EARLY TRIASSIC (SMITHIAN) PALEOGEOGRAPHY AND AMMONITES IN SOUTHWESTERN UTAH

In Iron and Washington Counties, southwestern Utah, the lowest unit of the Moenkopi Group (Formation) is marine strata up to 137 m thick termed Timpoweap Formation (Member) that disconformably overlie the Permian Kaibab Formation and are conformably(?) overlain by the siliclastic red beds of the Moenkopi lower red member. The marine interval is equivalent to and lithologically very similar to the Sinbad Formation of east-central Utah, so the name Timpoweap is a superfluous synonym of Sinbad and should be abandoned. Previous scattered reports of Smithian ammonites (Meekoceras assemblage) in the Sinbad Formation of southwestern Utah were of limited occurrences of poorly preserved specimens in shallow marine limestone facies of the Hurricane Cliffs area. We recently discovered an extensive and diverse ammonite fauna of Smithian age in the Sinbad Formation just east of Cedar City. These ammonites occur in a 76-m thick interval of calcareous shale, nodular wackestone and ledgy ammonite wackestone of the middle and upper Sinbad Formation. This ammonite fauna and a regional reappraisal of Sinbad lithofacies suggest that the Sinbad seaway shallowed rapidly along its southern margin in Washington County, and deepened to the north into Iron County. Indeed, deeper water facies of the Sinbad extend northeastward into Capitol Reef and the San Rafael Swell, where they rapidly shallow again onto the seaway’s eastern shoreline. These lithofacies thus identify a Sinbad bay in southern Utah of Smithian age based on its ammonite faunas. Identification of the Sinbad bay is a refinement of regional Early Triassic paleogeography.