Questioning carbonate diagenetic paradigms; evidence from the Neogene of the Bahamas

Author(s): Melim, L. A.; Westphal, H.; Swart, P. K.; Eberli, G. P.; Munnecke, A.
Author Affiliation(s): Primary:
Western Illinois University, Macomb, IL, United States
Other:
Universität Hannover, Federal Republic of Germany
Rosenstiel Scholl of Marine and Atmospheric Science, United States
Universität Tübingen, Federal Republic of Germany
Volume Title: Carbonate margin development (Bahama Transect, ODP Leg 166)
Volume Author(s): Reijmer, John J. G., editor
Source: Marine Geology, 185(1-2), p.27-53; Paleoceanology of reefs and carbonate platforms; Miocene to modern meeting (PRCP'99), Aix-en-Provence, France, Sept. 27-30, 1999, edited by John J. G. Reijmer. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0025-3227 CODEN: MAGEA6
Note: In English. 98 refs.; illus., incl. 1 plate, sketch map
Summary: Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100-150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the -120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ18O = +1 per mil) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution accompanied by calcite cementation. The sediments examined were deposited well above the aragonite compensation depth, so seawater entering the sediment is saturated with respect to aragonite. The under-saturation needed to drive diagenesis is likely the result of bacterial oxidation of organic matter using sulfate. (4) Microspar forms in these sediments as a cement based on petrographic and SEM examination of partly to completely altered samples. This contradicts the common assumption that microspar forms by aggrading neomorphism of micrite. (5) Strontium content of sediments altered in marine pore fluids can show an extreme range of values, formerly thought to indicate different environments. The opportunity to finally examine the diagenesis of aragonite-rich sediments buried in seawater challenges current diagenetic paradigms and emphasizes the importance of integrated studies. Abstract Copyright (2002) Elsevier, B.V.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Atlantic Ocean; Bahamas; Bahamas Drilling Project; Bahamas Transect; Carbonate sediments; Caribbean region; Cementation; Cenozoic; Chemical composition; Continental margin; Diagenesis; Dinoflagellata; Foraminifera; Geotraverses; Great Bahama Bank; Invertebrata; Isotope ratios; Isotopes; Leg 166; Lithofacies; Marine sedimentation; Marine sediments; Microfossils; Mineral composition; Neogene; North Atlantic; Northwest Atlantic; O-18/O-16; ODP Site 1003; ODP Site 1005; ODP Site 1007; ODP Site 1008; ODP Site 1009; Ocean Drilling Program; Oxygen; Palynomorphs; Protista; Reconstruction; Sedimentation; Sediments; Stable isotopes; Tertiary; West Indies
Coordinates: N243346 N243346 W0791408 W0791408
Record ID: 2002055814
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands