REINTERPRETATION OF THE GEOLOGY OF THE CHICAGO PASS AREA

The Chicago Pass thrust system in the northern Nopah Range has served as a significant piercing line in reconstructions of extensional systems in the Death Valley region, but significant questions have arisen over the correlation of thrust faults at Chicago Pass with faults in the Resting Spring Range and Panamint Ranges that are the basis of these reconstructions. We have conducted new detailed mapping together with compilations of older mapping in this area into a GIS, and these observations provide insights into the structural history. Two thrust faults, the Chicago Pass thrust and the Shaw thrust, crop out in the hills on the southeast side of the highway at Chicago Pass. East tilted Tertiary fanglomerates overlap both the Shaw thrust and the Chicago Pass fault indicating that if the Chicago Pass fault was reactivated it occurred long before deposition and east tilting of these fanglomerates. Similarly, megabreccias of Stirling Quartzite and the Bonanza King Formation exist as breccia crusts overlying younger rocks and overlapping faults, but field relationships suggest these megabreccias are landslide deposits emplaced late in the extensional history. Nonetheless, initial petrographic analysis suggests that some of the megabreccia deposits are, in fact, stratigraphically in place and therefore not landslides at all. Mapping north and west of Chicago Pass confirms a southeast-dipping thrust system with a footwall syncline that Wilhelms (1963) referred to as the Burro thrust. This thrust appears to be a hanging-wall backthrust above the Baxter Mine thrust of the central Resting Springs Range. A significant feature of this thrust fault, however, is that it is cut by a gently south-dipping normal fault that has a minimum offset of ~500m. This fault appears to project beneath Chicago Pass, and may account for the large stratigraphic offset across Chicago Pass. If true, this regional interpretation would place the rocks north of Chicago Pass in the footwall of the Chicago Pass thrust and the Baxter Mine thrust in the Resting Springs Range would be the equivalent of the Shaw thrust. Chicago Pass is a classic field area visited by numerous field classes and field trips. A daylong field trip for both high school and college level students will be suggested and outlined.