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Kortyna, C.D., 2011

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Kortyna, C.D., 2011, Sedimentary record of the transition from Late Cretaceous arc magmatism to Eocene spreading ridge subduction in southern Alaska: Lewisburg, Pennsylvania, Bucknell University, B.S. thesis (honors), 140 p., illust., maps, photos.


Upper Paleocene-Eocene boulder conglomerate, cross-stratified sandstone, and laminated carbonaceous mudstone of the Arkose Ridge Formation exposed in the southern Talkeetna Mountains record fluvial-lacustrine deposition proximal to the volcanic arc in a forearc basin modified by Paleogene spreading ridge subduction beneath southern Alaska. U-Pb ages of detrital zircon grains and modal analyses were obtained from stratigraphic sections spanning the 2,000-m-thick Arkose Ridge Formation to constrain the lithology, age, and location of sediment sources that provided detritus. Detrital modes from 24 conglomerate beds and 54 sandstone thin sections are dominated by plutonic and volcanic clasts and plagioclase feldspar with minor quartz, schist, hornblende, argillite, and metabasalt. Westernmost sandstone and conglomerate strata contain <5% volcanic clasts whereas easternmost sandstone and conglomerate strata contain 40 to >80% volcanic clasts. Temporally, eastern sandstones and conglomerates exhibit an upsection increase in volcanic detritus from <40 to >80% volcanic clasts. U-Pb ages from >1,400 detrital zircons in 15 sandstone samples reveal three main populations: late Paleocene-Eocene (60-48 Ma; 16% of all grains), Late Cretaceous-early Paleocene (85-60 Ma; 62%) and Jurassic-Early Cretaceous (200-100 Ma; 12%). A plot of U/Th vs U-Pb ages shows that >97% of zircons are <200 Ma and >99% of zircons have <10 U/Th ratios, consistent with mainly igneous source terranes. Strata show increased enrichment in late Paleocene-Eocene detrital zircons from <2% in the west to >25% in the east. In eastern sections, this younger age population increases temporally from 0% in the lower 50 m of the section to >40% in samples collected >740 m above the base.

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