FACIES AND FACIES ARCHITECTURE OF PALEOGENE FLOODPLAIN DEPOSITS, WILLWOOD FORMATION, BIGHORN BASIN, WYOMING, USA

Paleogene deposits of the Willwood Formation were analyzed in two area s of the Bighorn Basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various process es that influence floodplain construction and facies variability. Desp ite similar facies and facies organization, floodplain deposits in the two areas differ in grain size, hydromorphy and maturity of the paleo sols, 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, fo rmed on overbank deposits. The red paleosols were better drained and f ormed on less clay-rich parent material. Intense mottling and iron-oxi de nodules indicate that the purple paleosols, which formed on more cl ay-rich parent material, were poorly drained. The grain size differenc es reflect basin position, in particular, distance from a local sedime nt source. Avulsion deposits are volumetrically important, comprising about half of the floodplain deposits in both areas. The avulsion depo sits resemble the Stage III splays described by Smith et al. (1989) [S mith, N.D., Cross, T.A., 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 morp hologic differences interpreted as developmental differences and attri buted to differing rates of avulsion in the two study areas. This stud y shows that avulsion-belt paleosols provide a key to understanding av ulsion rate in the stratigraphic record. Other differences between the Willwood avulsion deposits in the two areas, and between these ancien t deposits and the modem avulsion deposits described by Smith et al. m ay reflect: (1) problems in clearly recognizing ancient avulsion depos its in the field, and (2) sampling bias resulting from the areally res tricted view provided by the stratigraphic record of an areally extens ive modem depositional feature. At the same time, because exposures ar e three-dimensional, the Willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the m odern ones.