Nature and origin of carbonate particles along a transect on the western margin of Great Bahama Bank (Middle Miocene): sedimentary processes and depositional model

Abstract

During ODP Leg 166, Middle Miocene sediments were collected along the western margin of the Great Bahama Bank (GBB) at four sites, distributed along a proximal-distal transect. Site 1006 is located in the basin, Site 1007 at the toe-of-slope and Sites 1003 and 1005 on the mid and upper slope. The carbonate slope deposits of GBB consist of periplatform oozes, an admixture of platform-derived aragonite and high-magnesium calcite particles, and pelagic low-magnesium calcite. An assessment of carbonate sedimentation is made in order to estimate the proportion of platform-derived versus pelagic components. The originality of this study is based on the application of a separation method giving access to homogeneous granulometric fractions, ranging from 63 to 3 µm in grain diameter. This method, associated with a multidisciplinary approach (micropaleontology, optical and electronic microscopy, mineralogy -X-ray diffractometry, and trace elements geochemistry -strontium and magnesium), allows the quantification and characterization of different kinds of carbonate particles. In Miocene sediments, three types of particles display a biogenic structure: planktonic foraminifera, calcareous nannofossils and fragments of neritic organisms. Two types of particles do not exhibit any structures that allow for a determination of their origin. Based on their size and their mineralogy, we have distinguished macroparticles (calcite and dolomite) and microparticles (calcite and aragonite).The detailed study of the composition of the separated fractions highlights major differences in carbonate ooze composition between the different sites along the transect. The unlithified samples of Sites 1006 and 1007 are dominated by pelagic components (planktonic foraminifera and calcareous nannofossils) and contain aragonite microparticles. In contrast, lithified sediments of Sites 1003 and 1005 (and 1007) are characterized by higher proportions of neritic debris and particles without biologic structure, the pelagic phase is impoverished and aragonite needles are absent. The origin of particles without biological structure has been demonstrated by their spatial distribution and by mineralogical as well as geochemical criteria. The rhombohedral calcitic microparticles mainly occur at slope sites. Their high magnesium contents support a formation on the bank implying an allochthonous origin. This suggests that calcitic microparticles correspond to the bank micrite exported towards slope environments. Rhomboedral calcitic macroparticles, which fill sediment voids, originate from in situ transformations of metastable carbonates due to pore fluids, which preferentially flow in the slope permeable sediments. Two sedimentary sources are proposed: 1) an autochthonous pelagic source with low-magnesium calcitic components and 2) an allochthonous neritic source with both aragonitic and calcitic components. Export and depositional processes can be differentiated for the allochthonous particles. The neritic debris and calcitic microparticles in slope Sites 100...

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

“…These particles have been differentiated based on crystallographic, granulometric, and mineralogical criteria: Macroparticles occur in the 20-12 lm, 12-8 lm, and 8-5 lm fractions and consist of calcite, which is mainly xenomorphic and sometimes rhombohedral or dolomite rhombohedra. In contrast, microparticles that are either composed of (1) calcite rhombohedra (\8 lm), which-from a granulometric point of view-are similar to the 'micarb particles' described by Cook and Egbert (1983), Erba (1992) and Mattioli and Pittet (2002), their high proportion in slope sediments and their high magnesium content suggest a bank-top origin (Turpin 2006;Turpin et al 2008), or of (2) needle-shaped aragonite (\12 lm), which only occurs at distal Sites 1006 and 1007 (Table 2; Fig. 2).…”

“…At these Sites 1005 and 1003, the mean amount of \63 lm fraction is equal to 67 and 69% and the \12 lm fraction represents 41 and 46%, respectively. For a detailed description of composition per fraction, please refer to Turpin et al (2008). At Site 1007, numerous authors (Betzler et al 1999(Betzler et al , 2000Frank and Bernet 2000;Isern and Anselmetti 2001;Reuning et al 2002) related the cyclic sedimentation pattern to sea-level fluctuations.…”

“…Their microstructures suggest a biological origin but the specific carbonate-producing organisms could not be identified (Minoletti 2002). Their high magnesium content and their higher abundance at slope sites (Sites 1003 and 1005), however, suggests a neritic origin (Turpin 2006;Turpin et al 2008). …”

“…1; Eberli et al 1997) have shown the occurrence of aragonite needles in the pelagic facies of the distal sites (basin: Site 1006 and toe-of-slope: Site 1007; Turpin et al 2008). In contrast, these needles are absent in the proximal sites (upper and middle slope: Sites 1005 and 1003 ;Turpin 2006;Turpin et al 2008).…”

“…1; Eberli et al 1997) have shown the occurrence of aragonite needles in the pelagic facies of the distal sites (basin: Site 1006 and toe-of-slope: Site 1007; Turpin et al 2008). In contrast, these needles are absent in the proximal sites (upper and middle slope: Sites 1005 and 1003 ;Turpin 2006;Turpin et al 2008). While modern Bahamian aragonite needles are present in all leeward settings and are linked to their production site on the bank top, the exclusive occurrence of Middle Miocene aragonite needles in the distal part of the sedimentary apron off the Great Bahama Bank questions their origin.…”