TIMING OF THE FLUVIAL RESPONSE TO THE PALEOCENE-EOCENE THERMAL MAXIMUM IN THE BIGHORN BASIN

The Paleocene-Eocene Thermal Maximum (PETM), a period of global warming ca. 56 Ma, is marked in the geologic record by a negative carbon isotopic excursion (CIE) linked to elevated atmospheric pCO₂. Global precipitation patterns altered in a spatially heterogeneous manner, with increased seasonality in mid-latitudinal regions. The Bighorn Basin of Wyoming likely experienced decreased precipitation and increased seasonality. These changes are thought to have generated an anomalously thick and laterally extensive fluvial sheet sandstone (the “Boundary Sandstone”). This study examines the timing of the cross-basinal fluvial response to the PETM in the Bighorn Basin by comparing a new carbon isotope record to existing records, all of which are derived from pedogenic carbonate nodules. The new record contains a CIE of -6.6‰, (offset between average baseline δ¹³C values and minimum excursion δ¹³C value), which is nearly identical to previous studies. Although the full magnitude of the CIE is captured in the new record, the release phase of the PETM, when pCO₂ was rising, is missing and the Boundary Sandstone occupies this stratigraphic level. In an isotope record ca. 7 km to the northwest, the sandstone occurs within the PETM’s main body, when pCO₂ was elevated but stable. One of three hypotheses explains this discrepancy. First, the Boundary Sandstone initiated deposition prior to the PETM, and laterally expanded and consistently aggraded during the PETM. Second, the strata recording the release phase in this section were scoured and removed, obscuring the sandstone’s deposition during the PETM’s main body. Third, the sandstone is time-transgressive; it was deposited earlier in this isotope section than the other. The timing offset represents a lateral migration of the sandstone during the PETM. In hypothesis two, the stable climate of the main body of the PETM instigates the Boundary Sandstone’s deposition; a threshold response in alluvial systems was initiated. In hypotheses one and three, the deposition of the sandstone began just prior or during the transitional phase of the PETM, with alluvial systems progressively responding to a changing climate. Additional isotope records from the basin and correlation of strata below the sandstone across the basin could clarify which of these hypotheses is most plausible.