KIMBELL, TOD N., Univ. of Texas at Dallas, Richardson, TX; JOHN D. HUMPHREY, Colorado School of Mines, Golden, CO; JAY L. BANNER and MARYLYNN MUSGROVE, Univ. of Texas at Austin, Austin, TX

ABSTRACT: Microdissolution Control on Distribution of Quaternary Mixing Zone Dolomite, Barbados, West Indies

Dolomite occurs in two distinct stratigraphic intervals (Intervals I and II) recovered in core from two boreholes (GD-4 and GD-5) drilled along the southeast coast of Barbados. High-resolution backscatter electron imaging (BEI) indicates that disseminated and patchy dolomite rhombohedra are associated with microdissolution cavities within fine-grained matrix. The important issue is the timing of dolomitization relative to formation of micro-dissolution voids, which has significant implications for early diagenetic modification of porosity and permeability distribution within reefal limestones.

Interval I (<1 to 25 wt. % dolomite) occurs as a 5 m interval in both cores and is entirely above the modem water table. Interval I also has undergone massive dissolution and has poor recovery. Dolomite Interval II ranges from <1 to 20 wt. % dolomite in GD-5 and is entirely within the modern mixing zone. Dolomite textures identified are essentially the same as those reported for most late Cenozoic dolomites. The most common dolomite texture is the partial replacement of fine-grained matrix material. SEM analysis indicates that dolomite rhomboherda (1-15 micrometer) precipitate within microscopic voids between matrix grains and continue to grow at the expense of the surrounding grains. Invariably, these patches of dolomite areassociated with intra-matrix micro-dissolution voids w th larger rhombohedra toward the center of some voids.

Isotopic and trace element variations (delta{13}C, delta{18}O, Sr) for leached dolomite separates suggest precipitation from variable mixtures of meteoric and marine waters. Carbon and oxygen isotope data range from -2.0 to 2.0 o/oo (PDB) and 0.5 to 2.7 o/oo (PDB), respectively. These data exhibit a positive correlation (r{2} = 0.89) and plot along a mixing line between meteoric and marine end members. Strontium concentrations are variable and range from 400 to 2000 ppm and inversely correlate to changes in (delta){13}C (r{2} = 0.80) and (delta){18}O (r{2} = 0.71). In addition, (delta){87}Sr for dolomite separates are statistically less radiogenic (-8.5 to -3.8 o/oo, n = 3) than modern seawater ((delta){87}Sr (approx.) 0 o/oo). These data indicate a meteoric component to the dolomitiz ng fluid since Barbados groundwaters contain elevated Sr/Ca ratios primarily from updip dissolution of older carbonates. Likewise, isotopic and trace element relationships suggest a primary geochemical signature reflecting precipitation within a water-dominated system. Lower (delta){87}Sr derived from meteoric waters result from interaction with older Pleistocene carbonates and underlying deep-sea Tertiary sedimentary rocks.

Early dissolution of fine-grained matrix, which enhanced fluid flow rates, likely controlled the initial distribution of dolomite. Since mixed meteoric-marine waters are simultaneously undersaturated with respect to aragonite and supersaturated with respect to dolomite over a large mixing range (up to 75% seawater), dolomitization probably occurred coeval with or shortly after matrix dissolution. Maintaining these mixed fluid saturation conditions would result in continued dissolution of aragonite and precipitation of dolomite. In such a scenario, the resultant dolomite texture would consist of a tight, interpenetrating mosaic of varying sized rhombohedra, as seen in locally 100% dolomitized matrix samples from elsewhere on the island (Golden Grove).

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.