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Stratigraphy, depositional systems, and provenance of the Lower Cretaceous Kahiltna assemblage, western Alaska Range: Basin development in response to oblique collision

By
James L. Kalbas
James L. Kalbas
1
ExxonMobil Development Company, 12450 Greenspoint Drive, Houston, Texas 77060, USA
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Kenneth D. Ridgway
Kenneth D. Ridgway
2
Department of Earth & Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47907-2051, USA
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George E. Gehrels
George E. Gehrels
3
Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 E. Fourth Street, Tucson, Arizona 85721, USA
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Published:
January 01, 2007

The Kahiltna assemblage of southern Alaska crops out in an 800-km-long belt that forms the core of much of the rugged Alaska Range. New sedimentologic, provenance, and geologic mapping data suggest that the Kahiltna assemblage exposed in the western Alaska Range represents a late Early Cretaceous to Late Cretaceous marine basin that formed in response to oblique collision between a composite island-arc terrane and the Mesozoic continental margin of North America. The Kahiltna assemblage in the study area crops out in two belts located north and south of the Denali fault system. Measured stratigraphic sections show that the Kahiltna assemblage in the southern outcrop belt has a minimum thickness of 5560 m and consists of eight siliciclastic lithofacies that represent tabular and weakly channelized mixed sand-mud submarine-fan systems that developed in a base-of-slope environment of deposition. Our analysis of the Kahiltna assemblage located north of the Denali fault indicates the presence of similar lithofacies along with additional strata that we interpret to represent outer-shelf and/or upper-slope (slope apron) depositional environments. Geologic mapping for this study identified the depositional basement of both outcrop belts as Upper Triassic to Lower Jurassic marine-volcanic and volcaniclastic strata that form the upper part of the Mystic subterrane. Detrital zircon data constrain the depositional age of most of the Kahiltna assemblage in the study area to Early Cretaceous time (Aptian or Albian) or later and suggest a significantly younger timing of basin development than previously recognized.

Compositional data indicate that sandstone and conglomerate of the Kahiltna assemblage were derived from both Mesozoic continental margin and composite island-arc terrane sources. Modal sandstone compositions (n = 41) are consistent with a mixed arc and recycled orogen provenance (Q23F9L68; Qm11F9Lt80). Detrital zircons from sandstone collected in the lower part of the Kahiltna assemblage yield Precambrian (32%), Paleozoic (12%), and Mesozoic (56%) U-Pb ages. Concordant ages are consistent with the age distributions of Proterozoic, Devonian, Mississippian, and Triassic-Jurassic plutonic rocks of the former continental margin that formed the northern boundary of the Kahiltna basin. Plutons of the Talkeetna and Chisana arcs, part of the composite island-arc terrane located south of the basin, also probably contributed to the abundance of detrital zircons with ages between 200 and 163 Ma and between 124 and 106 Ma, respectively. Our new findings indicate that by Early Cretaceous time, the North American continental margin and composite island-arc terrane were in close enough proximity for both to contribute sediment to the Kahiltna basin.

Stratigraphic, structural, and geochronologic relationships presented here, combined with previous regional studies of Mesozoic strata in the suture zone, suggest that the Kahiltna assemblage is the product of oblique island-arc terrane collision. Oblique collision resulted in the juxtaposition of continental margin and oceanic strata within thrust sheets along the closing suture zone. Dominantly west- and southwest-directed submarine-fan systems transported detritus axially away from the closing suture zone and into the along-strike marine basin represented by most of the Kahiltna assemblage exposed in the western Alaska Range. Comparisons with along-strike uplifted Mesozoic marine basins suggest westward time-transgressive closure of a suture zone that extends from British Columbia to southwestern Alaska.

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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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