--> ABSTRACT: Are Petrographic Textural Criteria Valid for Determining Timing of Dolomitization? An Example from Wahoo Formation, Prudhoe Bay, Alaska, by Jeremy Jameson; #91022 (1989)

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Are Petrographic Textural Criteria Valid for Determining Timing of Dolomitization? An Example from Wahoo Formation, Prudhoe Bay, Alaska

Jeremy Jameson

Textural criteria such as crystal size, shape, and fabric preservation may be misleading when trying to distinguish early from late dolomites. Data from the Wahoo formation, a typical Pennsylvanian shallow-marine carbonate, reveal that petrographically similar dolomites can have very different origins.

In the absence of other evidence, fine-grained (15-30 µm), subhedral-to-anhedral or hypidiotopic fabrics with well-preserved depositional textures are often regarded as having formed relatively early. Coarse, nonfabric selective dolomitization generally is considered to be of late origin. The problem of using textural criteria to determine timing is particularly acute in hypidiotopic fabrics, where more than one stage of dolomitization may be present.

Crosscutting relationships and geochemical and isotopic data reveal the range of origins of Wahoo formation hypidiotopic dolomites. The earliest dolomites were of mixing zone origin and probably formed during the Pennsylvanian. Dolomitization resumed in the Permian-Triassic as the Wahoo formation was buried to depths of 1,000-2,000 ft. Permian-Triassic burial dolomites are usually overgrowths of earlier dolomite. Trace element gradients reveal that burial dolomitizing fluids were sourced from the shales above the Wahoo formation.

In spite of their diverse origins, Wahoo dolomites are petrographically similar. Geochemical and isotopic data reveal that most dolomites that meet the early criteria formed relatively late at shallow to intermediate burial depths. Petrography incorrectly suggests only one episode of dolomitization. Textural criteria alone are a misleading guide to the origin of Wahoo dolomites.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.