349-3 Influence of the Carnian Pluvial Episode on the Popo Agie Formation.

Abstract:

Late Triassic terrestrial strata from the western USA preserve a relatively continuous record. High-resolution radioisotopic dates have constrained the lowermost Chinle Formation to an age no older than the earliest Norian (ca. 225––227 Ma). Although the Dockum Group does not have radioisotopic age constraints, biostratigraphic correlations have historically demonstrated that the lowermost Dockum Group pre-dates the oldest Chinle Formation strata (e.g., presence of non-leptosuchomorph phytosaurs), and is likely correlative (in part) to the Popo Agie Formation, Chugwater Group. Recent work has presented maximum depositional ages for the top and bottom of the upper Popo Agie (ca. 227 Ma and 229 Ma, respectively). Age-depth models calibrated to upper Popo Agie depositional rates suggest the base of the Popo Agie is around 231–232 Ma. Some models extend as early as 236 Ma, but without a clear hiatus this seems less likely. 

       The base of the Popo Agie (lower carbonate unit) is a microconglomerate largely composed of recycled soil carbonate nodules; bone clasts are not uncommon. Recent discoveries of vertebrate fossils in the lower Popo Agie suggest that these units are much more fossiliferous than previously supposed. Taken together, the sedimentology and vertebrate fauna of the lower Popo Agie displays a sharp environmental contrast to the underlying Jelm Formation.  

      There is a notable shift from the paleo-aridisol/inceptisol dominated deposits of the upper Jelm Formation, which also contain small estivating amphibians specialized to deal with seasonal hyperaridity (i.e., Ninumbeehan), to the much higher energy conglomerate-bearing fluvial deposits containing numerous terrestrial archosaur taxa, and the overlying fluviolacustrine deposits dominated by vertic paleosols and stereospondyl amphibians. The shift from an environment with strong seasonal aridity to a sub-humid environment with enough through flowing water to deposit the highest energy interval in the Chugwater Group suggests a significant increase in annual precipitation/accomodation. Although there is currently a dearth of quantitative evidence, both modeled temporal constraints and depositional environments are consistent with the effects of the Carnian Pluvial Episode (ca. 234–232 Ma), a period with globally increased precipitation and humidity. We hypothesize that the upper Jelm Formation may represent earliest Carnian deposition, while initial Popo Agie Formation deposition corresponds to the CPE, the formation preserves mid to late Carnian deposition at least partially overlapping with the lowest Dockum Group.

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