LITHOSTRATIGRAPHIC VARIATIONS IN THE NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON- INSIGHTS INTO PROVENANCE FROM GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF SHALES

The Neoproterozoic (c.a. 800-742 Ma) Chuar Group in the eastern Grand Canyon is a shale-dominated succession approximately 1600 m in thickness comprising seven members in two formations. Preliminary geochemical and petrographic analyses of Chuar shale samples show significant stratigraphic bulk-rock and mineralogical trends that suggest changes in provenance, basinal sediment distribution and/or weathering intensity in the source region.

Enriched Si, Al, Ti and depleted Fe abundances in the Awatubi and Walcott members indicate that the Kwagunt Formation is composed of sediment either of differing provenance or more highly weathered than the underlying Galeros Formation. BSEM data confirm textural (grain size and sorting) and mineralogical (abundant kaolinite, rutile and quartz) characteristics consistent with variation in weathering or provenance for shales of the Kwagunt Formation. In contrast, the Carbon Canyon, Jupiter and Tanner members of the underlying Galeros Formation are chemically (enriched Fe, Mg) and texturally (coarser and more poorly sorted) less mature. In addition, the Galeros Formation generally contains more detrital mica, chlorite and feldspar. These detrital mineral components suggest a significant plutonic or metamorphic provenance. If our interpretations are correct, the upper Chuar may record more intense silicate weathering, consistent with models for drawdown of atmospheric carbon dioxide and cooler conditions perhaps tens of millions of years prior to the first recognized "snowball" event (Sturtian glaciation).

Petrographic analysis further indicates a significant volcanic ash component in the Tanner, Carbon Canyon and Awatubi members. Both detrital monazite (<10 micrometers) and zircon (<10 micrometers) are present suggesting the possibility of dating of source region or (optimistically) ashfall components. These results, in conjunction with high-resolution lithostratigraphy, C-isotope stratigraphy, and the distribution of Chuaria and Melanocyrillium in the Chuar Group, provide a means of dating and better correlation with other Neoproterozoic shale-bearing sequences in the southwestern US.