THE OCCURRENCE AND DIVERSITY OF TRACE FOSSILS IN PALEOSOLS OF THE WILLWOOD FORMATION DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM), BIGHORN BASIN, WYOMING

The Paleocene-Eocene Thermal Maximum (PETM), a transient episode of global warming, has been recognized in the fluvial Willwood Formation based upon isotopic analyses of pedogenic carbonate nodules. The PETM interval in the Willwood Formation also records an abrupt and permanent turnover in fossil land mammal assemblages, with the first appearance of artiodactyls, perissodactyls, and primates in North America. The Willwood Formation contains a diverse and abundant ichnofossil assemblage. Soil biota, as represented by these trace fossils, may have also responded to the warming event.

Near McDermotts Butte in the southeastern Bighorn Basin, the PETM interval in the Willwood Formation contains well-exposed paleosols. The paleosols can be divided into two groups: more strongly developed paleosols that formed on overbank mudstones and more weakly developed paleosols that formed on mudrocks and fine sandstones deposited as the channel avulsed. Strongly developed paleosols have red, yellow-brown and purple matrix and mottle colors, abundant root traces, carbonate and goethite nodules, and well-developed slickensides. Weakly developed paleosols commonly show gray-green matrix colors with less intense mottling, relict bedding, and fewer root traces and nodules than well-developed paleosols.

The most common trace fossils are fine root traces, usually gray-green and purple in color, highlighted by red and yellow-brown mottles. Larger root traces often grade into powdery CaCO₃ rhizoliths. After root traces, adhesive meniscate burrows (AMB) are the most common trace fossil. Well-developed paleosols show abundant, small diameter (1-4 mm) AMB; weakly-developed paleosols contain fewer and larger (5-15mm diam.) AMB. Sand-filled burrows, some with distinct clay-linings, are present in both kinds of paleosol, especially near the tops of mudstone units capped by heterolithic units. Other ichnotaxa associated with better-developed paleosols include Coprinisphaera, Scaphichnium hamatum, and cf. Ancorichnus. Ichnogenera from weakly-developed paleosols include Planolites, Macanopsis and Edaphichnium. This preliminary work provides a framework for establishing ichnocoenoses within Willwood paleosols. This, in turn, will allow for the comparison of similar trace fossil and paleosol associations occurring before, during, and after the PETM interval.