Questioning carbonate diagenetic paradigms

Evidence from the Neogene of the Bahamas

L. A. Melim, H. Westphal, Peter K Swart, Gregor P Eberli, A. Munnecke

Research output: Contribution to journalArticle

157 Citations (Scopus)

Abstract

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 ‰) 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.

Original languageEnglish (US)
Pages (from-to)27-53
Number of pages27
JournalMarine Geology
Volume185
Issue number1-2
DOIs
StatePublished - Jun 15 2002

Fingerprint

Bahamas
aragonite
Calcium Carbonate
Carbonates
Neogene
carbonates
sediments
carbonate
diagenesis
Sediments
sediment
cementation
burial diagenesis
porosity
limestone
calcite
dissolving
fluid
fluids
dissolution

Keywords

  • Aragonite
  • Bahamas
  • Calcite cement
  • Carbonate diagenesis
  • Marine-burial diagenesis
  • Meteoric
  • Microspar
  • Mixing zone
  • Strontium

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geology
  • Geophysics

Cite this

Questioning carbonate diagenetic paradigms : Evidence from the Neogene of the Bahamas. / Melim, L. A.; Westphal, H.; Swart, Peter K; Eberli, Gregor P; Munnecke, A.

In: Marine Geology, Vol. 185, No. 1-2, 15.06.2002, p. 27-53.

Research output: Contribution to journalArticle

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