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Dinosaur-bearing hyperconcentrated flows of Cretaceous Arctic Alaska



Ben Creisler
bcreisler@gmail.com

A new paper:



Peter P. Flaig, Anthony R. Fiorillo, and Paul J. Mccarthy  (2014)
Dinosaur-bearing hyperconcentrated flows of Cretaceous Arctic Alaska:
recurring catastrophic event beds on a distal paleopolar coastal
plain.
PALAIOS 29(11): 594-611
doi: http://dx.doi.org/10.2110/palo.2013.133
http://www.bioone.org/doi/abs/10.2110/palo.2013.133


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°–85° 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.