Lower Triassic Transgressive-Regressive Sequences in the Rocky Mountains, Eastern Great Basin, and Colorado Plateau, USA

Abstract Lower Triassic marine strata in many parts of the world are transgressive deposits, produced by a global rise in sea level after the Permian lowstand. The relative change was greater than 200 m during the Early Triassic (Scythian) interval. Superimposed upon this general trend was a series of transgressive - regressive (T-R) cycles. Three Lower Triassic transgressive sequences are well-documented in the Canadian Arctic and the Cordilleran miogeocline of the western U.S.A. In both of these regions, transgressions were rapid and of relatively short duration. In the miogeocline of the western U.S.A., three Lower Triassic transgressions spread across the site of the former Permian Sublett basin centered in southeastern Idaho during the Griesbachian, early Smithian, and early to middle Spathian stages. Three corresponding T-R cycles are recorded from Arctic Canada during the Griesbachian-Dienerian, earliest Smithian, and late Smithian-Spathian. Nearly synchronous transgressive events are recorded in marine strata of other regions, although they are not always identical in relative magnitude. Biostratigraphic control for the T-R cycles in the Sverdrup Basin of Canada is provided by ammonoid, bivalve, and palynological data. In the western U.S.A., diagnostic macrofauna are scarce, and transgressive conodont faunas and their associated lithofacies provide information on the timing and extent of transgressive sequences. Periods of faunal expansion were linked to relative increases in sea level. Radiation of cosmopolitan Lower Triassic conodont species also reflects these eustatic changes.