PETROGRAPHIC ANALYSIS OF THE CARBON CANYON AND CARBON BUTTE MEMBERS OF THE CHUAR GROUP, EASTERN GRAND CANYON: POSSIBLE PROVENANCE CHANGES

A petrographic pilot study of the middle Chuar Group sandstones indicates a change in source area(s) between Carbon Canyon and Carbon Butte deposition. The Carbon Canyon Member (~650 m thick), Galeros Formation, and overlying Carbon Butte Member (~40-70 m thick), Kwagunt Formation, comprise the middle Chuar Group (1600 m thick). The Carbon Canyon Member is composed of dominantly variegated mudrocks with subordinate m-scale interbeds of sandstone and dolomite. Sedimentary structures in the Carbon Canyon sandstones (horizontal and low-angle planar laminations, trough and planar tabular crossbeds, and mud-cracked mud-draped symmetric ripples) indicate wave- and tide-influenced sand flat deposition. The Carbon Butte Member is dominantly composed of variegated mudrocks and £~10-m-scale sandstone beds. Sedimentary structures in the Carbon Butte sandstones (horizontal and low-angle planar laminations, rare bipolar trough crossbeds and small-scale dunes showing reverse-current flow, epsilon crossbeds, and mud-cracked mud-draped symmetric ripples) indicate wave- and tide-influenced deposition, perhaps within a barrier-bar complex.

Preliminary results from petrographic analysis (Gazzi-Dickinson Method) indicate that the Carbon Canyon and Carbon Butte members are mineralogically and texturally distinct. Carbon Canyon sandstones are subfeldarenites, sublitharenites, quartz arenites and wackes, contain ~5% feldspar, and are texturally more immature than the Carbon Butte sandstones (more matrix, moderately sorted). Carbon Butte sandstones are sublitharenites to quartz arenites, contain no feldspar, show more variability in monocrystalline and polycrystalline percentages, and are more texturally mature (moderately to well sorted, less matrix, more cement).

These results identify subtle but notable provenance changes across the Galeros-Kwagunt Formation contact and indicate either: 1) an unroofing sequence; 2) a change in the locus of the source area (e.g.; local derivation vs. long-shore drift); or 3) an increase in weathering of the source area. These textural and mineralogical changes might be utilized as a marker horizon for regional correlation with other mid-Neoproterozoic siliciclastic-bearing rocks. A Qm-F-Lt plot suggests a cratonic interior/continental block provenance.