VOLCANICLASTIC SEDIMENTS IN THE UPPER RED MEMBER OF THE MOENKOPI FORMATION, CEDAR CITY, UTAH AREA

The upper red member of the Moenkopi Formation is a heterogeneous assemblage of fluvial and volcanic clastic sediments. Three beds of volcaniclastics were studied in the upper part of the member. The lowermost bed occurs at 68 feet below the top of the upper red member. This first bed is nine feet in thickness and is composed of iron-rich clay, altered from a tuff protolith. A zircon from this bed yielded a date of 233 Ma while a xenocrystic zircon yielded a date of 1.6 Ga. The middle bed is one foot of poorly cemented sand-sized lapilli and contained a xenocrystic zircon that yielded a date of 1.2 Ga. This bed is separated from the lower volcaniclastic bed by six feet hard red siliceous clay. The uppermost layer is one foot of moderately cemented, burrowed, fine-grained lithic sandstone. It is separated from the middle unit by six feet of hard red siliceous clay. Source for this early Triassic volcanism remains problematic and is the subject of ongoing study.