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The Oneonta Formation (Catskill Magnafacies) in south-central New York is composed of two lithofacies associations: (1) Medium- to very fine-grained, cross- and planar-stratified sandstone bodies, with bedsets (lithofacies) arranged into one or (usually) more erosively based storeys. Individual storeys generally have upward-fining lithofacies, also lateral-accretion bedding and channel fills: (2) Interbedded mudstones and erosively based sandstones with a diversity of primary sedimentary structures, calcareous concretions, plant remains and trace fossils. Upward-fining bedsets (lithofacies) are sheet-like or channel-filling, and are arranged in meter-scale lithofacies sequences.

The sandstone bodies are interpreted as deposits of laterally migrating and aggrading single-channel (perennial) rivers. Vegetated point-bar tops were subjected to both sheet floods and chute-channel formation. Minor low-flow deposition occurred on bar surfaces. Quantitative reconstruction of bankfull channel geometry and hydraulics gives channel widths, mean depths and slopes of approximately 60 m., 2.5 m., and 10−4 respectively: low reconstructed bend sinuosities (1.1 to 1.2) are supported by paleocurrent data, and help to explain the dominantly coarse-grained channel fills associated with chute cut-off. Sinuosity changes during lateral migration are documented quantitatively.

The sandstone-mudstone lithofacies association is interpreted as overbank flood deposits, on levees, crevasse channels and splays, and flood basins. Plant and faunal activity, and soil-forming processes, were abundant. Periodic channel-belt diversions (avulsions) caused the meter-scale lithofacies sequences in this association, also the cyclicity of the two lithofacies associations. If avulsion frequency averaged about once per 103 years, floodplain deposition rates are estimated at about 2 × 10−3 m/year.

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