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Middle Pennslvanian rocks of the immensely thick Oquirrh Group in the Stansbury and Onaqui Mountains, Tooele County, Utah, consist primarily of thick, fine-grained siliciclastic rock units with relatively thin interbedded limestone units. A total of 12 microfacies are recognized; 3 are primarily siliciclastic, and 9 are primarily carbonate. The relatively pure siliciclastic rock composed of coarse silt and very fine sand-size quartz and feldspar is here called the “siltstone microfacies.” Two other microfacies, composed primarily of quartz and feldspar grains, are (1) peloidal siltstone (siltstone and very fine grained sandstone containing micritic peloids), and (2) fossiliferous siltstone (siltstone and very fine grained sandstone containing rounded fossil fragments). The carbonate microfacies, assigned to modified standard microfacies, are shallow spiculite (smf-1-s); boundstone (smf-7); whole-fossil wackestone (smf-8); bioclastic wackestone (smf-9); fusulinid-bearing bioclastic wackestone (smf-9-f); coated bioclasts in sparite, grainstone (smf-11); coquina, bioclastic grainstone or rudstone, shell hash (smf-12); laminated to bioturbated pelleted lime mudstone-wackestone (smf-19); and coarse lithoclastic-bioclastic rudstone or float-stone (smf-24).

The vertical arrangement of microfacies, fauna, flora, and sedimentary structures indicates that each carbonate sequence, from base to top, represents a transgression of the sea over shoreline deposits, followed some time later by reestablishment of these environments. The thick sequences of siltstone sandwiched between marine limestone sequences represent deposition in various shoreline and, probably, terrestrial environments.

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