PATTERNS IN FLOODPLAIN DRAINAGE DURING THE EARLY EOCENE IN THE SOUTHERN BIGHORN BASIN (WYOMING, U.S.A.)

The Eocene Willwood Formation in the Bighorn Basin of Wyoming is an alluvial stratigraphic formation characterized by fluvial channel sandbodies, heterolithic avulsion deposits (i.e. crevasse splay-rich lithofacies), and fine-grained overbank deposits. The overbank facies consists mainly of mudrocks with moderate to strong pedogenic development, which is documented by complex mottling patterns, ped structures, clay cutans, rhizoliths, and slickensides amongst other sedimentary structures characteristics of soil horizonation. The degree of paleosol development has been attributed to variations in sedimentation rate, floodplain drainage, and overarching climate within the basin by previous researchers. Specifically, previous researchers documented better-drained conditions during Paleocene-Eocene Thermal Maximum (PETM) at ~56 Ma, which was a severe global warming lasting ~200,000 years, using a semi-quantitative soil morphology index. This study focuses on a ~80 meter stratigraphic section that post-dates the PETM in the early Eocene, and establishes a longer history of floodplain drainage in the basin.

The index includes measurements of matrix chroma, pedogenic carbonate, and ferruginous nodules. Matrix chroma, or color intensity, decreases with increasing duration of soil saturation. The presence of reduced iron gives periodically or seasonally waterlogged soils a grey color (low chroma), while brightly colored soils, often red, indicate oxidation under well-drained conditions (high chroma). Paleosol chroma was described in the field using Munsell color charts on dry samples in natural light. Pedogenic carbonate forms in higher abundance in drier conditions, and absence of soil carbonate indicates soil moisture above a certain threshold. In the field, numerical values were assigned to paleosol profiles according to the amounts of carbonate. Ferruginous, yellow-brown nodules form during changing redox conditions and increase in abundance with soil wetness, indicating frequent waterlogging. The scores for chroma, carbonate, and ferruginous nodules were added together for each paleosol profile to obtain scores ranging from waterlogged soils to very well drained soils.