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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 3. (March)

First Page: 453

Last Page: 454

Title: Dolomite is Evaporite Mineral--Evidence from Rock Record and from Sea-Marginal Pools of Red Sea: ABSTRACT

Author(s): Gerald M. Friedman

Article Type: Meeting abstract

Abstract:

Despite recent pleas to consider dolomite a product involving fresh water, especially the reaction between fresh water and seawater, more recent work in the rock record and in sea-marginal pools of the Red Sea commands a return to the earlier hypothesis that most dolostones owe their origin to hypersaline brines and that dolomite is an evaporite mineral. Schizohaline dolostones, as well as other examples, commonly lack evaporites; yet these dolostones probably accumulated under hypersaline evaporitic conditions although the evaporite minerals have since vanished. However, the imprint of evaporite minerals and other evidence for hypersalinity have been preserved. Evidence includes (1) abundant authigenic feldspar; (2) calcitized anhydrite nodules; (3) euhedral quartz cryst ls; (4) solution-collapse breccias; (5) ghosts and pseudomorphs of former crystals of gypsum or anhydrite, now preserved as molds, calcite, or pyrite, in some places preserving the

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outlines of the former sulfate crystals; (6) relict inclusions of anhydrite, barite, or celestite; (7) enterolithic folds; (8) various kinds of chert, including length-slow chalcedony; (9) saddle-shaped dolomite crystals; (10) dedolomite; and (11) fluorite. The Dorag model was developed from study of the classical mid-Ordovician authigenic feldspar-bearing strata, where hypersalinity must have prevailed.

Research in modern sea-marginal pools of the Red Sea shows that dolomite forms only where gypsum and/or anhydrite is likewise present. Among submerged algal mats where gypsum is absent, the carbonate minerals are aragonite or high-magnesian calcite; by contrast, where gypsum is abundant in deeper parts of pools, or among submerged algal mats, dolomite is present. Likewise, in a pool-marginal salina, not only halite, form a cement between constituent particles. The high salinities at which gypsum precipitates (up to 330 × 103 mg/L in the summer) and the observation that dolomite prefers sulfate association suggest that both minerals owe their origin to hypersaline brines.

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