Abstract

The Devonian Guilmette Formation of southeastern Nevada is composed of cyclic peritidal carbonates deposited on a broad, shallow, semi-arid platform. The Guilmette is divided into lower, middle, and upper members based on lithofacies. The lower member is composed of interbedded limestone and dolomite deposited as shallowing-upward cycles. Overall the lower member shows an increasing predominance of subtidal facies upward to the erosional contact with the overlying middle member, In general the limestones represent subtidal deposition while the dolomites represent intertidal to supratidal deposition.

The chaotically-bedded middle member is a floatstone, composed predominantly of limestone stromatoporoid reef debris and dolomite intraclasts supported in a lime wackestone matrix. The origin of this member is enigmatic. It is characterized by debris flows and collapse structures interbedded with stromatoporoid bioherms. The basal contact of the middle member is an erosional surface that is locally filled by a flat-pebble lag.

The upper member is composed of interbedded limestone, dolomite, and sandstone deposited as shallowing-upward cycles. The limestones represent subtidal deposition. The dolomites occur as either recrystallized limestone or as laminated dolomitic mudstone deposited in an intertidal to supratidal environment. The sandstones are dolomite-cemented quartz arenites that are commonly cross-bedded. They represent deposition in tidal channels and on tidal flats. In general the cycles of the upper member progressively shallow upward.

A relative sea level curve for the Guilmette at this location was constructed using computer-generated Fischer (accommodation) plots. Due to its deepening-upward stacking pattern, the lower member is interpreted as the transgressive systems tract. The basal contact of the middle member is interpreted as the transgressive surface of erosion and it represents the point of maximum transgression. The middle member is interpreted as the highstand systems tract because it represents a stillstand that is characterized by an aggradational stacking pattern. The upper contact of the middle member represents the subsequent sea level fall. The influx of terrigenous clastics in the upper member indicates a lowering of base level and progradation of the shoreline, thereby reflecting a regressive systems tract.