ALLUVIAL DEPOSITS FROM THE LOWER CANTWELL FORMATION, SABLE MOUNTAIN, DENALI NATIONAL PARK, ALASKA

The lower Cantwell Formation is a late Cretaceous (Campanian/Maastrichtian) terrestrial succession located on the north side of the Alaska Range in south-central Alaska. The formation is bounded by the Denali Fault to the south and the Hines Creek Fault to the north. Numerous successions of upward-fining shallow alluvial deposits in the Sable Mountain area in Denali National Park consist of well-indurated conglomerate, sandstone, siltstone, shale and local coal seams. Recent finds of abundant dinosaur and bird footprints have heightened interest in the paleoenvironments. Rocks in the study area display a heterogeneous fining and coarsening lithology interpreted here as braided channel, sandy channel, overbank, crevasse splay, and lacustrine deposits. Conglomeratic and sandy channel deposits are shallow and are encased in abundant floodplain fines. Contacts are generally sharp with little or no erosion. Conglomerates are commonly matrix-supported, poorly to moderately sorted and grade into medium to fine sandstone. Abrupt channel abandonment is indicated by flat upper surfaces, commonly capped with mud drapes. Shallow tabular sandstone bodies, often 100’s of meters long, are interpreted as sheetfloods. Finer-grained rocks represent overbank, abandoned channel and lacustrine deposits. Interspersed debris and mud flow deposits identify the depositional environment as an alluvial fan system. In places, the alluvial fan deposits interfinger with axial braided river facies. Trace fossils including Mermia, Scolicia, Cochlichnus, Haplotichnus, Sagitichnus and cf. Arenicolites occur in a variety of facies and indicate episodic subaerial reworking, a shallow vadose zone, and high depositional rates. Combined sedimentological and ichnological data suggest sediments were largely wet resulting in frequent disruptions of biogenic activity.

The Cantwell Formation was likely deposited in a compressional setting during the latest phase of the Wrangellia Composite Terrane collision with the former North American margin. The vertical aggradation and preservation of abundant floodplain fines (~2000 m thick) suggests that accommodation space was readily created by rapid subsidence due to thrust loading along an active thrust fault system.