2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 8
Presentation Time: 8:00 AM-12:00 PM


SCHOENBORN, William A., Dept. of Earth and Environmental Sciences, The George Washington University, Washington, DC 20052 and FEDO, Christopher M., Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, schoenborn.william@epa.gov

Neoproterozoic to Cambrian strata in southeastern California mantle a craton-margin hinge-zone that formed in response to the fragmentation of Rodinia. Major-, trace-, and rare-earth-element geochemistry of the Johnnie Formation are used to reconstruct the sediment provenance and determine controls on the mineralogic and chemical characteristics of the siliciclastic sediments. A multi-element plot shows modest depletion in Ba, Sr, Pb, Ta, T, Cr, Co, and Ni relative to average Proterozoic cratonic shale. Coupled with variation in values of the chemical index of alteration (CIA) from 56 to 79 suggests that Johnnie sediments were derived from variably weathered and recycled bedrock. CIA values appear to be lowered from diagenetic addition of potassium most likely to kaolinite. Sample scatter in Th/Sc vs. Zr/Sc suggests Johnnie sediments experienced zircon enrichment due to sedimentary recycling with the largest ratios associated with massive fluvial and shallow-marine sandstones in the middle of the Johnnie Formation, which is consistent with passive margin deposition. Chondrite normalized REE concentrations of Johnnie Formation muds display averages that are within the range of values for average upper continental crust: Eu/Eu*=0.67; LaN/SmN=4.59;GaN/YbN=1.35. Preliminary U-Pb ages from single zircons define a complex provenance for Johnnie sands. As expected in an early post-rift setting, there are modes at ~1.4 Ga and ~1.7 Ga that represent local Mojave bedrock sources. Additionally, there is a mode at ~1.0-1.1 Ga that cannot be explained by local sources, which likely represents a Grenville provenance. Clearly this mode requires a source from outside the Mojave region and indicates that the rift shoulder had been breeched and sediment from the interior of the craton was reaching the margin. An outside source is also suggested by the fact that REE abundances cannot be duplicated by mixing known Mojave bedrock end members. In combination with the geochemistry (and stratigraphy), the data are consistent with the Johnnie Formation being part of a early, post-rift passive margin.