STRATIGRAPHIC AND CHRONOLOGIC RELATIONSHIPS OF THE POPO AGIE FORMATION, UPPER CHUGWATER GROUP

The Triassic Chugwater Group comprises the Upper Jelm/Popo Agie (?late Carnian–earliest Norian) and Bell Springs formations (?Rhaetian). In contrast with the Chinle Formation, a robust radioisotopic chronology has yet to be established for the upper Chugwater. The Popo Agie Formation commonly is considered coeval, if not synonymous with, the Blue Mesa Member of the Chinle Formation based on presupposed lateral continuity. Some authors argue for an older age for the Popo Agie Formation based on vertebrate biostratigraphy. There is little faunal overlap between the Chinle Formation and Chugwater Group; of the 14 vertebrate genera present in the Popo Agie Formation, only 2 are known from the Chinle Formation. Furthermore, genera known from the lower portions of the Chinle Formation are hypothesized to be more derived than those from the Chugwater Group (e.g. phytosaurs). However, a large faunal overlap does exist between the lowest Dockum (Santa Rosa, Colorado City, and Tecovas formations) and Popo Agie Formation, in which 8 of 14 genera are shared.

We hypothesize that the upper Chugwater and lowest Dockum share a depositional history prior to the onset of early Norian Chinle deposition, which may be linked to the initiation of Andean-style subduction along the cordilleran margin. This would have increased accommodation space throughout the back arc basin resulting in the termination of the Middle Triassic hiatus. We propose that initial sediment accumulation began in the Popo Agie and lowest Dockum. Previous data suggested a distinct paleodrainage was shared between the lowest Dockum (Santa Rosa Formation) and the lowest Upper Triassic strata in northern Utah (Gartra Grit). The slightly older ages of both the Dockum Group and Popo Agie Formation compared to the Chinle Formation, and the genetic relatedness of the two with the Upper Triassic strata in northern Utah imply that the proposed drainage likely ended prior to Norian Chinle Formation deposition.