THE CHESTERIAN AND MORROWAN ENVIRONMENTS AND ECOLOGY OF THE WESTERN BIRD SPRING BASIN (NEVADA)

Analysis of conodont faunas in the Chesterian and Morrowan strata of the Bird Spring Basin in southern Nevada resulted in the recognition of four zones based on those of the midcontinent (Lane, 1977): Zone 1 (equivalent in its upper part to the Kladognathus-Cavusgnathus naviculus Zone); Zone 2 (equivalent to the Adetognathus unicornis and Rhachistognathus muricatus Zones); Zone 3 (equivalent to the R. primus Zone); and Zone 4 (equivalent in its lower part to the Idiognathoides sinuatus Zone). The base of Zone 3, marked by the lowest stratigraphic occurrence of either R. primus or A. lautus, is here interpreted to be the Mississippian-Pennsylvanian boundary.;Five sedimentary facies were recognized from petrographic and field data: Facies I (poorly-fossiliferous sandstones, siltstones, shales and minor dismicritric limestones interpreted as supratidal and shoreface deposits); Facies II (intraclastic and siliciclastic-rich biosparites and sandstones interpreted as shallow shelf deposits just above normal wave base); Facies III (oolitic and oncolitic biosparites deposited in carbonate sand bars and back-bar lagoons); Facies IV (fossiliferous peloidal limestones interpreted to have formed below normal wave base on an open marine platform); and Facies V (organic-rich micrites deposited on the outer, deeper parts of a marine shelf). The nearshore conodont biofacies is restricted to Facies I-III, and the offshore biofacies is present in Facies IV and V.;Analysis of the succession of sedimentary facies through time indicates that the northwestern and southern portions of the basin subsided more rapidly than the center, which remained for a short time as a broad, low-lying arch. At the end of the Early Morrowan, subsidence was apparently consistent along the axis of the basin.;Fossil distribution in the lower Bird Spring Group shows a trend from dominantly brachiopod and conodont faunas in the north to coral and foraminferan faunas in the south. This may be related to a restriction of normal marine circulation in the south, which resulted in elevated salinity. This restriction diminished during the Early Morrowan.;Two conodont biofacies were revealed, one nearshore and one offshore. Both are similar to those noted in the midcontinent and Appalachian regions.;The distinct megafossil communities were recognized. One is a productid brachiopod association in Facies II; the other is a compositid-bryozoan community in Facies IV. An excellent bedding-plane exposure of this community demonstrates an ecological system in which succession was controlled by the baffling of current-transported bioclastic debris.