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Abstract

The Prince Creek Formation is an Upper Cretaceous, dinosaur-bearing, high-latitude alluvial succession deposited on an ancient coastal-plain that crops out in bluffs along the Colville, Kogosukruk, and Kikiakrorak Rivers of northern Alaska. Studies that document the complex stratigraphy and architecture of high-latitude alluvial systems deposited under greenhouse conditions are extremely rare. It is exceptionally uncommon to find extensive, accessible outcrops that also contain numerous Arctic dinosaur fossils; hence the Prince Creek Formation is of great significance not only to sedimentologists but also to paleontologists involved in reconstructing high-latitude dinosaur habitats.

Maastrichtian strata of the Prince Creek Formation record deposition on a tidally influenced high-latitude coastal-plain in (i) first-order meandering trunk channels, (ii) second-order meandering distributary channels, (iii) third-order fixed (anastomosed?) distributary channels, and (iv) on floodplains. Conglomerate and medium- to coarse-grained multistory sandbodies are found exclusively in regionally restricted 13–17-m-thick fining-upward successions (FUSs) that display inclined heterolithic stratification (IHS) capped by finer-grained, organic-rich facies. These relatively thick FUSs are interpreted as first-order meandering trunk channels. Thinner (2–6-m thick), single-story, heterolithic sheet sandbodies composed predominantly of IHS and including abundant mud-filled channel plugs are the most frequently encountered channel form. Trough cross-lamination at the base of the IHS records paleoflow at high angles relative to the dip of the inclined beds, indicating that lateral accretion of point-bars was the principal depositional mechanism. These single-story sandbodies are interpreted as second-order meandering distributary channels. Fine-grained 1.5–3.0-m-thick, ripple cross-laminated ribbon sandbodies deposited mainly by vertical accretion above an arcuate erosion surface and containing only minor IHS are interpreted as third-order fixed (anastomosed?) distributary channels. Thinner (0.2–1.0-m-thick) current-rippled sheet sands and silts are interpreted as small-scale crevasse splays and levees. Organic-rich siltstone and mudstone, carbonaceous shale, coal, bentonite, and tuff are interpreted as deposits of lakes, ponds, swamps, marshes, mires, paleosols, and ashfall on floodplains.

Heterolithic sheet sandstones deposited by small, sinuous meandering distributary channels typically appear lenticular along strike, commonly incise into pre-existing distributary channels, and interfinger with and incise into organic-rich floodplain facies. Fixed, ribbon-form (anastomosed?) distributaries incise either into meandering distributaries or into floodplain facies, with numerous ribbons typically preserved in tiers at the same stratigraphic level. Spatial relationships between channel types, and between channels and floodplain facies, indicate that the bulk of deposition took place on crevasse-splay complexes adjacent to trunk channels. Crevasse-splay complexes were constructed by the lateral migration of sinuous meandering distributaries and the vertical filling of fixed (anastomosed?) distributaries, with splay complexes separated from each other by organic floodplain facies. Flow in meandering distributaries and fixed (anastomosed?) distributaries may have been contemporaneous. Alternatively, fixed (anastomosed?) distributaries may record the initial or waning stages of flow during splay-complex formation or abandonment.

IHS composed of rhythmically repeating, coarse-to-fine couplets of current-rippled sandstone and siltstone or mudstone is found in all three types of channels. The rhythmic and repetitive nature of these couplets together with relatively thick, muddy fine-grained members in couplets suggest that flow in channels was likely influenced by tidal effects.

Drab colors in fine-grained sediments, abundant carbonaceous plant material, and common siderite nodules and jarosite suggest widespread reducing conditions on poorly drained floodplains influenced, in more distal areas, by marine waters. However, carbonaceous root traces found ubiquitously in all distributary channels and most floodplain facies along with common Fe-oxide mottles indicate that the alluvial system likely experienced flashy, seasonal, or ephemeral flow, and a fluctuating water table. The flashy nature of the alluvial system may have been driven by recurring episodes of vigorous seasonal snowmelt in the Brooks Range orogenic belt as a consequence of the high paleolatitude of northern Alaska in the Late Cretaceous.

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