WEST-VERGENT DEFORMATION IN THE CAMBRIAN-ORDOVICIAN PREBLE FORMATION IS PENNSYLVANIAN AT EDNA MOUNTAIN (HUMBOLDT COUNTY, NEVADA)

An unusual age and style of deformation has long been recognized in the Cambrian-Ordovician Preble Formation exposed at Edna Mountain, the Osgood Mountains, and the Sonoma Range, in Humboldt County, Nevada. Unlike most rocks of similar age in the state, which are relatively unmetamorphosed ,the Preble Formation is metamorphosed to greenschist grade. This metamorphic fabric is then folded into west-verging folds. Both the metamorphism and the sense of vergence are anomalous for this part of the Cordillera; they were recognized as such by Erickson and Marsh (1974), who interpreted them to record a pre-Antler (and possibly as old as Cambrian) deformation event. Other workers in the region have interpreted the west-verging deformation in the Preble to be pre-Pennsylvanian in age, based on the observation that it is unconformably overlain by "Antler Overlap Sequence" rocks, including a basal conglomerate which they assumed to be lower Pennsylvanian.

Detailed structural and biostratigraphic analyses of the late Paleozoic "Antler Overlap Sequence" rocks at Edna Mountain reveal a complex deformation history, and provide the opportunity to better understand the deformation in the Preble Formation. Distinctive deformation events, separated by angular unconformities, are recognized within the late Paleozoic section; their ages are bracketed by the ages of the over- and underlying units. A distinctive folding event characterized by west-vergent, asymmetric to overturned folds is unequivocally mid-Pennsylvanian in age. Morrowan to mid-Atokan rocks are involved in this folding, and late Missourian to early Virgilian rocks unconformably overlie it. The west-vergent folding of the metamorphic fabric in the underlying Preble Formation has the same geometry and orientation as this deformation within the "Antler Overlap Sequence". We therefore interpret the post-metamorphic, west-vergent folding in the Preble Formation to be mid-Pennsylvanian, not older as previously thought.