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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 53 (1969)

Issue: 1. (January)

First Page: 55

Last Page: 72

Title: Edwards Formation (Lower Cretaceous), Texas: Dolomitization in a Carbonate Platform System

Author(s): W. L. Fisher (2), Peter U. Rodda (2)


The Edwards Formation is characterized by rudist bioherms, carbonate grainstone and mudstone, and evaporites which were deposited on an extensive, shallow-water, marine platform bounded by deeper water basins in which chiefly carbonate muds were deposited. Rudist bioherms were constructed principally along platform edges peripheral to an evaporitic lagoon. Main dolomite deposits are in a concentric belt marginal to the lagoonal facies.

Two main types of dolomite are present: (1) stratal dolomite--fine grained, tightly knit fabric, laminated to thin bedded, very slightly porous and permeable, associated with thin-bedded, mud-cracked, stromatolitic carbonate mudstone, ripple-marked carbonate grainstone, and thin evaporite-collapse layers; dolomite units are generally less than 2 ft (0.6 m) thick; magnesium carbonate content ranges irregularly from low to high; and (2) massive dolomite--fine- to coarse-grained loosely knit euhedral crystals, moderately to highly porous and permeable, replacing thick-bedded, fossiliferous carbonate grainstone; dolomite units are commonly more than 10 ft (3 m) thick and underlie prominent evaporite-solution units; magnesium carbonate content is low or high, with few intermediate values.

Both stratal and massive dolomites are judged to be products of metasomatic replacement of calcium carbonate which resulted from contact with magnesium-enriched brines. Features of stratal dolomite indicate prelithification replacement of carbonate muds and grains in extensive, low-relief, supratidal and intertidal zones along the southern margin of the lagoon. Massive dolomite was created by postlithification replacement of reef-trend carbonate grainstone along the northern margin of the lagoon as a result of seepage refluxion of lagoon brines. Both dolomitization processes were part of the original depositional system, and type of dolomitization was controlled by specific depositional facies of the system.

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