Sources of Middle Proterozoic to Early Cambrian siliciclastic sedimentary rocks in the Great Basin: A Nd isotope study

Abstract

The Nd isotopic compositions of Middle Proterozoic to Early Cambrian terrigenous sedimentary rocks in the Great Basin, United States, were determined to identify the source areas of the siliciclastic detritus and to constrain models for the early tectonic evolution of the western margin of the continental United States. In the southwestern Great Basin and vicinity, Nd isotope data from the intracratonic, Middle to Late Proterozoic Pahrump Group reveal that the coarse-grained detritus from the Crystal Springs and the younger Kingston Peak Formations, including apparent glacial dropstones, was derived from the local Proterozoic basement (province 1, as defined by Nd isotope data). Fine-grained detritus derived from other Proterozoic crustal terranes (provinces 2 or 3) was identified in the southernmost exposed portions of the Pahrump Group, and corresponds to more distally derived material originally eroded from Proterozoic basement farther to the south and east. Coarse- and fine-grained sedimentary material composing the overlying Early Cambrian miogeoclinal strata were also derived primarily from local Province 1 basement rocks, but the occurrence of detritus derived from Province 2 or 3 crust is widespread geographically. The deposition of sediment derived from province 2 or 3 across the developing continental shelf presumably reflects thermal subsidence and progressive marine encroachment of the ancient continental margin. Within the Middle Proterozoic to Early Cambrian section, only the arkosic middle member of the Wood Canyon Formation and the correlative deeper marine Andrews Member of the Campito Formation have measured ϵ Nd values greater than −15 (−6 to −4). These units were apparently derived from a local, ∼1 b.y. old, granitic source exposed during the Early Cambrian Period to the northeast of present-day southern Nevada. The tectonic settings of the formation and subsequent uplift of this granitic terrane are unknown, but erosion of the granite generated excellent Nd isotopic marker beds that should prove useful in correlating Early Cambrian miogeoclinal sedimentary rocks throughout southwestern North America. In the north-central Great Basin, Late Proterozoic miogeoclinal sedimentary rocks of the Trout Creek and McCoy Creek groups show a marked shift from province 2 or 3 to province 1 sources through time. This shift is consistent with a change in topography related to the emergence of the Tooele-Uinta arch as an area of positive relief. The Nd data provide the first evidence that this structure was present in this region as early as latest Proterozoic time. As in the southern Great Basin, however, the isotope data indicate that sediments deposited in the Late Proterozoic Era in the northern regions were dominantly derived from sources in the local Precambrian basement.