DINOSAUR-BEARING HYPERCONCENTRATED FLOWS OF CRETACEOUS ARCTIC ALASKA: RECURRING CATASTROPHIC EVENT BEDS ON A DISTAL PALEOPOLAR COASTAL PLAIN

The Cretaceous coastal plain of Arctic Alaska contains the richest concentration of high-latitude dinosaurs on Earth. Three bonebeds (Liscomb, Byers, Sling Point) are found in paleopolar (82 degrees-85 degrees N) coastal-plain deposits of the Prince Creek Formation on Alaska's North Slope. 40Ar/39Ar analysis of a tuff below the oldest bonebed (Sling Point) returned an age of 69.2 +/- 0.5 Ma indicating a maximum early Maastrichtian age for these bonebeds. Bonebeds are overwhelmingly dominated by partially articulated to associated late-stage juvenile Edmontosaurus sp. Bone is rarely found in channels; instead high-density accumulations are preserved on floodplains in laterally extensive, muddy alluvium. Bone size grading is vertically nonuniform and most bones are in hydraulic disequilibrium with the surrounding clay-rich matrix. Bones exhibit little evidence of rounding, weathering, predation, or trampling, suggesting short-distance transport and rapid burial. Because these bonebeds are unlike typical debris-flow or streamflow deposits, the mechanism for bonebed emplacement remained poorly understood. All bonebeds contain a current-rippled siltstone containing the largest bone overlain by a distinctive mudstone encasing smaller bones, bone fragments, and subparallel-aligned plant fragments that appear frozen in flow within the muddy matrix. We recognize that these bonebeds exhibit a recurring facies pairing and bipartite division of flow consistent with deposition by fine-grained viscous hyperconcentrated flows. We suggest that exceptional discharge events entrained mud and ash stored on point bars and floodplains, increasing suspended- sediment concentrations in rivers and generating erosive hyperconcentrated flows that transported the remains of scores of juvenile dinosaurs onto floodplains adjacent to distributary channels.