Stratigraphy, Sedimentology, and Sequence Stratigraphy of the Lower Triassic Sinbad Formation, San Rafael Swell, Utah

The Lower Triassic Sinbad Formation (Thaynes Group) in the San Rafael Swell of east-central Utah (11-18 m thick) represents the maximum flooding zone and at least part of the highstand systems tract of a 185+ m thick depositional sequence. The Sinbad Formation conformably overlies siliciclastic tidal flat, fluvial channel and carbonate peritidal deposits of the Black Dragon Formation, a transgressive systems tract, and is overlain by prodelta, delta front and fluvial deposits of the Torrey Formation. Vertical facies relationships in the Sinbad Formation in the San Rafael Swell and Capitol Reef indicate initial deepening followed by shallowing. The carbonate peritidal deposits of the upper part of the Black Dragon Formation represent the regional onset of carbonate sedimentation during the Early Triassic. These intertidal deposits are overlain by an offshore facies at the base of the Sinbad Formation that represents maximum flooding. The offshore facies thickens to the north (oceanward) and contains an open-marine fauna dominated by ammonoids, bivalves, gastropods and scaphopods. The overlying deposits are restricted foreshoal/shoal, peloidal-oolitic grainstone, backshoal/lagoonal skeletalintraclastic-oolitic grainstone and mudstone (with a depauperate fauna composed of gastropods, bivalves and ostracods). These deposits also contain tidal channel skeletal-intraclastic-oolitic grainstone. A shoreface/shoal, skeletal-peloidal-oolitic grainstone caps the Sinbad Formation regionally.