Late Cretaceous (Campanian-Maastrichtian) Paleoenvironments from the Lower Cantwell Formation at Sable Mountain , Denali National Park , Alaska

Preliminary research from the lower Cantwell Formation in south-central Alaska suggests that multiple sub-environments are preserved in the Sable Mountain area in Denali National Park, Alaska. The formation is a Late Cretaceous fluvial sequence that crops out along the northern foothills of the Alaska Range. Laterally discontinuous conglomerates and lenticular and sheet sandstone bodies are interbedded with extensive floodplain fines. Recently established paleontological sites contain dinosaur tracks, invertebrate trace fossils and plant megafossils. Abundant fragments of fern fronds, Equisetites, the branches, cones and wood of Metasequoia, and a variety of dicotyledonous angiosperm leaf impressions including hamamelid-like forms imply a diverse flora. Ichnofossils comprise Mermia, Cochlichnus, cf. Skolithos, and Planolites among others. Dinosaur footprints are found exclusively at depositional breaks. Floral and faunal fossils occur within distinct lithological facies and provide site-specific information for local habitat and drainage conditions. Sedimentary facies, alluvial paleosols, and the fossil data are used to designate small fluvial channel, crevasse splay, splay channel, lake, and low-lying floodplain deposits. Better drained floodplain deposits are inferred from vegetation types and pedorelicts. Angiosperm leaves preserved in fine-grained sandstone suggest that floodplain uplands drained by small channels were covered by hardwoods whereas diverse fossil assemblages within what are interpreted as channel margin sediments imply that some dicots were part of a hydrophilic floodplain flora. Pollen and spores in mudstones signify low-lying floodplain vegetation consisting of conifers and shrubs with an understory comprised of a variety of ferns and horsetails. Local occurrences of Mermia ichnocoenoses designate lake margin settings for this flora. Overall, the depositional environment in the Sable Mountain area is interpreted as a distal, low-gradient, stream-dominated alluvial fan or part of an extensive floodplain of an axial river system. Frequent depositional breaks and incipient paleosols imply rapid aggradation probably as a result of episodic higher discharge and recurring avulsions.