A NEW, UNUSUAL TERRESTRIAL SECTION THROUGH THE PALEOCENE-EOCENE THERMAL MAXIMUM IN WESTERN WYOMING, USA

We report the first discovery of the Paleocene-Eocene Thermal Maximum (PETM) in the overthrust belt of western Wyoming, near La Barge. Its identification is based on a large negative carbon isotope excursion (CIE) of ca. -4.5‰ in paleosol carbonates, its stratigraphic placement in the Chappo Member of the Wasatch Formation, and the presence of typical Paleocene mammals below it. We sampled micrite from 109 paleosol carbonates over an 84 meter section. The CIE here is ~23 m thick and the main body ~14 m. The Chappo Member at this exposure consists of brightly colored, variegated, orange, red, and purple beds of siltstone and sandstone, associated with layers of massive to inversely graded coarse-to-medium grained pebbly sandstone. These beds are deeply incised by multiple thick, tabular conglomeratic units up to 6 meters thick with clasts exceeding 20 cm in diameter. We preliminarily interpret the depositional environment as a fluvial fan or alluvial fan subjected to high energy, ephemeral debris flows and intermittent channel activity followed by relatively long periods of stasis. This contrasts with other PETM localities in Laramide basins, which are associated with lower energy meandering fluvial and floodplain systems in basin centers. Importantly, the magnitude of the CIE at La Barge is about 1‰ smaller than in equivalent sections in the Bighorn Basin, and pre-CIE baseline δ¹³C values are 2 to 3‰ higher. A phenomenon occurs in most paleosol carbonates whereby the magnitude of the CIE is amplified over the CIE in bulk organics and marine carbonates. The lack of major amplification in the La Barge CIE suggests that the La Barge section was not subject to the same local, environmental processes that result in amplification in other depositional settings. The higher baseline δ¹³C values may be linked to water availability, which is the primary control on δ¹³C values in C₃ plants, and to the secondary control of vegetation density. We suggest that the higher overall δ¹³C values in the La Barge section are consistent with a drier, better-drained environment with ephemeral water discharge, coupled with more open vegetation, consistent with sedimentologic evidence.