THE PERMIAN-TRIASSIC BOUNDARY IN THE VICINITY OF CURRIE, NEVADA

In western North America the Permian-Triassic boundary has long been thought to be absent due to a global regression and subsequent erosion. The occasional presence of a conglomerate between strata of the two systems has been cited as evidence of subaerial erosion of the latest Permian sediments within the Cordilleran miogeocline. Petrographic and cathodoluminescence studies of two sites within 20 km (~13 miles) of the small town of Currie, Nevada suggest that the area was a site of relatively continuous marine deposition. The youngest brachiopods reported from the Gerster Formation are Wordian (Middle Permian) in age, but dolomitization has entirely obscured all fossil remains from the uppermost meter or so of the Gerster Formation, and significantly obscured many of the fossils in the immediately underlying sediments at one site. Study of the >2 meters of sediments that crop out between the characteristic brachiopod and bryozoan-rich cherty limestones of the Gerster Formation and the bivalve-rich silty limestones of the Smithian (mid-Early Triassic) Thaynes Formation has yielded significant information. Overlying the Gerster is a 0.5 meter thick conglomerate composed of silicified brachiopod shells and shell fragments, sub-angular chert pebbles, and sub-rounded silicified carbonate lithoclasts, probably derived from Gerster sediments, in a completely dolomitized matrix. Overlying this conglomerate is a laminated unit containing rip-up clasts and desiccation cracks, indicative of an inter-tidal environment, which is overlain by a largely recrystallized grainstone. These units suggest that the area became very shallow, and probably hypersaline, but no evidence for long-term subaerial exposure such as hardgrounds, truncated veins, or meteoric cements was observed. The absence of globally recognized Upper Permian (Lopingian) index fossils such as conodonts or ammonoids can be more easily explained by an inhospitable environment of deposition than by a complete lack of deposition or the more commonly invoked erosional hiatus.