Depositional Environments Of The Late Cretaceous (Maastrichtian) Dinosaur-Bearing Prince Creek Formation: Colville River Region, North Slope, Alaska

Abstract

The Prince Creek Formation contains first-order meandering trunk channels, second-order meandering distributary channels, third-order fixed anastomosed(?) distributary channels, crevasse splays, levees, lakes, ponds, swamps, paleosols, and ashfall deposits. Trampling by dinosaurs is common. Most deposition occurred on crevasse splay-complexes adjacent to trunk channels. Rhythmically-repeating coarse-to fine-grained couplets in inclined heterolithic stratification suggest tidal-influence in channels. Cumulative to compound soils similar to Entisols, Inceptisols, and potential acid sulfate soils formed on levees, point bars, crevasse splays, and on the margins of lakes and swamps. Frequent overbank flooding is evidenced by silt and sand dispersed throughout paleosol profiles and fluctuations with depth in several molecular ratios. Drab colors, organics, siderite, depletion coatings, and zoned peds indicate waterlogged, anoxic conditions while ferruginous and manganiferous features, insect and worm burrows, and rare illuvial clay coatings and infillings suggest drying and oxidation of some soils. Repeated wetting and drying is tied to fluctuating river discharge. Marine influence is evidenced by jarosite, pyrite, and gypsum which become increasingly common up-section near the contact with the shallow-marine Schrader Bluff Formation. Pollen includes Peridinioid dinocysts; algae; projectates; Wodehouseia edmontonicola; pollen from lowland trees, shrubs and herbs; Bisaccates; fern and moss spores; and fungal hyphae and indicates that all strata are Early Maastrichtian and that sediments become progressively younger to the north. 40Ar/39Ar analysis of a tuff returned an age of 69.2 +/- 0.5 Ma. World-class dinosaur bonebeds are encased in muddy overbank alluvium overlying floodplains. No concentration of bone was found in channels. Bonebeds are laterally extensive except where truncated by distributaries. At the Sling Pont, Liscomb, and Byers bonebeds alluvium encasing bone exhibits a bipartite division of flow and a massive mudstone facies containing flow-parallel plant fragments that "float" in a mud matrix suggesting deposition by fine-grained hyperconcentrated flows. Exceptional floods driven by seasonal snowmelt in the Brooks Range increased suspended sediment concentrations, generating hyperconcentrated overbank flows that killed and buried scores of juvenile dinosaurs occupying this high-latitude coastal plain. This unique killing mechanism likely resulted from fluctuating discharge tied to extreme seasonality brought about by the near polar latitude of northern Alaska in the Late Cretaceous.