Cal Poly Geology Club Death Valley Field Trip – 2004

152.8(13.6) Road forks, stay on the paved road CA 178 (Jubilee Pass Road)160.0 (7.2) Park on road shoulder; Amargosa Chaos (STOP 12).Noble (1941) observeda style of faulting in thissubject area so intricateand complex that hereferred to the faultedrock units as “chaos.”He referred to these, aswell as other similarlyfaulted terranes in theDeath Valley region, asthe “Amargosa chaos.”The Virgin Spring areain the west-central partof the Black Mountains is the type locality for the Amargosa chaos.The geologic features of the Amargosa Chaos record four major deformationalevents. The first, about 1.7 Ga, accompanied and followed the metamorphismof the crystalline basement. It contributed to the angular discordance in thecomplex with the overlying later Precambrian sedimentary units. The secondbegan with the deposition of arkosic conglomerate near the base of the CrystalSpring Formation and continued through Noonday time, spanning an interval ofprobably 400 Ma. This event was accompanied by vertical crustal movementcausing facies changes in the Pahrump Group and Noonday Dolomite, and theangular unconformity beneath. Folds preserved in the later Precambrian andCambrian sedimentary rocks mark a third period of deformation in Mesozoic orEarly Tertiary time. The fourth deformational event can be related to faultingcaused by crustal extension in late Cenozoic time.Noble (1941) recognized three phases of the Amargosa chaos and named themthe Virgin Spring, Calico, and Jubilee phases. The Virgin Spring phase iscomposed almost entirely of units of the Pahrump Group and of the overlyinglatest Precambrian and Cambrian units. The Calico phase consists mostly ofTertiary volcanic units. The Jubilee phase comprises Tertiary conglomerate,finer-grained strata, and bodies of mono!ithologic breccia.The best exposure of the Virgin Spring chaos along California 178 occurs atthis stop. The chaos underlies the steep north face of a hill about 300 ft high anddisplays most of the features that are commonly ascribed to the lower part ofthe chaos in general.The lower part of this face is underlain by the gray weathering, locally redstainedcrystalline complex. Within it are sheared masses of dark green diabasedikes and nearly white granitic pegmatite dikes. All are thoroughly sheared andbecome progressively more so upward to the nearly horizontal contact with theoverlying chaos. The strong evidence of shearing along this contact impressed