GEOMETRY OF POST-LOWER TRIASSIC DEFORMATION, COAL MINE BASIN, ADOBE RANGE, ELKO COUNTY, NEVADA

A thick section of Paleozoic and Mesozoic rocks in Elko County, Nevada has been deformed by several orogenic events beginning in late Devonian. New data suggest it may be possible to separate kinematically distinct events. A late phase of deformation, post-Lower Triassic, is clearly recorded in Permian to Lower Triassic rocks exposed in Coal Mine Basin, in the northern Adobe Range. The Lower Triassic rocks are silts and shales with interbedded limestones; they are dated based on ammonoids and conodonts. The Lower Triassic rocks crop out for >15 km along strike and throughout this distance are folded into a macroscopic asymmetric syncline known as the Adobe Syncline. Timing of the folding is bracketed by the Lower Triassic rocks and the Eocene Elko Formation, the oldest sedimentary overlap.

All structures in the Lower Triassic rocks are consistent with a single southeast-vergent folding event. Mesoscopic fold axes in the Lower Triassic rocks plunge up to 10 degrees toward S60W. Folding style ranges from kink folds to sub-isoclinal folds. The southeast limb of the macroscopic syncline dips 45 degrees northwest; the northwest limb dips steeply and is locally overturned. Axial plane cleavage in the shale dips steeply northwest.

Because several Late Paleozoic deformation events have recently been recognized elsewhere in the Adobe and Piñon Ranges, the initial objective of this study is to establish the geometry of the later (post-Triassic) deformation, in order to distinguish it from the earlier events. Results to date suggest that the Mesozoic deformation in Coal Mine Basin is geometrically distinct from late Paleozoic deformation recognized elsewhere in the Adobe Range, and may be distinguishable in areas where age relations are not clear.