--> Abstract: Structurally Collapsed part of Laurentian Margin in the Northeastern Brooks Range, Arctic Alaska, by Thomas Moore; #90177 (2013)

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Structurally Collapsed part of Laurentian Margin in the Northeastern Brooks Range, Arctic Alaska

Thomas Moore

It has been long recognized that two contrasting Neoproterozoic to Early Paleozoic stratigraphic sections, one carbonate dominated, the other clastic dominated, are juxtaposed in the NE Brooks Range in angular unconformity beneath the Upper Paleozoic and Lower Mesozoic Ellesmerian Sequence. The carbonate sequence underlies the eastern North Slope coastal plain and is exposed in the Sadlerochit and Shublik Mountains and has been interpreted as being exotic to Arctic Alaska with a possible origin in Siberia or Baltica. The clastic sequence is exposed in the Romanzof and Franklin Mountains to the south and contains fossils that have been linked to northwestern North America. Rocks of both sequences are deformed by north-vergent basement-involved thrusts of the Tertiary northeastern Brooks Range orogenic belt. Investigation of the boundary between the two sequences in the Plunge Creek area indicates that the clastic sequence was thrust northward onto the carbonate sequence prior to the development of the angular unconformity beneath Ellesmerian strata. Devonian metaclastic rocks that consist predominantly of carbonate debris eroded from the carbonate sequence crop out in the footwall of the pre-Mississippian thrust, but this unit also contains numerous blocks of black chert that may be olistoliths shed from the clastic sequence in the hanging wall. We interpret these relations as those of a north-vergent foreland fold-thrust belt and Devonian foreland basin. Detrital zircon U-Pb age populations from the clastic sequence in the hanging wall block are dominated by 1.8-2.0 Ga zircons with lesser populations at 2.4-2.7 Ga and 1.0-1.5 Ga, whereas those from the carbonate sequence in the footwall are dominated by 1.0-1.6 Ga zircons, although samples from the Sadlerochit Mountains yield larger populations at 1.8-1.9 Ga and 2.6-2.7 Ga. The zircon populations of both sequences are consistent with derivation from Laurentian sources but differ in consisting of varying proportions of zircons derived from the Archean and Paleoproterozoic parts of the craton (Laurentia type 1 signature of Hadlari et al., 2012) and zircons derived ultimately from sources in the Grenvillian orogen (Laurentia type 2 signature). The structural framework and detrital zircon populations of the pre-Mississippian rocks in the NE Brooks Range suggest that the two sequences once formed part of the continental margin of Laurentia but were collapsed by thrusting of its clastic-dominated basinal facies onto its more proximal carbonate-dominated facies during a Devonian collisional event. The present east-west trend of Devonian structures in this part of the Brooks Range is consistent with the hypothesis that rifting during counterclockwise rotational opening of the Amerasia Basin in the Cretaceous dislodged a forward part of the Devonian orogen and rotated it with the Arctic Alaska microplate away from northeast-trending Paleozoic structures of the northern margin of North America.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013