Paper No. 32
Presentation Time: 8:00 AM-12:00 PM
TIMING AND ORIGIN OF REDDENING IN CARBONATE LENSES OF THE JURASSIC NAVAJO SANDSTONE
Carbonate lenses in the Navajo provide a unique lithology to test timing of reddening. Previous work has shown the importance of alteration of ferromagnesium minerals as a source for reddening in sandstones. These lenses, formed in ponds, are found on first and second order bounding surfaces throughout much of the Navajo and vary in complexity. Individual ponds left dense carbonate lenses 10 cm to 1 m in thickness and up to 100 m in diameter. From their initial deposition, red carbonate lenses had lower permeability than the surrounding sandstones. Three locations in the Navajo around the Lake Powell region of Arizona and Utah were studied. Dominant lithologies in the lenses are grain-supported sandstones cemented by dolomite and crystalline dolomites with less than 5 % detrital grains. Clays are found surrounding all detrital grains, in intercrystalline areas of dolomite, as patches and pellets, and as irregular laminae. Reddening is associated with the clays. All clays in thin sections appear dark red in reflected light. SEM analysis using backscatter and EDS revealed most hematite crystals are euhedral and embedded in clays. Nearly all clay flakes are poorly crystalline and appear detrital. Hematite crystal sizes are typically less than 1 µm. A single example of an altered detrital ferromagnesium mineral was observed and showed a distinctive and localized distribution of secondary hematite. The timing of reddening may be constrained by early carbonate precipitation. Remnants of calcite in dolomite crystals and relict allochems indicate replacement. Carbonate clasts have been incorporated into breccias in the overlying sandstones. Although other timing scenarios are possible, the relationship between the red clay and the dolomite suggests that reddening occurred prior to dolomitization. Paleomagnetic analysis showed weak intensities and generally inconsistent directions. Even distribution of pigment-sized hematite in clays disconnects the source of iron from ferromagnesium minerals. The source of the reddening is the oxidation of iron in detrital clays and the most favorable scenario for the timing of the reddening is early, prior to dolomite formation.