Graptolite-Bearing Ordovician Siliceous and Volcanic Rocks, Northern Independence Range, Nevada

Abstract

The northern Independence Range of northeastern Nevada has a well-exposed succession of siliceous and volcanic rocks that are rich in Ordovician graptolites. This succession is divided into three new formations that are considered subdivisions of the widespread Valmy Formation, which is raised to group status in this paper. The highest peaks of the northern Independence Range are composed of about 1000 feet of massive white quartzite containing interbedded chert and graptolitic shale, the McAfee Quartzite of medial Ordovician (Llandeilan to Caradocian). A similar thickness of bedded chert, pillow lava, and argillaceous rock, the Snow Canyon Formation, containing older lower Ordovician (Arenigian) graptolites is exposed in deep canyons below the quartzite succession. Locally, the upper part of the Snow Canyon Formation has many interbeds of dark quartzite bearing somewhat younger, probably earliest medial Ordovician (Llanvirnian or Llandeilan) graptolites. The Jacks Peak Formation, overlying the McAfee Quartzite, consists of a lower, predominantly black chert member about 200 feet thick and an upper quartzite member of more than 200 feet. Graptolites from the lower member of the Jacks Peak are late medial Ordovician (upper Caradocian). The Jacks Peak Formation is overlain unconformably along Jack Creek north of the high range by the Carboniferous and Permian? Schoonover Formation (Fagan, 1962) of radiolarian chert, mudstone, conglomerate, and volcanic rocks. To the south, the Seetoya sequence of cherts, argillaceous rocks, and volcanic rocks that is thrust over lower Paleozoic carbonate rocks contains middle Ordovician (Caradocian) as well as the lower Silurian (Llandoverian) graptolites previously reported (Kerr, 1962). In the northern Independence Range, representatives of the standard graptolite zones are recognized, permitting correlation of the new formations of the Valmy Group with the following graptolite-bearing siliceous assemblages in widely separated parts of Nevada and Idaho: undivided Valmy and Vinini Formations and Basco, Petes Summit, Sams Springs, Perkins Canyon, and Phi Kappa Formations. Contrasting with this siliceous assemblage is about 4000 feet of Permian and Carboniferous? calcareous sandstone, siltstone, and limestone that unconformably overlies Cambrian carbonate rocks in a triangular fault block at the northeastern tip of the range. This block seems autochthonous with respect to the surrounding siliceous and volcanic rocks. The placement of the siliceous sequence into the present position apparently was accomplished by post-Carboniferous thrusting, for the late Paleozoic rocks in the two sequences contrast in facies, and each lies unconformably on similarly contrasting early Paleozoic rocks although they are of different systems. A fault zone exposed along the eastern base of the range has blocks of Ordovician, Silurian?, and Carboniferous limestones in fault-bounded wedges within the Snow Canyon Formation allochthon and has autochthonous Carboniferous-Permian limestone on the east. This fault may be the northern continuant of the Seetoya thrust that is exposed farther to the south.