Abstract

Massive, banded, black and light brownish gray lime mudstone and coarsely crystalline dolostone containing millimeter to centimeter size, discrete white carbonate spar features known as "bluebird structures" are typical of the Middle Cambrian in the eastern Great Basin. Association of the lime mudstones with grainstones and stromatolitic algal boundstones indicates that the carbonate fines accumulated in calm water areas within an island and shoal complex and to some extent on tidal flats. Banding appears to be a function of the preservation of organic material due to varying redox relationships in poorly circulated waters of intershoal basins and island embayments. "Bluebird Structures" were formed by the calcitization of scattered patches of coarsely crystalline dolomite. The structures typically show centripetal and centrifugal fabrics which were produced by pore filling and replacement processes. Pore-filling fabrics are associated with the centrifugal replacement of ferroan dolomite; nonferroan dolomite appears to be centrifugally replaced by the growth of internal calcite inclusions.

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