Facies and facies architecture of Paleogene floodplain deposits, Willwood Formation, Bighorn Basin, Wyoming, USA

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Paleogene deposits of the Willwood Formation were analyzed in two areas of the Bighorn Basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability. Despite similar facies and facies organization, floodplain deposits in the two areas differ in grain size, hydromorphy and maturity of the paleosols, and scale and organization of ribbon sandstones in the avulsion deposits. These differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas. Two kinds of cumulative floodplain paleosols, red and purple paleosols, formed on overbank deposits. The red paleosols were better drained and formed on less clay-rich parent material. Intense mottling and iron-oxide nodules indicate that the purple paleosols, which formed on more clay-rich parent material, were poorly drained. The grain size differences reflect basin position, in particular, distance from a local sediment source. Avulsion deposits are volumetrically important, comprising about half of the floodplain deposits in both areas. The avulsion deposits resemble the Stage III splays described by Smith et al. (1989) [Smith, N.D., Cross, IA., Dufficy, J.P., Clough, S.R., 1989. Anatomy of an avulsion. Sedimentology 36, 1–24.] in terms of characteristics of their ribbon sandstones and the presence of thin, sheet sandstones or siltstones. Paleosols that formed on the fine-grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas. This study shows that avulsion-belt paleosols provide a key to understanding avulsion rate in the stratigraphic record. Other differences between the Willwood avulsion deposits in the two areas, and between these ancient deposits and the modern avulsion deposits described by Smith et al. may reflect: (1) problems in clearly recognizing ancient avulsion deposits in the field, and (2) sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature. At the same time, because exposures are three-dimensional, the Willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones.