GEOCHEMICAL AND ISOTOPIC CONSTRAINTS ON THE PROVENANCE OF NEOPROTEROZOIC THROUGH MIDDLE CAMBRIAN UNITS IN THE DEATH VALLEY REGION, EASTERN CALIFORNIA

Geochemical data from siliciclastic units exposed in the Death Valley region, which were deposited during the Neoproterozoic through Middle Cambrian, were examined to determine their provenance characteristics. Trace-element and isotopic studies of sandstone and shale samples reveal that the sources for the studied samples (upper Johnnie through Carrara Formations) all lie within the range of each other. REE patterns are fairly consistent throughout the package and have fractionated light REEs (La-Sm), small negative Eu anomalies, and typically flat heavy REE (Gd-Lu). Such characteristics are common to post-Archean shales and point to a continental source, with the sediments having been homogenized in a passive margin setting. The Th/Sc ratio, which is sensitive to subtle compositional changes in source rocks, ranges from 0.5 to 2.5, with an average of 1.6. In contrast, the Zr/Sc ratio, which represents a measure of sediment recycling, ranges from <10 to ~200. The data plot along a line typical of recycled sediment in passive margins. Measured eNd values from seven samples from the Johnnie Fm to Zabriskie Quartzite range from -15 to -22, which is generally consistent with the derivation of the detritus from Precambrian crust of Mojavia and adjacent Nd Province 2/3. The underlying Crystal Spring through Kingston Peak Formations were derived from these same source regions (Farmer and Ball, GSA Bull, 1987). Therefore, no obvious changes occurred in the sources of sediments deposited in this region during the time interval that presumably spanned the transition between rift and passive margin sedimentation. In contrast, the middle member Wood Canyon Formation and correlative units elsewhere in the region are characterized by high eNd values (eNd ~ -4 to -8) and the occurrence of 1.0 to 1.1 Ga old detrital zircons. These sediments can be interpreted as representing an influx of distantly derived detritus to the miogeocline. The data suggest that considerable local variations may exist in the provenance of siliciclastic sediments deposited along the axis of this portion of the Cordilleran miogeocline.