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Decker, J.E., Jr., 1980

Geology of a Cretaceous subduction complex, western Chichagof Island, southeastern Alaska

Bibliographic Reference

Decker, J.E., Jr., 1980, Geology of a Cretaceous subduction complex, western Chichagof Island, southeastern Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 199 p., illust., maps.


This thesis deals with several aspects of the geology of a Cretaceous subduction complex, the Chugach terrane, on Chichagof and Baranof islands in southeastern Alaska. The thesis is divided into six independent chapters that are modifications of manuscripts that are intended to be published separately. Detailed mapping on western Chichagof Island reveals that five distinctive units can be recognized; they are, from east to west, the Goon Dip Greenstone, the Whitestripe Marble, the Kelp Bay group, the Ford Arm Formation, and the Sitka Graywacke. The Goon Dip Greenstone and the Whitestripe Marble are pre-Late Jurassic in age and possibly correlate with Triassic rocks in the Wrangell Mountains to the north. The Kelp Bay Group is a chaotic metasedimentary and metavolcanic terrane that I have subdivided into genetically significant subduction units based on deformational style, textural grade, and lithologic association. The Kelp Bay Group is correlative with the Lower Cretaceous McHugh Complex in the Chugach Mountains and the Lower Cretaceous Uyak Complex on Kodiak and adjacent islands. The Ford Arm Formation consists mainly of flyschoid rocks that I interpret to represent a middle-fan facies association. The Ford Arm Formation is continuous with Upper Cretaceous rocks of the Valdez Group in the Chugach Mountains and correlative with the Kodiak and Shumagin Formations in southwestern Alaska. These units are suggested to have formed in a single trench system, fed from a dominantly southeastern source. The Sitka Graywacke consists mainly of massive sandstone that I interpret to represent an inner-fan facies association. Except for metamorphic effects, the Sitka Graywacke is petrographically similar to the Ford Arm Formation, and since no fossil age control is available from either unit, it could not be determined whether they are both parts of the same or of different depositional systems. The flyschoid rocks have mineral assemblages indicative of prehnitepumpellyite or lower greenschist metamorphic facies. This low-grade metamorphism caused, most notably, metasomatic loss of potassium, first from felsic volcanic rock fragments and then from potassium feldspar. The Kelp Bay Group was metamorphosed under conditions of the prehnitepumpellyite, greenschist, and glaucophane schist facies. Glaucophane occurs locally throughout a central zone on western Chichagof Island, outside of thermal aureoles around tertiary plutons. The occurrence, geochemistry, and petrology of metavolcanic rocks from Chichagof Island, and elsewhere in the Chugach terrane, indicate that basaltic ocean-floor volcanism was contemporaneous with deposition of continental-derived sediment.

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