WHITEROCKIAN (MIDDLE ORDOVICIAN) GRAPTOLITES OF THE LOWER MEMBER OF THE VININI FORMATION, ROBERTS MOUNTAINS, EUREKA COUNTY, NEVADA

The Ordovician strata of the Lower Member of the Vinini Formation comprise a sequence of greenstone, sandstone, shale, and siltstone representing the prograding and retrograding of submarine fans along the continental margin. Although graptolites are normally preserved within shale beds in the Lower Member of the Vinini Formation, the greatest abundance of well preserved graptolites is found within the sandstone turbidite beds. These graptolites are uniquely preserved in full relief as opposed to being flattened on shale. It is interpreted based on fragmentation and species composition within the sandstone that the graptolites flourished in an upwelling zone on the continental margin and that as their remains accumulated on the underlying seafloor, were swept downslope in turbidity currents. Furthermore, given the very low diversity of graptolites in the intervening shales, it is concluded that graptolites did not flourish in the oceanic waters directly above the site of deposition.