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Sedimentary record of the tectonic growth of a collisional continental margin: Upper Jurassic–Lower Cretaceous Nutzotin Mountains sequence, eastern Alaska Range, Alaska

By
Jeffrey D. Manuszak
Jeffrey D. Manuszak
1
Department of Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907-2051, USA
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Kenneth D. Ridgway
Kenneth D. Ridgway
1
Department of Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907-2051, USA
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Jeffrey M. Trop
Jeffrey M. Trop
2
Department of Geology, Moore Avenue, Buckness University, Lewisburg, Pennsylvania 17837, USA
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George E. Gehrels
George E. Gehrels
3
Department of Geosciences, Gould-Simpson Building, University of Arizona, 1040 E. Fourth Street, Tucson, Arizona 85721, USA
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Published:
January 01, 2007

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and least-metamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin.

New sedimentologic and stratigraphic data from the ∼3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) mudstone with distinctive fossil-rich horizons and minor interbedded sandstone. The overall stratigraphy of the Nutzotin Mountains sequence represents a general upward-shallowing and upward-coarsening package that represents a general transition from distal mud-rich submarine-fan strata to more proximal sand-rich submarine-fan strata that are in turn overlain by marine shelf strata. Feldspathic sandstone compositions (Q6F67L27), eastward and northeastward directed paleocurrent indicators, diagnostic clasts in conglomerate, and detrital zircon U-Pb ages of 151–147 Ma (n = 8) and 159–156 Ma (n = 2) indicate that sediment in the Nutzotin basin was derived primarily from the Wrangellia terrane and the Chitina and Chisana arcs that intrude the Wrangellia terrane.

The stages of deformation documented in the Nutzotin Mountains sequence provide insight into the growth of collisional continental margins by the tectonic incorporation of basinal strata. Our data show that strata of the Nutzotin basin have been deformed into an accretionary wedge by north-dipping thrust faults and related overturned folds above a north-dipping décollement. Displacement on this décollement was the product of northward underthrusting of basinal strata beneath the former continental margin and resulted in southward tectonic transport of distal basinal strata of the Nutzotin Mountains sequence strata over both more proximal basinal strata and the Wrangellia terrane. Previously published K-Ar ages from plutons that cross-cut both the décollement and folded Nutzotin Mountains sequence strata indicate that contractional deformation ended between 117 and 105 Ma. Regionally, the Nutzotin Mountains sequence represents part of a series of Mesozoic sedimentary basins located along the inboard margin of the Wrangellia composite terrane that have similar depositional styles and were all subsequently incorporated into accretionary wedges that dip toward the former continental margin. These deformed strata define a continental-scale suture zone that extends along the northwestern Cordillera for over 2000 km.

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GSA Special Papers

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska

Kenneth D. Ridgway
Kenneth D. Ridgway
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Jeffrey M. Trop
Jeffrey M. Trop
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Jonathan M.G. Glen
Jonathan M.G. Glen
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J. Michael O'Neill
J. Michael O'Neill
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Geological Society of America
Volume
431
ISBN print:
9780813724317
Publication date:
January 01, 2007

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