Depositional Environments of the Upper Devonian Oneonta Formation of South-Central New York

The Oneonta Formation is a fluvial, nonmarine component of the Upper Devonian Catskill molasse sequence exposed in south-central New York. Stratigraphic sections, primary sedimentary structures, and lithological characteristics were examined with the intent of determining the depositional environments and type of fluvial system represented by the formation. Detailed analysis resulted in identification of nine lithofacies. Lithofacies interpreted to be channel and bar deposits include: crude, large-scale trough cross-stratified pebbly intraclast sandstone; large-scale trough cross-stratified medium- and fine-grained sandstone; large accretionary troughs or lenses of cross-stratified pebbly or medium-grained sandstone, generally trough cross-stratified; solitary sets of large-scale planar - tabular cross-stratified medium-grained sandstone; wide, shallow troughs filled with large-scale inclined planar stratified fine-grained sandstone; horizontally bedded and very low-angle stratified fine-grained sandstone; and gray colored small-scale cross-stratified and riple- cross-laminated sandstones and siltstones. Lithofacies interpreted as alluvial flood basin deposits include: red colored massive, small-scale cross-stratified, and ripple-drift cross-laminated very fine-grained sandstones and siltstones; blocky fracturing siltstone; and disrupted small-scale cross-stratified and irregularly stratified fine-grained sandstones and siltstones. The large-scale trough cross-stratified lithofacies are interpreted to result from in-channel magaripple deposition. The large accretionary units, planar cross-strata, and inclined planar stratification reflect bar deposition. The large accretionary units of large-scale cross-stratification represent bars formed at high-stage flow with subsequent modification during falling stages. Solitary sets of planar – tabular cross-strata and inclined planar stratification respectively were formed by the migration of solitary and multilobate high-stage linguoid bars which were largely unmodified. Horizontally bedded and low-angle stratified sandstones reflect plane bed or antidune deposition on bar surfaces. The gray colored small-scale stratified and ripple-drift cross-laminated sandstones and siltstone lithofacies results from bar-top deposition during waning and low-stages. The red colored member of the fine-grained sandstone and siltstone lithofacies is interpreted as levee deposits and the blocky lithofacies to be overbank deposits which have been intensely bioturbated. The disrupted small-scale cross-stratified and irregularly stratified sandstones and siltstones reflect crevasse splay deposition. Paleocurrent data indicate a northwest paleoflow direction. The grand vector mean azimuth determined from the orientation of cross-strata is 293°. Flow was unimodal with low dispersion. Similar paleoflow patterns have been attributed to braided systems. The Oneonta Formation sandstones are immature phyllarenites (Q39 - F1 - L60). They were derived from the collision orogen tectonic provenance of the contemporaneous Acadian Mountains. Sedimentation was rapid with little detrital reworking. Substantial and rapid discharge fluctuations of a flashy nature are suggested. The fluvial system responsible for deposition of the Oneonta Formation is proposed to have been a sandy, braided system which was operative in a distal alluvial fan delta environmen