BURIED CANYONS: END-LARAMIDE PALEOTOPOGRAPHY OF NORTHEASTERN NEVADA PRESERVED BENEATH EOCENE TO MIOCENE VOLCANIC SEQUENCES

The regional paleoelevation of the Western U.S. is crucial to understanding the potential energy of the lithosphere during the transition from regional contraction to crustal extension at the end of Laramide time. Henry (Geosphere, in press) has recently proposed a model for Mid-Cenozoic paleogeography of the northeastern Great Basin that envisions an Altiplano-type highland deeply incised by eastward-draining paleovalleys that acted as distributary channels for major Middle to Late Eocene ignimbrite flows. Mapping in the southern part of the Jarbidge Wilderness Area in northeastern Nevada supports Henry's hypothesis by delineating a deeply incised canyon system filled with Middle Eocene ignimbrite. The Jarbidge Wilderness is thickly blanketed by Early Middle Miocene Jarbidge Rhyolite, but in the southern part of the area erosion exposes an underlying terrain of stacked thrust sheets with allochthonous western facies rocks thrust over parautochthonous Mississippian to Triassic rocks that are in turn thrust over autochthonous Ordovician strata (Coats, 1977). Twisting through this terrain is a discontinuous linear belt of distinctive pink ignimbrite 30+ km long 1 - 3 km wide and up to 300 m thick that we correlate with a similar sequence bracketed between 41.3 and 42.7 Ma in the Dead Horse tuff in Copper Basin 10 km to the west. Where its basal contact is exposed, the ignimbrite rests not on the allochthonous western facies rocks but on the underlying parautochthon, suggesting a deeply incised paleovalley that had cut through at least 550 m of allochthonous rocks before incising >300 m into the underlying parautochthon. Furthermore, detailed mapping near the headwaters of Camp Creek reveals a newly recognized basal sequence of river gravel buried beneath a massive cooling unit of the ignimbrite at least 100 m thick. In Copper Basin, a lake system that formed shortly after 41 Ma may record the onset of extension and the decapitation of the inferred canyon system to the east. Based on regional correlations, the ignimbrite appears to originate from near Bull Run Basin approximately 60 km farther west. These exposures preserve a valuable snapshot of Eocene paleogeography at a critical juncture synchronous with the ignimbrite flare-up and the transition from Contraction to extension near the end of Laramide time.