The Paleocene‐Eocene Thermal Maximum Recognized in Distal Alluvial Fan Strata of the Sevier Fold‐And‐Thrust Belt (Southwestern Wyoming, USA)

Abstract

Fifty‐six million years ago a massive flux of exogenic carbon was rapidly released into Earth's oceans and atmosphere during the Paleocene‐Eocene Thermal Maximum (PETM). The warming associated with this release persisted for ∼200 kyr. It had severe impacts on ocean environments, global ecosystems, and the hydrologic cycle. Yet, regional patterns of response within continental settings are not well constrained. Here we describe the first recognition of the PETM in the Greater Green River Basin of western Wyoming, USA. We present new sedimentologic, chemostratigraphic, and paleontologic data from the Chappo Member of the Wasatch Formation in the Sevier fold‐and‐thrust belt. The Chappo Member contains lithofacies indicative of debris flows, ephemeral and incisional stream channels, and well‐drained overbank deposits, which, along with the geographic position of the study area, indicate a distal alluvial fan setting. Using pedogenic carbonates, we find a negative carbon isotope excursion (CIE) of ∼3.8‰ associated with the first appearance of Celtis endocarps stratigraphically above beds containing late Paleocene mammals. The CIE magnitude here is considerably smaller, compared to records from the Bighorn Basin, which we interpret to represent drier, better‐drained conditions. High baseline carbon isotope values suggest dry conditions in the study area, which likely resulted from well‐drained sediments associated with the distal alluvial fan. Low amounts of precipitation, as suggested by previous climate models, may have contributed to dryness. We also find sedimentologic evidence consistent with an enhanced hydrologic cycle during the PETM, as seen in other basins in the region.