Metamorphosed and Nonmetamorphosed Paleozoic Rocks in the Wood Hills and Pequop Mountains, Northeast Nevada

Abstract

The Wood Hills and Pequop Mountains of northeastern Nevada are located within the Cordilleran miogeosyncline of Paleozoic age and in the Basin and Range province of Cenozoic age. Two structural units or plates, designated Plates I and II, are juxtaposed along the Wood Hills thrust and underlie the area. Plate II overlies Plate I in both ranges and contains nonmetamorphosed Paleozoic carbonates and clastics ranging from Early Ordovician through Middle Permian in age and exceeding 15,000 ft in thickness. Within Plate II are two stratigraphically different tectonic units that have been juxtaposed along the Wells fault, a major east-southeast-trending transcurrent fault with possibly 40 mi of right slip. Rocks typical of the western portion of the miogeosyncline—Roberts Mountains Formation (Silurian)—occur north of the fault, whereas rocks typical of the eastern portion—Laketown Dolomite (Silurian)—occur south of the fault. In contrast to the upper structural unit (Plate II), Plate I consists of regionally metamorphosed Cambrian through Devonian miogeosynclinal strata. Metamorphism produced banded tectonitic marble, micaceous marble, schist, and metaquartzite. Though the rocks have undergone considerable deformation, the metamorphosed succession can be correlated almost member by member with Plate II strata. This correlation is based on stratigraphic sequence and fossils preserved locally in the meta-sedimentary rocks. The metamorphism occurred in two phases, a synkinematic phase that attained the kyanite-staurolite zone, followed by a post kinematic phase interpreted as a record of continued recrystallization after penetrative deformation had ended. During Jurassic-Cretaceous time, the area was subjected to several episodes of orogenesis, beginning with regional metamorphism and ending with large-scale thrusting. Depth of burial during metamorphism for kyanite-bearing rocks of Plate I in the Wood Hills may have been about 5 to 6 mi. Following metamorphism, the Paleozoic succession, both metamorphosed and nonmetamorphosed, was apparently thrust northwestward over Precambrian crystalline rocks, developing large overturned folds in the metamorphic rocks. Subsequently, the non metamorphic rocks (Plate II) were sheared off from the underlying metamorphic rocks (Plate I) along the Wood Hills thrust and moved east to southeast, truncating previously formed structures in the metamorphic rocks; displacement may be as much as 20 to 30 mi or only 5 to 10 mi. While attaining its present position, Plate II was broken up into several smaller structural subdivisions. The Wood Hills thrust represents the regional decollement zone that Misch (1960) considers to be present throughout most or all of the central-eastern Nevada portion of the Great Basin. During the Cenozoic the area underwent intermittent erosion and deposition until Mio-Pliocene? time, when considerable local relief was developed, so that landslide deposits accumulated on both Plates I and II in the Wood Hills. Tertiary volcanic rocks occur in the Pequop Mountains predating range uplift. Sometime during the Cenozoic, possibly before the events referred to above, two north-trending domes formed, one centered under the Wood Hills, and the other, just west of the Pequop Mountains. Uplift of the Wood Hills appears to have occurred without major faulting, whereas the Pequop Mountains is an east-tilted fault block that includes the eastern half of a dome.