Early Triassic Stratigraphy and Paleogeography of the Cordilleran Miogeocline

Abstract Lower Triassic rocks represent the last widespread episode of marine sedimentation in the Cordilleran miogeocline displaying a depositional and structural pattern inherited from the late Paleozoic, and possibly the late Precambrian. Early Triassic time-stratigraphic correlations which use a basin-wide conodont zonation permit definition of facies patterns for restricted time-rock units and subsequent recognition of persistent paleogeographic and paleotectonic features. In addition to the classic north-south trending miogeocline, a relatively deep basin, centered in southeastern Idaho, influenced depositional patterns throughout the Early Triassic. The depositional basin was bordered on the north, east, and south by shallow shelves. To the north and east Lower Triassic rocks thin and intertongue with red bed formations. The southern boundary between shelf and basin generally coincided with the Permian Oquirrh-Uinta uplift, and may have been related to vertical movement on a major east-trending flaw in the crust, along which periodic tectonic adjustment occurred. The progressive onlap of older Lower Triassic rocks onto the Permian unconformity in eastern Nevada is well documented by the work of others. However, farther to the west in central Nevada, the last known exposures of younger Lower Triassic rocks (Spathian Stage) show little evidence of shoaling on postulated highlands. Four transgressive episodes are recognized in the Griesbachian, Smithian, mid-Spathian, and latest Spathian. Marine sedimentation ceased on the shelves after the mid-Spathian, but continued within the basin until the latest Early Triassic or earliest Middle Triassic. Termination of marine sedimentation in the miogeocline may be related to a collision of the Sonomia microcontinent with the passive margin of western North America.