The fluvial lower Eocene Willwood Formation (Bighorn Basin, Wyoming) is composed largely of red, oxidized, overbank sediment that has been extensively altered by soil formation. Within this matrix are two different types of dominantly fine grained, drab- to dark-colored deposits that contain abundant plant remains. The first are areally restricted, lenticular bodies that truncate underlying mudstone layers. These are interpreted as having formed in abandoned sections of channels. Deposits of the second type are tabular, as much as 10 km in lateral extent, and rest conformably on other floodplain sediment. These units show a cyclic arrangement of sedimentary subunits. Sedimentologic and paleobotanic evidence suggests that these tabular deposits accumulated under a sequence of vegetational types including, in succession, a floodplain marsh, a swamp forest, and a drier-ground floodplain forest. These interpretations stress the interaction of vegetational succession, soil characteristics, and fluvial morphology in creating the chemical conditions that led to the preservation of organic matter. The stratigraphic distribution of the two types of deposits within the Willwood Formation suggests that in the Bighorn Basin, tectonism exerted a greater influence on the geometry of carbonaceous deposits than did climate.

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