Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
The Arctic Alaska–Chukotka microplate is a large Mesozoic–Cenozoic composite terrane that resides at the northern limit of the North American Cordillera. Although its Mesozoic origins are assuredly linked to the opening of the Amerasian Basin of the Arctic Ocean, its Paleozoic origins can be linked to at least three separate paleocontinents, including northern Laurentia, Baltica, and Siberia. Across the Arctic Alaska portion of the microplate, an internal, mid-Paleozoic suture zone presumably separates rocks of the North Slope subterrane (Laurentian affinity) from a collection of smaller subterranes in the southern Brooks Range and Seward Peninsula (Baltic affinity).
The mountains of the northeastern Brooks Range expose a thick assemblage of Neoproterozoic–Lower Cretaceous rocks that belong to the North Slope subterrane. New data from geological mapping, coupled with zircon U-Pb and muscovite 40Ar/39Ar radiometric ages, reveal that the Neoproterozoic–Ordovician rocks in the NE Brooks Range, assigned to the Firth River Group, Neruokpuk Formation, and the informal Leffingwell formation, record deep-water, slope- to basin-floor sedimentation along the ancient passive margin of northern Laurentia. Stata of the Ordovician–Lower Devonian(?) Clarence River Group (new name) disconformably overlie these passive margin units and record a major shift in the sedimentary source. Detrital zircon U-Pb ages from Clarence River Group strata closely resemble the deep-water, syn-orogenic strata exposed in the Franklinian Basin of northern Ellesmere Island, and are interpreted to reflect erosion and transport of sediment sourced from the Caledonian orogen.
A rootless thrust sheet places a Cambrian–Middle Ordovician structural complex of basalt, limestone and chert, herein named the Whale Mountain allochthon, above the upper strata of the Clarence River Group. Igneous geochemistry and trilobite paleontology suggest that the Whale Mountain allochthon formed as a series of remote volcanic islands or seamounts that established outboard the Laurentian margin. The emplacement of the allochthon occurred in concert with the locally-defined, Early–Middle Devonian Romanzof orogeny, and it may be linked to the closure of the Iapetus Ocean and the collision between Baltica and Laurentia in the Caledonian orogeny. This major collisional event is responsible for the assembly of the Arctic Alaska–Chukotka microplate, implicating the Whale Mountain allochthon as a potential relic to the suture zone that separates the North Slope subterrane from the rest of Arctic Alaska and Chukotka.
Johnson, Benjamin G., "Structural Style And Stratigraphic Architecture Of The Northeastern Brooks Range, Alaska" (2019). Graduate Theses, Dissertations, and Problem Reports. 3949.
Preliminary geological map of the Kongakut River, Arctic National wildlife refuge, northeastern Brooks Range, Alaska
Plate_2_small.pdf (13014 kB)
Preliminary geological map of the upper Jago and Aichilik rivers, Arctic National wildlife refuge, northeastern Brooks Range, Alaska