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The Twin Creek Limestone in northern Utah was deposited in a shallow sea during the Middle Jurassic. Paleocurrent patterns determined from study of ripple marks and cross-stratification, and the distribution of facies, indicate that the average shoreline orientation was northeast-southwest.
Two major transgressive-regressive cycles, which are correlated with sea level curves proposed by P. R. Vail and others for the Jurassic, are delineated. The lower cycle in the Twin Creek Limestone unconformably overlies the Lower Jurassic Nugget Sandstone. The Twin Creek Limestone is represented by deposits of the Gypsum Springs, Sliderock, Rich, and Boundary Ridge Members. Lithofacies in this cycle sequence grade upward from reworked Nugget Sandstone at the base into oosparite, micrite, and silty ripple-marked micrite, to an overlying discontinuous red siltstone unit. The cycle is correlative with the J2.1 global cycle of relative change of sea level.
The upper cycle is separated from the lower one by a hiatus of unknown duration. This boundary is recognized on the basis of the thin discontinuous red bed at the top of the Boundary Ridge Member. The cycle consists of the Watton Canyon, Leeds Creek, and Giraffe Creek Members. Lithofacies grade from oosparite at the base up into micrite, silty micrite interbedded with sandy oosparite, and sandstone. The upper boundary is marked by the base of the Middle Jurassic Preuss Sandstone. The cycle is correlative with the J2.2 global cycle.
Duration of these global cycles is about 8-10 m.y. Recognition of the cycles leads to improvement in stratigraphic and structural interpretation incorporating the effects of sea level changes.
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