--> Abstract: Dolomitization Preserves or Enhances Porosity of Lime-Mud Precursors in Sucrosic Dolomites, by P. W. Choquette; #90987 (1993).

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CHOQUETTE, PHILLIP W., Univ. of Colorado at Boulder, Boulder, CO

ABSTRACT: Dolomitization Preserves or Enhances Porosity of Lime-Mud Precursors in Sucrosic Dolomites

Replacement dolomitization of CaCO<3> clearly does create new kinds of pore space, but whether it also causes an increase in pore volume from that of CaCO<3> precursors is difficult to demonstrate in Phanerozoic carbonates. Study of sucrosic, planar -e and -s dolomites of "lime-mud" origin [from mudstone-wackestone (MS-WS) precursors], in several Paleozoic and Mesozoic shallow-shelf sequences in cores and outcrops, reveals a common sequence of changes as lime MS-WS is converted to dolomite: 1) dissolution microvugs (order of 10{0}-10{1} micron diameter) appear next to scattered rhombs; 2) with increasing amounts of dolomite (i.e., more rhombs), intercrystal pores and micropores develop by microvug coalesce; 3) remaining lime-mud CaCO<3> disappears, resulting in porou /microporous dolomite. Any CaCO<3> grains that were present dissolve next. Dolomite overgrowths, similar compositionally to the "matrix" dolomite, may then precipitate on walls of resulting grain molds, apparently with CO<3>{-2} from the dissolved grains. Dolomite in these examples is enriched in Mn, Fe and Sr relative to the lime MS-WS.

Why do closely associated lime MS-WS of precursor types in Phanerozoic sequences typically have such low porosities (<3-5%) and permeabilities (<0.1 md) by comparison with their dolomite neighbors? In these examples, limestone porosity has typically been all but destroyed, after dolomitization, by calcite micro-cement. Amounts of micro-cement should give conservative estimates of the amounts of porosity in the precursor lime MS-WS at the onset of dolomitization. Physical and chemical compact ion have less readily measurable effects on porosity reduction.

In sucrosic lime-mud dolomites, the sequence of fabric changes just described is compatible with, but does not unequivocally prove, mole-for-mole replacement of CaCO<3> (aragonite or calcite) by dolomite, i.e., the precursor can be assumed to have supplied most or all of the carbonate. Multiple zonation of rhombs in CL shows that many lime-mud dolomites had multi-stage pore-water histories.

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