OCCURRENCE OF DOLOMITE BEDS, CLASTS, OOIDS AND UNIDENTIFIED MICROFOSSILS IN THE COCONINO SANDSTONE, NORTHERN ARIZONA

The Permian Coconino Sandstone of Arizona has typically been interpreted as an eolian deposit. However, at several locations along its northern edge there are bedded dolomites, dolomite-cemented sandstones, sand-sized detrital dolomite clasts and dolomite ooids in flat- and cross-bedded portions of the formation. All of these features have been observed at Andrus Point and some can be found at other localities. Carbonate beds in this vicinity were first recognized by Fisher (1961) but not described in detail. New petrographic analyses and X ray diffractometry (XRD) allow positive identification of the carbonate as dolomite. XRD analysis of the flat-bedded dolomite (5 cm thick) showed that it was 98% pure. Unidentified circular and tube-shaped microfossils and possible ghosts of microfossils occur in these beds (up to 500 µm in diameter). The detrital dolomite grains were medium sand-sized, and in many cases larger than the fine to medium quartz sand that was also present. Euhedral dolomite rhombs occur as porosity replacement cements. Two types of ooids occur: those with quartz and feldspar cores and a concentric dolomite rim and those without cores. The ooids (up to 1 mm in diameter) occur in poorly sorted flat- and cross-bedded sands. The ooids are pristine with no physical compaction or chemical dissolution features. Occurrence of dolomite in the Coconino at this location has significance in interpreting the depositional environment of the formation along its northern edge.