FLOODPLAINS AND PALEOSOLS OF PAKISTAN NEOGENE AND WYOMING PALEOGENE DEPOSITS - A COMPARATIVE-STUDY

Comparative study of fossil-bearing fluvial deposits in the Eocene Wil lwood Formation of northern Wyoming and the Miocene Chinji Formation o f northern Pakistan indicate how tectonic and climatic processes opera ting at different scales controlled physical and chemical features of floodplain environments and affected preservation of the paleontologic al record. The architecture of Willwood Fm. floodplain deposits repres ents a combination of avulsion-belt sediment packages and overbank sed iments that formed alluvial ridges. The architecture of the Chinji Fm. floodplain deposits was controlled by widely distributed crevasse-spl ay deposition and floodplain topography. Similarities in individual pa leosol-bounded overbank sequences from the two formations indicates th at the internal structure of such deposits can be independent of chann el belt proximity to areas of aggradation. Chinji Fm. paleosols have l ittle vertical zonation and show no consistent pattern of lateral chan ge in relation to major channels, while overbank paleosols in the Will wood Fm. exhibit considerable soil horizon development and a pattern o f increasing maturity from alluvial ridge to distal floodplain. The '' pedofacies model'' of Bown and Kraus (1987) based on such lateral tren ds in the Willwood paleosols is not applicable to the Chinji Fm. Plant and animal fossils are abundant in the Willwood overbank deposits, wi th vertebrate remains concentrated in paleosol A. horizons. Plant rema ins are rare in the Chinji Fm. and vertebrate fossils occur primarily in channel fills rather than in paleosols. These differences relate to contrasting patterns of floodplain deposition and to levels of oxidat ion that controlled penecontemporaneous recycling of organic material, particularly in paleosols. Different large-scale climatic and tectoni c controls on temperature and rainfall, water table fluctuations, and soil biota are proposed to account for the differences in organic pres ervation. Large and small-scale environmental processes also affected spatial and temporal resolution of the organic record, resulting in im portant differences in the paleoecological and evolutionary informatio n that can be reconstructed from the two sequences.