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1 the Origin of Dolomites in Tertiary Sediments From THE ORIGIN OF DOLOMITES IN TERTIARY SEDIMENTS FROM THE MARGIN OF GREAT BAHAMA BANK PETER K. SWART1 AND LESLIE A. MELIM2 1-Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 2- Department of Geology, Western Illinois University, Macomb, IL 61455 Abstract Based on an integrated geochemical characterized by extremely high Sr and petrographic investigation of dolomites concentrations, which reflect high from two cores drilled on Great Bahama concentrations of Sr 2+ in the pore fluids. Bank, we have determined three different The high concentrations of Sr 2+ in the pore mechanisms of formation for the dolomites fluids arise through the continued which are common throughout the Pliocene recrystallization of meta-stable aragonite and and Miocene aged portions of these cores. high-Mg calcite to dolomite and LMC driven The first mechanism of dolomitization occurs by the oxidation of organic material by in association with development of non- sulfate. Sulfate reduction not only provides depositional surfaces. Dolomite typically the thermodynamic drive for recrystallization, forms below each of these surfaces, the but as the absolute concentration of strontium concentration and extent of which is governed in the pore fluids is governed by the solubility by the length of the period of non-deposition. of celestite, allows the Sr2+/Ca2+ ratio of the These dolomites are recognized by their interstitial fluid to become much higher than association with the non-depositional normally encountered. The final type of surfaces, characteristic heavy oxygen isotopes dolomite is a massive dolomite which occurs indicative of formation from cold bottom in coarse grained reefal sediments. The waters, and d18O and Sr profiles with depth pervasive nature of the dolomitization and the which suggest formation in the presence of relatively normal Sr concentrations, suggest diffusive temperature and Sr gradients. The the circulation of normal marine water in a second mechanism of dolomitization, occurs relatively open system. in pore fluids where the cation and anion profiles are governed by diffusive processes INTRODUCTION and forms what we term background It has long been known that a large dolomite. This is a microsucrosic dolomite proportion of the rocks in the subsurface of and forms both by the recrystallization of the Bahamas are pervasively dolomitized. The existing sediment and precipitation directly presence of dolomite was established through into void space. Dolomitization by this a series of cores, up to several hundred meters mechanism uses a local source of Mg2+ and in thickness, taken through Tertiary sediments consequently the dolomite never comprises (Beach and Ginsburg, 1980; Supko, 1977; more than between 5 and 10% of the Gidman, 1978; Pierson, 1982; Williams, sediment. This type of dolomite is c 1985). Although numerous modes of 1 Swart and Melim BAHAMAS DRILLING PROJECT SUMMARY e in L rn Drilling Operations te s e W Cores Clino and Unda were obtained using a diamond coring system mounted Unda Clino aboard a jackup barge. The two cores were located approximately 5 and 13.5 km respectively from the edge of Great Bahama Bank. They were drilled along a seismic Cay Sal profile composed of Western Geophysical lines Bank GBB-82-03 and 82-03x previously interpreted by Eberli and Ginsburg (1989). Core Clino was drilled 677.71 m below the mud pit datum (7.3 m above sea level) and recovery averaged 80.8%. Core Unda was drilled 454.15 m below mud pit (5.2 m above sea level). Recovery in Unda averaged 82.9% In this paper all depths are reported in meters as Figure 1: Site location map, showing the depths below the mud pit. position of Clino and Unda near the western margin of Great Bahama Bank. Facies and Chronostratigraphy Clino, the more distal core, penetrated formation have been suggested for these inclined slope deposits overlain by a reef to dolomites including mixing-zone (Supko, platform sequence. The upper platform to 1977), normal seawater (Swart et al., 1987), reefal interval (197.4 to 21.6 m) consists of 7 reflux (Kaldi and Gidman, 1984), and Kohout sequences, each capped by sub-aerial exposure convection (Simms, 1984), the precise surfaces (Kievman, 1998). The reefal unit mechanism of formation remains uncertain. includes a deeper forereef facies that shallows to reef and eventually backreef facies. The This paper reports on the origin of remainder of the core is a 480 m thick dolomite in Tertiary sediments retrieved from sequence of slope sediments composed of fine- two cores, Clino and Unda, drilled near the sand to silt-sized skeletal and non-skeletal western margin of Great Bahama Bank (GBB) grains interrupted by intervals of coarse- (Fig. 1). These two cores were drilled as part grained skeletal sands. Three hardgrounds are of the Bahamas Drilling Project on a Western present (256-263, 367, and 536.3 m.), each of Geophysical seismic line (Eberli et al., 1997) in which represents a break in deposition, the order to date the seismic sequences identified longest of which (2 to 3 Myrs) occurs at 536.3 by Eberli and Ginsburg (1989) and to m. This surface represents the transition from investigate the nature of the carbonate the late Miocene to the early Pliocene. Based diagenesis in deeper water facies. on a combination of biostratigraphy (Lidz and McNeill, 1995a, 1995b), magnetostratigraphy (McNeill et al., in press) and strontium isotope 2 Dolomitization in Great Bahama Bank Figure 2: Summary of the sedimentology, chronostratigraphy, mineralogy, and isotopic composition for Clino and Unda. Data are from Eberli et al. (1997), Kenter et al. (In press), Lidz et al. (1995a, b), Melim et al. (1995; In Press). stratigraphy (Swart et al., in press) the Plio- exposure surfaces overlying a reefal unit based Pleistocene boundary can be placed at on a marine firmground (Kievman, 1998). approximately 110 m (Fig. 2). The middle shallow-water unit (354.7 to Core Unda, the more proximal of the 292.82 m) is a somewhat deeper water reef two, consists of three successions of shallow- with platy corals and rhodoliths (Budd and water platform sands and reefal deposits, that Kievman, in press) overlain by a sub-aerial alternate with sand and silt-sized deeper exposure surface that also is a phosphatic marginal deposits. The Plio-Pleistocene marine hardground (Melim et al., in press). shallow-water interval (60 to 8.6 m) has 14 The deepest shallow-water unit (454.0 to platform sequences capped by sub-aerial 443.5 m) consists of shoaling-upward 3 Swart and Melim packages of coarse-grained skeletal to non- level caused development of a subaerial skeletal grainstones to rudstones (Kenter et al., exposure surface in Unda (at 292.82 m) and in press). The two deeper marginal deposits continued hardground development in Clino. sandwiched between the shallow-water units During the early Pliocene a major sea-level rise are fine-sand to silt-sized grainstones to forced eastward backstepping of the shallow- packstones that alternate with coarse-sand water platform and renewed deeper water intervals. Hardgrounds occur at 270.36, facies in Unda and formed a condensed 292.82, and 393.81 m and a firmground tops interval in Clino (sequence f). The Unda the deeper water facies at 108.1 m. The Plio- subaerial exposure surface (292.82 m) was Pleistocene boundary can be placed at overprinted by marine-hardground diagenesis approximately 200 m and the Mio-Pliocene during the transgression. Before progradation boundary at 292.82 m (Fig. 2). could bring highstand deposits to the margin locations of Clino and Unda, another sea level rise further backstepped the platform renewing Sequence stratigraphy transgressive deeper margin and slope facies. Facies successions document several The subsequent highstand (sequence e) hierarchies of changes in relative sea level in resulted in major progradation of the western cores Clino and Unda (Eberli et al., in press). margin of GBB. The late Pliocene began with These changes resulted in pulses of a relative sea-level fall then rapid rise, resulting progradation of the western margin of Great in a hardground (later partly eroded) in Clino Bahama Bank that are seen on seismic lines as (at 367 m) and a firmground in Unda (at 108.1 seismic sequences (Eberli et al., in press) and m). In Unda, the following sequence (d) is a in the cores as depositional sequences (Kenter reef, while in Clino a thick package of et al., in press; Kievman and Ginsburg, in proximal slope facies documents rapid press). The sequence boundaries are indicated progradation of the margin. An early by discontinuity horizons (subaerial exposure Pleistocene lowstand resulted in a lowstand on the platform, marine hardgrounds and reef in Clino and platform top facies and firmgrounds on the slope), changes in facies subaerial exposure in Unda (sequence c). and changes in diagenesis (Melim et al., in Sequences b and a were deposited during the press). high frequency, high amplitude sea level Eight seismic sequences (a – i) record changes of the Pleistocene. The margin of the relative sea level changes of the middle GBB was located to the west of the cores by Miocene to Recent (Eberli et al., 1997) (Fig. this time resulting in only highstand platform 2). Platform facies of possible middle Miocene facies and numerous subaerial exposure age (sequence i) were deposited during a surfaces in both cores. relative lowstand. The following late Miocene highstand (sequence h) deposited a thick METHODS package of deeper margin facies in Unda and Samples were taken at 1.5 m intervals deeper slope facies in Clino. Sequence g throughout the two cores for X-ray diffraction deposited a late Miocene lowstand reef in (XRD) and stable carbon and oxygen isotopic Unda while a marine hardground was forming analysis.
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