Notes on the Moenkopi Formation in the salt anticline region of Colorado and Utah

Abstract

The Moenkopi formation of Triassic age is widely exposed in the salt anticline region in southeastern Utah and southwestern Colorado* The distribution of the Moenkopi is that of a blanket of irregular thickness several large holes wrn in it,, due chiefly to pre-Chinle erosion* Four members of contrasting lithology have been mapped in the Moenkopi in the salt anticline region. In ascending order these members ares l) the Tenderfoot member^ composed dominantly of muddy or silty poorly sorted sandstone | 2) the Ali Baba member 9 composed of interstratified arkosie conglomeratic sandstone and fissile siltstone| 3) the Sewemup member $ composed dominantly of fissile siltstone with minor beds of conglomeratic sandstone and gypsum 5 and 4) the Pariott member composed of interstratified sandstone and siltstone* Part of the Tenderfoot member of the Moenkopi formation correlates the Hoskinnini member of the Moenkopi formation of southeastern Utahj, a unit which may be either Permian or Early Triassic in age or possibly both. If equivalents of the type Moenkopi of northeastern Arizona are present in the salt anticline region, they may include the upper part of the Sewemup member and part or all of the Pariott The Moenkopi formation of Trlassie age, a unit long recognized on the Colorado Plateau (Ward, 1901, p0 4-03-4-041 Gregory, 1917, p. 23-31$ and McKee, 1954-) was first delimited in the salt anticline region of southeastern Utah and southwestern Colorado (fig. l) Baker and others (1927, p. 796-798)„ It was subsequently mapped in the Utah part of the region by Baker (1933) and by Dane (1935), who extended stratigraphie observations on the Moenkopi Into adjacent parts of Colorado. Dane (1935, p, 51) noted and called attention to an unusually thick section of the Moenkopi formation in Sinbad Valley, Colorado. In 194-8, mapping -was begun In the vicinity of Sinbad Valley, Colo., by the senior author under a program of mapping undertaken by the U. S. Geological Survey on behalf of the Division of Ra¥ Materials of the TJ 0 Ba Atomic Energy Commission* In order to determine the nature of the unusual lateral changes in thickness of the Moenkopi formation, three members were distinguished and mapped In Sinbad Valley, -where the formation Is composed of three major units of contrasting lithology (Shoemaker, 1955 and 1956). Subsequently the units were traced northward (by the senior author) to the vicinity of Gateway, Colo. (Cater, 1955a), southward into Paradox Valley, Colo. (Shoemaker, 1956), and westward into the areas previously mapped by Baker and Dane in Utah (Shoemaker, 1952), A fourth member was recognized in Utah<, In 1952, additional field and laboratory work was undertaken In collaboration the junior author to complete a study of the Moenkopi throughout the salt anticline region. The purpose of these notes is to set forth the salient features of the stratigraphy of the Moenkopi formation of this region and to supplement new