BIOSTRATIGRAPHIC RELATIONSHIPS AND STRUCTURAL IMPLICATIONS OF PALEOZOIC SEDIMENTS OF THE ROBERTS MOUNTAINS ALLOCHTHON, NORTHERN CARLIN TREND, NEVADA

A detailed biostratigraphic study was conducted on the Dee and Rossi properties of the Santa Renia Fields 7.5’ Quadrangle, Elko County, Nevada, with the purpose of reconstructing the local Paleozoic stratigraphy of the Roberts Mountains allochthon (RMA) and producing revisions of an existing lithologic map. Siliceous rock units of Ordovician, Silurian and Devonian age were differentiated using radiolarian biostratigraphy. Additional siltstone and mudstone beds were assigned ages based on their stratigraphic relationships to dated strata and on correlation to units of known age from the neighboring Ren property. Measured stratigraphy amounts to 100 meters thickness with an estimated 200 meters additional stratigraphy unmeasured due to poor exposure. Biostratigraphy allows for a geologic subdivision of the allochthonous stratigraphy into previously established stratigraphic groupings within the RMA. Differentiated units include the upper Vinini Formation, Cherry Spring chert, Elder Sandstone and the Slaven Chert. Biostratigraphy also reveals that all occurrences of bedded barite are Devonian or younger. The oldest occurrence of these beds is equivalent to or younger than the Emsian Helenifore laticlavium assemblage.

Reconstruction of a measured stratigraphic column with the aid of biostratigraphic sampling allows for detailed revisions of a part of the existing lithologic map at a scale of 1:6,000. This area is designated the Dee-Rossi reference section. Applied at a larger scale, the reference section stratigraphy is used to differentiate mapped lithologic units over a wider area. The map reveals that the stratigraphic reference section is indicative of a distinct stratigraphic domain with two additional, structurally bound, stratigraphic domains occurring to the north. These domains may represent separate plates, or thrust sheets, within the RMA stratigraphy. Several structural features recognized in the adjacent Beaver Peak 7.5’ Quadrangle are seen to extend into the Dee and Rossi property, including the Little Jack Thrust and the Coyote Thrust. These structures and the Emsian-or-younger bedded barite deposits are interpreted to be of regional significance with respect to tectonic reconstructions of the Paleozoic western Cordilleran margin.