A NEW RICH RECORD OF DINOSAUR TRACKS FROM AN UNNAMED CRETACEOUS ROCK UNIT ALONG THE YUKON RIVER (WEST-CENTRAL, ALASKA)

We report on a rich, new record of dinosaur tracks from 100-90 Ma Cretaceous rocks exposed along the Yukon River of west-central Alaska. Our survey covered approximately 120 river miles and we recorded 93 new occurrences of dinosaur tracks in an area where none had been documented previously. Ten stratigraphic sections were measured which display primarily fluvial-delta plain facies. Main fluvial channels, up to 18 m thick, consist of well-developed fining-upward successions of medium- to fine-grained, trough cross-bedded, and ripple cross-laminated sandstones. Large-scale lateral accretion surfaces are visible in larger exposures. Main channels may be single-story or multi-story and are interpreted as meandering river deposits. Smaller channels, consisting of lenticular, fine- to very-fine, trough cross-bedded sandstones up to 2 m thick, represent crevasse or distributary channels.Thin tabular, fine- to very-fine-grained sandstones interbedded with siltstone, mudstone, and shale represent vegetated floodplains. Tabular, trough cross-bedded, and laminar, fining upward sandstones less than 1 m thick are interpreted as sheet flood deposits. Interbedded, fine-grained sandstones, siltstones, and mudstones are interpreted as distal floodplain deposits. Root traces are pervasive throughout floodplain mudstones and crevasse splays. Thin coals or coaly shales represent peat swamps, and thick mudstone deposits represent small ponds or lakes. Small coarsening-upward successions of interbedded tabular sandstone and mudstone probably represent crevasse deltas and/or crevasse splays into interdistributary bays. The broad ichnotaxonomic groups represented are non-avialan theropods, at least three examples of avialan theropods, hadrosauroids, and nodosaurids. Of these ichnotaxa, the most encountered track type was attributable to young to adult hadrosauroids, a frequency pattern like that found in a roughly correlative study several hundred kilometers north in the Nanushuk Formation of North Slope, Alaska. Our study not only provides new insights into a biologically productive ancient, high-latitude, terrestrial ecosystem, but also demonstrates the need for additional high-latitude studies to expand our knowledge of ancient biodiversity in the greenhouse Cretaceous Arctic.