Cal Poly Geology Club Death Valley Field Trip – 2004

flanking deposits of mud and sand have been metamorphosed to gneiss. Elevenmillion years ago, the gneiss was injected by the Willow Spring pluton. Thedioritic to gabbroic composition of the Willow Spring pluton blends well withthe dark Precambrian gneiss, so you have to look carefully to see the contactrelationship between the two rock types. Badwater is also the northernmost ofthree “turtlebacks” that lie along the Black Mountains western flank. We willbe discussing the origin of the turtlebacks at the next stop.139.2 (~15) Stop along the road for a view to the north of the Copper Canyon Turtleback(STOP 11). (Note: this is onlyan approximation of thelocation of this stop; Don willselect a location that bestshows the geologicrelationships.)Mormon Point affords anexcellent view of the profileof the Copper Canyon turtlebackand the overlyingTertiary sedimentary andvolcanic rocks that have beentransported northwestward over the turtleback surfaces. From Mormon Pointthe view to the northeast is almost at a right angle to the northwest plunge andtrend of the crest of the turtleback surface.Poorly visible from this viewpoint is the relatively thin skin of carbonate rockdirectly beneath the turtleback surface. This is because the southwest flank ofthe turtleback surface has been cut by younger faults along the mountain front.Precambrian foliated gneissic rock and foliated Mesozoic dioritic rock formmost of the range front in view beneath the turtleback. Patches of carbonaterock are exposed at the crest of ridge. A few dikes of Tertiary intrusive rockscross the turtleback at nearly right angles to the trend of the ridge crest.The turtleback surfaces were first recognized and named by Curry (1938). Sincethen they have been the subject of considerable debate. Today it is recognizedthat most of the crustal extension that has occurred in Death Valley can beattributed to low-angle normal (detachment faults). These faults juxtapose,older, strongly metamorphosed, ductility deformed mid-crustal rocks (lowerplate core complexes) and younger, largely unmetamorphosed, brittleydeformed, volcanic and sedimentary rocks (upper plate). Typically the upperplate rocks are cut by a series of high angle normal faults. The Black Mountainfault zone is an example of a major detachment fault. Death Valley hasdropped downward and tilted to the east along the detachment surface. Thefault zone remains active today as evidenced by numerous high angle faultscarps that can be seen dissecting the alluvial fans.