LATERAL VARIATION IN LITHOSTRATIGRAPHY ALONG A SHELF TO SLOPE TRANSITION FOLLOWING A LATE PROTEROZOIC GLACIAL EVENT, BLACK MOUNTAINS OF SOUTHERN DEATH VALLEY

Excellent exposures in the Black Mountains of eastern California provide a key section linking disparate facies of Neoproterozoic age.  Sudden lateral as well as vertical changes in lithofacies and mineralogy characterize the late Proterozoic succession in this region.  Extensive geologic mapping reveals seven distinct lithologic units exposed along a previously inferred platform to slope transition.  The lowermost lithologic unit is a basal diamictite of disputed age and origin.  The diamictite thickens to the south as the overlying lower Noonday Dolomite (commonly considered a cap carbonate) thins from a tube-bearing microbial dolomite (> 80 m) to a thin mechanically laminated dolomite (< 5 m).  The Ibex Formation (considered a time equivalent unit to the lower Noonday) consists of a lower arkosic member composed of turbidite and siltstone deposits which rest conformably above the dololaminite in down-dip localities to the south.  The arkosic unit wedges out to the north, truncating distinct dolomite beds conformable with the lower Noonday.  Thin massively bedded limestones containing amalgamated intraformational conglomerates overlie the lower Noonday and the purple arkosic member in both up-dip and down-dip localities across the transition.  Overlying the limestone unit is a massive grey, sandy quartz dolomite, consisting of rip-up clasts and amalgamated beds representing gravity driven deposition on an ancient slope.  Several packages of laminated carbonate siltstone occur within this unit suggesting ambient sedimentation.  A possible explanation for the abrupt variations in lithology, mineralogy, and grain-size is the isostatically driven exposure of lithologically distinct terrains coupled with eustatic sea-level rise and tectonic subsidence along the Noonday margin following one of the largest glacial episodes in Earth history.