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Paper No. 3
Presentation Time: 2:15 PM


RIDGWAY, Kenneth, Department of Earth and Atmospheric Sciences, Purdue University, 500 Stadium Mall Drive, West Lafayette, IN 47907, BRENNAN, Patrick, Chevron Energy Technology Company, 1400 Smith St, Houston, TX 77002, KALBAS, James L., ExxonMobil Development Company, Houston, TX 77381, HAMPTON, Brian, Dept. of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, MI 48824-1115 and GEHRELS, George E., Department of Geosciences, University of Arizona, Tucson, AZ 85721,

Mesozoic sedimentary strata in southern Alaska contain a rich record of the tectonic development of this convergent margin. A key unresolved question is the link between Mesozoic strata exposed in remote southwestern Alaska and more accessible strata exposed in southcentral and southeastern Alaska. The Upper Jurassic and Cretaceous stratigraphic units in southwestern Alaska are represented by up to ~ 18 km of strata known as the Kahiltna assemblage and the Kuskokwim Group. Measured stratigraphic sections from the Kahiltna assemblage indicate deposition in submarine fan systems. Detrital zircons from the Kahiltna assemblage are dominated by Mesozoic (56%) and Precambrian (32%) age grains. Many of the Precambrian detrital zircons can be matched to sources within the Yukon composite terrane suggesting limited transport of the Kahiltna assemblage. The detrital zircon age distribution of the Kahiltna assemblage from southwestern Alaska is similar to detrital zircon ages from both Triassic metasedimentary strata and the Kahiltna assemblage exposed in the Alaska Range of southcentral Alaska but is different from the Kahiltna assemblage (and stratigraphic equivalents) in the northeastern Talkeenta Mountains and southeastern Alaska.

The Kuskokwim Group ranges in age from Cenomanian to Campanian and crops out in a greater than 70,000 km2 belt. Measured sections of the Kuskokwim Group document deposition by mainly deltaic and submarine fan deposystems. Detrital zircon age populations are dominated by a wide range of Precambrian (54%) and Mesozoic age grains (24%) that contain few peak populations. We interpret the wide range of Precambrian ages to be a product of a regional fluvial system that fed the deltaic and submarine fans systems of the Kuskokwim Group. In our hypothesis this fluvial system would be analogous to a Late Cretaceous version of the modern Yukon River and delta system.

From a regional basin perspective, the Kahiltna assemblage in southwestern Alaska may represent the more distal parts of a remnant ocean basin that was connected to coarser-grained submarine fan deposystems now exposed in southcentral Alaska. A westward shift in deposystems during deposition of the Kuskokwim Group resulted in the establishment of a regional fluvial/deltaic depocenter that drained much of what is now southern Alaska.

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