PALEONTOLOGICAL AND STRATIGRAPHIC RELATIONSHIPS OF THE JELM AND POPO AGIE FORMATIONS (CHUGWATER GROUP, WY)

Over the last three years, we have conducted a paleontological survey of Middle and Late Triassic strata within the Chugwater Group, WY. The two units of interest, the Jelm and Popo Agie Formations, overlie the late-Early Triassic Alcova Limestone; however, their age and stratigraphic relationship are poorly understood. There are significant differences between the two units, in terms of both sedimentology and vertebrate paleontology, that support their stratigraphic distinction. The observed vertebrate assemblage from the Jelm is dominated by rhynchosaurs, while the Popo Agie is dominated by temnospondyls and basal phytosaurs. In addition to body fossils, vertebrate tracks from both the Popo Agie and the Jelm further support their faunal differences. It is suggested that there is no lateral relationship between these two units, but rather that the Jelm is unconformably overlain by the Popo Agie.

The vertebrate paleontological assemblage from the Popo Agie Formation also differs significantly from that of the better studied Chinle Group of southwestern USA. Of the 14 recognized vertebrate genera in the Popo Agie, only 2 are represented within Chinle strata. These differences are suggestive of either little to no temporal overlap between these stratigraphic units, or a strong paleoecological gradient. The latter seems unlikely, as the lowermost strata of the Dockum Group (i.e., Santa Rosa Fm) of west Texas contain 8 of the 14 Popo Agie genera. It is suggested that the Popo Agie and the lowermost Dockum strata are slightly older than the lowest strata of the Chinle Group. The utility of vertebrate biostratigraphy is generally limited to local or regional studies; however, it is currently the only means to compare the Chugwater with other Late Triassic strata across the western USA. Detrital zircon geochronology is planned to further test temporal relationships of Chugwater strata.