IMPLICATIONS OF NEW CARBON ISOTOPE STRATIGRAPHY FOR THE PALEOCENE-EOCENE THERMAL MAXIMUM, SAND CREEK DIVIDE, BIGHORN BASIN, WYOMING

The Paleocene-Eocene Thermal Maximum (PETM) was an episode of global climate change that is well documented in alluvial strata of the Bighorn Basin, Wyoming. The PETM interval is recognized by a dramatic negative carbon isotope excursion (CIE) that has been identified using both pedogenic carbonate nodules and paleosol organic matter. New isotope results from one location have intriguing implications for the PETM event in the Bighorn Basin.

In the northern part of the basin at Polecat Bench, the CIE interval is ~ 40 m thick. The interval is in the Willwood Formation, which is distinguished by bright red paleosols that indicate moderately well-drained conditions. The Fort Union Formation, recognized by its somber-colored paleosols that reflect poor soil drainage, lies ~ 430 m below the PETM interval. At Cabin Fork, ~160 km southeast of Polecat Bench, the base of the PETM interval sits directly above the Fort Union-Willwood contact. The interval is also thicker at ~50 m thick. New isotope data obtained from dispersed organic carbon show that a PETM interval at Sand Creek Divide (130 km southeast of Polecat Bench) is thicker than originally thought (~ 60 m) and the thickest of the three PETM intervals. In addition, the isotope record shows that this section differs from the other two in that the lower 20-25 m of the PETM interval are in the Fort Union Fm. The lower part of the interval here is dominated by the gray paleosols typical of the Fort Union Fm; however, the gray paleosols grade laterally into areally restricted red and purple paleosols that hint at a transition to better-drained soils.

The results suggest that, in the southeastern part of the basin, onset of the PETM caused the transition from the Fort Union Fm to the Willwood Fm; however, there was a temporal lag in developing laterally persistent red soils at Sand Creek Divide. In the northern basin, the formation boundary precedes the PETM; it may have been controlled by a local event such as rise of the adjacent Beartooth Mountains. The variations in thickness of the PETM intervals indicate spatial variations in sediment accumulation rates and suggest that the Sand Creek Divide section provides a more complete record of the PETM. The new data also suggest that faunal changes associated with the PETM may have lagged the event by longer than originally believed.