island in the centre of the fi gure is Andros Island. Latitude and longitude indicated at the side. Kilometre bar for scale. 1/7/2009 1:32:02 PM A re-evaluation of facies on Great Bahama Bank I 31 (mainly faecal pellets) dominate the interior of the et al. (1995) showed that the average water platform. Very fi ne-grained sediments predomin- transport through the Santaren Channel ate in platform interior zones that are protected amounts to 1.9 Sv or 1.8 Sv, respectively. The trans- by topographic barriers such as marginal islands port through the Northwest Providence Channel and submarine topography (‘pelletoidal sands reaches 1.2 Sv (Leaman et al., 1995). On the west- with lime mud matrix and lime muds’, Ginsburg ern, leeward sides the current system on GBB is et al., 1958; ‘muddy sand and mud’, Newell et al., connected with the currents forming the sources 1959; ‘pellet-mud facies’ and ‘mud facies’, Purdy, of the Gulf Stream. On the windward sides a link 1963a; ‘pelletoidal packstones’ and ‘pelletoidal exists with the North Atlantic Subtropical Gyre wackestones’, Enos, 1974). Some differences, how- situated eastward in the Sargasso Sea. The GBB ever, exist in the interpretation and, as seen on the has a great lateral extent and shows a well-defi ned maps, in the distribution of the facies types. For marginal escarpment with islands. Shallow waters example, Ginsburg et al. (1958) and Enos (1974) cover most of GBB and consequently, the inter- show that the sediments in the platform interior action with water masses from the open ocean is mainly contain pellets, peloids or pelletoids. limited (Smith, 1940). Islands such as Andros Newell et al. (1959) and Purdy (1963a,b), how- Island defl ect westward-moving currents gener- ever, described sediments containing a larger per- ated by the trade winds to the north and south centage of grapestones. Enos (1974) also described (Bathurst, 1975). As a result protected areas with the occurrence of grapestones without showing limited water exchange are present on the western them on the map as a particular facies type. Enos side of these islands (Bathurst, 1975). Water move- (1974) was the only study in which the Dunham ment on GBB is mostly infl uenced by the wind and classifi cation (1962) was used and thus showed tides, but also by waves and storms (Smith, 1940; the components to matrix ratio along the plat- Cloud, 1962; Purdy, 1963a; Traverse & Ginsburg, form. All maps known from the literature were 1966; Winland & Matthews, 1974; Gonzalez & based on sampling along a series of profi les cross- Eberli, 1997). ing the bank and additional datasets. All samples Long-term net fl ow measurements over GBB were taken without modern precision positioning show a very slow current velocity of 2 cm s1 systems and thus might contain signifi cant uncer- toward the north (002°N; Smith, 1995). This is tainties in the exact location of facies types. thought to be the outcome of two almost oppos- The major question, however, concerns the ing forces, a tide-induced fl ow toward the processes that control the present facies distribu- east-southeast and a wind- and density-driven tion on western GBB, the Andros lobe. This paper northward fl ow (Smith, 1995). Tidal currents attempts to determine the links between the sedi- vary from near 0 to 4 km h1 in the tidal channels ment distribution and environmental para meters (Newell & Rigby, 1957; Bathurst, 1975) and infl u- such as currents, tides, topography, evaporation ence the entire platform to some degree. (salinity), precipitation, water exchange with the surrounding ocean water masses and sediment Water temperature, wind and precipitation export. The surface seawater temperature (SST) on GBB ranges from 18.5°C during winter to 28.5°C in Currents and circulation summer (Cloud, 1962; Bathurst, 1975). During the Most of the surface water masses of the Caribbean summer, higher SSTs are reached locally. Mixing Sea, which eventually form the Florida Current of bank and ocean waters is very low (Purdy, and the Gulf Stream, enter in the Lesser Antilles 1963a). Smith (1940) noted that the difference in through various passages (Wajsowicz, 2002). A SST between bank and surface ocean waters may smaller portion (approximately 20%) enters the be as large as 3.6°C. The lateral variation in the Caribbean Sea through the Windward Passage SST on the bank itself is very low, but on bank between Cuba and Hispaniola (Schmitz & margins the slow mixing with ocean water is dem- Richardson, 1991). The Florida Current is fed as onstrated by changes in SSTs (Purdy, 1963a). well by water masses through the Old Bahama The Bahama Platform is infl uenced by the Channel and Santaren Channel and through the trade winds blowing from east or southeast in Northwest Providence Channel (Fig. 1a; Leaman the summer (March to August). In the winter et al., 1995). Atkinson et al. (1995) and Leaman months, the winds have a northeasterly direction. Swart_C003.indd 31 1/7/2009 1:32:07 PM 32 J.J.G.