Geologic map of part of the Marble Mountains, San Bernardino County, California: Supplement 4 from "I. Geology of the Wildrose area, Panamint Range, California. II. Geochronologic studies in the Death Valley-Mojave Region, California" (Thesis)

The bedrock in the Wildrose area is predominantly a sequence of metamorphosed sedimentary and possible volcanic rocks that is more than 15,000 feet thick. All the metamorphic rocks are Precambrian in age on stratigraphic evidence. The rocks are divided into three age groups, early Precambrian, later Precambrian, and Precambrian (?), that are separated by unconformities. The early Precambrian Panamint metamorphic complex has been divided into two series that are separated by a profound unconformity. Small stocks and dikes of granitic rocks, which are presumably Cretaceous in age, intrude the metamorphic rocks. Tertiary sedimentary rocks and Quaternary alluvium are also present in the area. The structural features of the area are ascribed to four periods of deformation. The large folds in the early Precambrian rocks and associated minor folds were produced during the first two periods of deformation of early Precambrian age. Low angle faults are evidence of the third deformation which is considered to be Cretaceous in age. The fourth period of deformation produced high angle faults. Movement along the high angle faults began in late Tertiary time and has continued to the present. All the Precambrian rocks of the area have been affected by middle or lower middle grade metamorphism. Even though structural evidence suggests that metamorphism must have accompanied the earlier periods of deformation, the present mineral assemblages reflect the third period of deformation which, on the basis of mineral age measurements, was Cretaceous in age. Narrow contact metamorphic aureoles surround the larger masses of granite. Radioactive ages were measured in five areas in the Death Valley-Mojave Desert region. The rocks are Precambrian in age on stratigraphic evidence in three areas, the Wildrose area of the Panamint Range, the Mountain Pass district, and the Marble Mountains. The rocks in Joshua Tree National Monument and the Kilbeck Hills are presumably Precambrian, but no stratigraphic evidence of their age has been found. The geochronologic studies in the five areas indicate three distinct groups of K - Ar and Rb-Sr ages. Ages of approximately 1650 million years are obtained on metamorphic rocks and associated pegmatite s in the Mountain Pass district. Younger igneous rocks at Mountain Pass and in the Marble Mountains have age patterns that are interpreted to indicate intrusion in the 1350 to 1450 million year interval. Metamorphic rocks in the Wildrose area, Joshua Tree National Monument , and the Kilbeck Hills have ages in the 75 to 85 million year interval which are interpreted as the age of regional metamorphism related to the Cretaceous Nevadan orogeny. An age of 73 million years was obtained on a post-metamorphic granite.