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Evolution of Early Middle Ordovician Carbonate Platform, Basin Ranges of U.S.A.

Reuben J. Ross, Jr., Noel P. James, Lehi F. Hintze, Forrest G. Poole

The Whiterockian carbonate platform of western Utah, Nevada, and southern California grew during a single onlap-offlap cycle, 15 m.y. in duration, when most of the North American plate was exposed to subaerial weathering. Either tectonic subsidence was the main factor controlling its evolution or sea level changes ascribed to this time were not eustatic. Growth of the platform reflects three phases during which sedimentation initially lagged behind, kept pace with, and finally exceeded the rate of relative subsidence. The margin of the platform was a carbonate sand shoal complex initially rich in Nuia and calathid algal reefs but at its climax composed mostly of Girvanella- rich oncolites and receptaculitid algae. Seaward of the margin the slope was characterized by fine- rained carbonate and terrigenous clastic sediments as parted to ribbon limestones with hardgrounds and slumped horizons, punctuated by limestone conglomerates. Rapid accretion of the massive prograding oncolite sand shoals during the second phase impeded water circulation to the open platform behind them, resulting in euxinic lagoonal accumulation of black shales and intercalated storm deposits as tongues of coquina, oncolite, and fossiliferous mudstone. Slowing of relative subsidence on the shelf during the third phase is indicated by accumulation of thoroughly burrowed carbonates and laterally equivalent development of peritidal flats, which prograde westward.

Quartz sands prograded from the northeast toward the southwest and west, eventually overwhelming most of the platform. Dolomitization and karstification appear to have taken place prior to deposition of quartz sands in a seemingly anomalous southwest-northeast narrowly elongate area.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.