Proposed nomenclature of part of Upper Triassic strata in southeastern Utah

Abstract

The Triassic rocks in southeastern Utah have been divided into the Moenkopi formation of Early and Middle (?) Triassic age, and the Shinarump conglomerate, Chinle formation, and Wingate sandstone of Late Triassic age. Recent work has shown that the strata formerly called Shinarump conglomerate in southeastern Utah consist, in places, of a lower sandstone and conglomerate unit, a middle claystone and clayey sandstone unit, and an upper sandstone and conglomerate unit. The lower and upper units are Shinarump-type deposits whereas the middle unit is a Chinle-type deposit. Deposits of Shinarump-type and Chinle-type are interstratified and intertonguing. The writer proposes that the two types be grouped together in the Chinle formation and that the Shinarump conglomerate be re efined as the Shinarump member of the Chinle formation. The lower unit is correlative with strata in southwestern Utah, that are designated as type Shinarump conglomerate, and the writer proposes that the term Shinarump member be restricted in southeastern Utah to this lower unit. The middle and upper units, although formerly included in the Shinarump conglomerate, are considered as separate members of the Chinle formation. The middle unit has been defined as the Monitor Butte member of the Chinle formation, and the name Moss Back member is here proposed for the upper unit. In southeastern Utah, the Shinarump conglomerate included, in places, all of the Shinarump, Monitor Butte, and Moss Back members of the Chinle formation. In other places, the Shinarump conglomerate consisted only of the Moss Back member, and in still other parts of southeastern Utah the Shinarump conglomerate consisted of only the Shinarump member. Locally in the San Rafael Swell area, Emery County, a thin unit now called Temple Mountain member was included by previous workers in the Shinarump conglomerate. In addition to subdividing the strata previously included in the Shinarump conglomerate, the overlying part of the Chinle formation has been subdivided into members. As now recognized, the Chinle formation consists of seven members in southeastern Utah. They are, in ascending order, the Temple Mountain, Shinarump, Monitor Butte, Moss Back, Petrified Forest, Owl Rock, and Church Rock members.