Spatial characteristics and kinematics of precession-driven floodplain aggradation cycles in the lower Eocene Willwood Formation of the Bighorn Basin, Wyoming, USA

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Youwei WANG, Timothy Baars, Joep Storms, Allard Martinius, Philip Gingerich, Magda Chmielewska, Simon Buckley, Hemmo Abels


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. Astronomical forcing of alluvial stratigraphy is poorly documented so far as this driver strongly interacts with autogenic and other allogenic processes making it difficult to trace astronomical climate changes in these laterally highly variable sediments. 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. One floodplain aggradation cycle consists of two phases: (1) a longer overbank phase with relative channel stability and strong paleosol development on fine clastic sediments; and (2) a shorter avulsion phase characterized by channel instability and weak to no pedogenesis on heterolithic sandy avulsion-belt deposits. 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 floodplain aggradation cycles remains largely unknown, which determines 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 using an unmanned aerial vehicle (UAV) to develop a photogrammetric model covering a geographic area of ~10 km2 and spanning a stratigraphic succession of ~300 m. The 44 cycles have an average thickness of 6.8 m with a standard deviation of 2.0 m, which is in line with previous studies. Most cycles are consistently traceable over the entire model, indicating spatial consistency and in line with allogenic climate forcing by the astronomical precession cycle. Individual floodplain aggradation cycles may change in thickness rapidly when traced 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 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 statistically related to that at another locality over an average distance of 1.3 km in the paleoflow direction and 0.6 km perpendicular to the paleoflow direction. These different directional trends are interpreted to result from morphological elements oriented in paleoflow directions in the fluvial landscapes shaping more consistency of the sedimentary elements in paleoflow direction. Two different metrics suggest that full-compensational stacking occurs after deposition of 6 to 7 cycles, or timescales of ca. 120 to 140 kyr, although strong thickness compensation is shown to start at the subsequent one and two floodplain aggradation cycles, so at ca. 20-40 kyr time scales.



Geology, Sedimentology, Stratigraphy


compensational stacking, floodplain aggradation cycle, alluvial stratigraphy, Bighorn Basin


Published: 2021-06-08 02:31

Last Updated: 2021-06-08 19:13

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