THE TYPE SECTION AND AGE OF THE TIMPOWEAP MEMBER AND STRATIGRAPHIC NOMENCLATURE OF THE TRIASSIC MOENKOPI GROUP IN SOUTHWESTERN UTAH

Abstract—The name Timpoweap Member of the Moenkopi Formation has long been applied to the lowermost stratigraphic interval (mostly limestone and shale of marine origin) of the Moenkopi Formation in southwestern Utah. We describe in detail a section (lectostratotype) of the Timpoweap Member in Timpoweap Canyon near Hurricane, Utah. At this section, the Timpoweap Member is ~ 20 m thick and is mostly limestone and shale. Petrographic study of the lectostratotype indicates that Timpoweap deposition mostly took place in shallow marine and restricted tidal flat paleoenvironments. Ammonoid assemblages from the Timpoweap near Hurricane and Cedar City include Anasibirites kingianus (Waagen) and Wasatchites sp. and indicate it is of late Smithian age. We endorse removal of the basal, conglomeratic interval long included in the Timpoweap and its recognition as the Rock Canyon Conglomerate. The Timpoweap Member has long been recognized as laterally equivalent to the Sinbad Member of the Moenkopi Formation to the east, and both units are in a similar stratigraphic position, of similar lithology, and are the same age. Therefore, two names are not needed for this stratigraphic unit, the older name Sinbad should be used, and the younger name Timpoweap should be abandoned. The nearly 700 m thick section of Moenkopi Formation strata in southwestern Utah includes seven readily mappable stratigraphic units, so we endorse Poborski’s suggestion that Moenkopi be elevated to group rank. However, we only include conglomeratic and red-bed siliciclastic strata in the Moenkopi Group (Rock Canyon, lower red, middle red and upper red formations), and assign carbonate- and evaporite-dominated intervals (Sinbad, Virgin and Shnabkaib formations) to the Thaynes Group