Spatial characteristics and kinematics of floodplain aggradation cycles in the lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA
Authors: WANG Youwei , Baars Timothy , Storms Joep , Martinius Allard , Gingerich Philip , Chmielewska Magda , Buckley Simon , Abels Hemmo 
Published: June 2021

Interaction of allogenic and autogenic forcing in building alluvial stratigraphy remains a complex subject that is critical for paleoenvironmental and paleoclimate reconstruction and subsurface rock property prediction. Autogenic processes may act at similar vertical and lateral scales at which astronomical climate forcing drives alluvial stratigraphic deposition, making it difficult to disentangle these drivers in the rock record. In the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, USA, a lot of evidence has been gathered to relate dominant floodplain aggradation cycles to precession-scale climate change. Previous studies have analyzed these cycles to be consistently developed in multiple areas of the basin of different ages and, in one study, in two parallel one-dimensional (1-D) stratigraphic sections spaced several kilometers apart. However, the 3-D geometry of these floodplain aggradation cycles remains largely unknown. Building upon previous studies, these cycles are viewed to be driven by allogenic forcing in concert with autogenic factors in this research. Our goal is to reveal to what extent allogenic climate forcing produces regionally consistent sedimentary patterns and autogenic processes produce lateral variability. Here, 44 floodplain aggradation cycles were mapped and measured in 3-D space using an unmanned aerial vehicle (UAV) to develop a photogrammetric model covering a geographic area of ca10 km2 and spanning a stratigraphic succession of ca 300 m. The 44 cycles have an average thickness of 6.8 m with a standard deviation of 2.0 m and a coefficient of variation of 29% in line with previous studies. Most cycles are consistently traceable over the entire model, indicating spatial consistency in line with previous studies exemplifying allogenic climate forcing by the climatic precession cycle. Individual floodplain aggradation cycles may change in thickness rapidly laterally, with rates up to 1 m over a lateral distance of 100 m and a maximum of 4 m. Detailed mapping of seven successive cycles that are extensively present in the outcrops reveals differences in their regionally-averaged thicknesses of 3.7 m to 9.7 m, with their coefficients of variation ranging between 17% and 28%. Variogram analysis demonstrates that the thickness of a cycle at one locality is significantly related to that at another locality within an average distance of 1.3 km in the paleoflow direction and 0.6 km perpendicular to the paleoflow direction. This is interpreted as a result of morphological elements oriented in paleoflow directions in the ancient fluvial landscapes shaping more consistency of the sedimentary elements in paleoflow directions. Strong thickness compensation of floodplain aggradation cycles is observed at 20-40 kyr time scales and full compensation occurs between 120 and 240 kyr. In compacted floodplain stratigraphy, the measured vertical scale of autogenic processes impacting stratigraphy is 4.0 to 5.2 m at 95% significance. This is below the scale of precession-climate drivers occurring at around 7.0 m thickness, which is probably why autogenic and allogenic drivers can be disentangled in this series.