Stratigraphic Relations of Hoskinnini Member (Triassic?) of Moenkopi Formation on Colorado Plateau

Abstract

Recent work on strata of Permian and Triassic age on the Colorado Plateau indicates that the unit originally defined as the Hoskinnini tongue of the Cutler formation extends into parts of southeastern Utah where it had not been previously reported. In addition, the Hoskinnini is correlated with part or all of the unit defined as the Tenderfoot member of the Moenkopi formation in the salt anticline region of east-central Utah and west-central Colorado. As a result of this work, the Hoskinnini is redefined as the Hoskinnini member of the Moenkopi formation. The Hoskinnini and laterally continuous strata in the Tenderfoot member are exposed within a north-northeast oriented area about 180 miles long and 50 miles wide extending from the Monument Valley area, in northeastern A izona and southeastern Utah, to west-central Colorado. Previously the Hoskinnini had been recognized only in the Monument Valley area. The Hoskinnini member and laterally continuous strata in the Tenderfoot member are pale reddish brown sandy siltstone grading to silty very fine-grained sandstone and contains disseminated fine, medium, and coarse quartz grains. The Hoskinnini is composed of horizontal beds, which generally range from 1 to 2 feet in thickness. Individual beds are marked by indistinct discontinuous wavy laminae bounded by grayish red claystone or siltstone films. The Hoskinnini is 50-120 feet thick in most areas. The combination of fine to coarse sand grains in a silt or very fine-grained asnd matrix and of discontinuous wavy laminae serves to differentiate the Hoskinnini member from the underlying and overlying strata. These features also make possible the correlation of the member with rocks not previously correlated with the Hoskinnini in southeastern Utah and adjoining parts of Colorado.