Toward a Basin-Scale Paleofloral Analysis of the Late Cretaceous Cantwell Formation In Denali National Park, Alaska: Tying Facies Analysis to Plant Fossil Occurrence

In addition to its increasingly well-known vertebrate and invertebrate ichnofossil record, the Late Cretaceous (Campanian-Maastrichtian) lower Cantwell Formation (Cantwell Basin, Alaska Range, Alaska) preserves a plant megafossil assemblage in fluvio-lacustrine depositional settings of varying grain-size and energetics of deposition. By coupling newly obtained stratigraphic sections that describe lithological and environmental heterogeneity across the Cantwell Basin with floral remains that are tied to facies changes in the sections, it is becoming possible to discern depositional environment ties with paleofloral community structure within the basin.

Metasequoia-dominated communities including members of the Ginkgoales are preserved as impressions among coarse-grained fluvial-alluvial deposits possibly interpreted as “upland” or proximal depositional settings. Assemblages dominated by members of the Equisetales are often preserved three dimensionally and in association with freshwater bivalves and gastropods in finer-grained deposits that are interpreted as relatively lowland and distal lacustrine settings. Transitional ecotones with an abundant and more diverse set of leaf morphotypes as well as permineralized wood remains typify siltstones and sandstones in channel, floodplain, and crevass-splay facies at varied study sites in the Cantwell Basin. Preliminary collections of these floral elements include the cycad Nilssonia, Cladophlebis fern foliage, the monocot Sparganium, and the dicot genera Grewiopsis, Viburnum, Planera, Cocculus, Dicotylophyllum, Pseudoprotophyllum, and Menispermites among others. Wood remains are of the conifer genus Metasequoia and show stark growth rings with abrupt early to late wood transition and very few false rings.

Leaf margin and leaf size analyses of all dicot foliage collected at all newly sampled localities indicate temperate biome conditions with warmer than present mean annual temperatures. By coupling the paleobotanical record of the lower Cantwell Formation with the ichno- and body fossil faunal record, a more complete glimpse into Late Cretaceous ecosystems at mid- to high-latitudes is possible.