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Sample, J.C., 1986

Structure, tectonics, and sedimentology of the Kodiak Formation, Kodiak and adjacent islands, Alaska

Bibliographic Reference

Sample, J.C., 1986, Structure, tectonics, and sedimentology of the Kodiak Formation, Kodiak and adjacent islands, Alaska: University of California, Santa Cruz, Ph.D. dissertation, 157 p., illust., maps.


This thesis presents results from the Kodiak Formation bearing on its depositional history, structural evolution, and tectonic development. The Kodiak Formation is a slate belt underlying Kodiak and adjacent islands in the Gulf of Alaska and constitutes 70% of the Kodiak accretionary complex. Measured paleocurrents trend south or north-south and indicate deposition in a trench for part of its history. Occurrences of Inoceramus kusiroensis or Inoceramus balticus at five localities restrict the age of the formation to the early Maastrichtian. The volume of the Kodiak Formation suggests it was deposited at a rate comparable to the modern Amazon Cone. Petrographic data show a quartz-rich component with a probable recycled orogen source (Q56F19L24]) and a lesser volcaniclastic component with a probable volcanic arc source (Q26F19L55). Considerations of paleomagnetic data and a terrane tie suggest that the Kodiak Formation was deposited thousands of kilometers south of its expected position relative to North America. The Coast Plutonic Complex of western British Columbia is a likely source because it satisfies many of the lithologic and paleolatitudinal requirements of the Kodiak source and began substantial uplift in the Late Cretaceous. The sequential structural development (and inferred tectonic origin) includes: (1) boudinage and cataclastic shearing (underthrusting); (2) development of slaty cleavage, thrust faulting, and folding (underplating); (3) southeast-dipping crenulation and crenulation cleavage development (intra-wedge shortening); (4) right lateral strike-slip faulting (oblique subduction or northward translation of the complex?); and (5) brittle thrusting (uplift?). Intrusion of early Paleocene granodioritic and tonalitic plutons and sills intercedes between 2 and 3. Geometries of accretion-related structures indicate that underplating occurred by duplex accretion. Duplexes exist at the outcrop scale, and I interpret the central belt as a floor thrust zone beneath the landward belt duplex; the roof thrust has been eroded. Imbrication of thick packets of turbidites allowed the formation to remain coherent during underplating. Pre- and syn-accretion metamorphic conditions indicate that underplating occurred at a minimum depth of 10 km. A modern analog for this style of accretion is the Barbados Ridge complex, where coherent sediment packets are being imbricated at the base of the accretionary wedge as duplexes.

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