--> Dolomitization of Simonson (Middle Devonian) Carbonate Platform, Eastern Nevada, by W. C. Dawson; #90986 (1994).

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Abstract: Dolomitization of Simonson (Middle Devonian) Carbonate Platform, Eastern Nevada

William C. Dawson

The Simonson Formation is a Middle Devonian, shallow-marine, cratonic platform sequence deposited along the eastern margin of the Cordilleran miogeocline. Outcrops of Simonson carbonates in the South Egan and Hiko ranges of east-central Nevada consist of interstratified microfacies arranged into shallowing-upward sequences: pelmatozoan, coral, brachiopod, bryozoan packstones/grainstones; stromatoporoid bioherms/biostromes; peloidal, intraclastic, bioclastic, Amphipora packstones/wackestones; and algal laminites. Mixed carbonate/ siliciclastic microfacies characterize the lower Simonson. Collectively, these Simonson carbonates record subtidal to intertidal deposition on a semi-restricted, westward-sloping carbonate ramp.

The Simonson carbonate platform underwent a complex diagenetic history and was pervasively dolomitized during progressive burial. The generalized paragenetic sequence included: mechanical compaction, marine cementation, meteoric dissolution/cementation; dolomitization; chertification; stylolitization, fracturing, and hydrothermal mineralization. Petrographic and geochemical data revealed evidence of multiple dolomitization/recrystallization events. Early (mixing zone) dolomites underwent extensive recrystallization at shallow to moderate depths by circulating meteoric fluids. These recrystallized, coarse, sucrosic dolomites were altered by hydrothermal fluids and cemented by saddle dolomite during late deep burial. Secondary porosity (intercrystalline, vugs, and fractures/breccias) oc urs preferentially in the sucrossic dolomites. These pore systems developed during late burial diagenesis, prior to the precipitation of saddle dolomite. Bitumen coatings and droplets are commonly present within porous Simonson dolomites. Comparable dolomitization fabrics and porosity types occur in subsurface samples of Simonson carbonates.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994