MID-CRETACEOUS VOLCANISM AND FLUVIAL-DELTAIC SEDIMENTATION ASSOCIATED WITH ACCRETION OF THE WRANGELLIA COMPOSITE TERRANE, CHISANA ARC, GRAVINA-NUTZOTIN BELT, ALASKA

Cretaceous tectonic models for accretion of the oceanic Wrangellia terrane include closure of an ocean basin and magmatism, however the nature of the magmatism and its tectonic settings along strike are unclear. An ideal area in south-central AK to test disparate models for Wrangellia accretion is in the Nutzotin Mts. where the Chisana Formation (Kc) crops out. Prior and new geochronologic and geochemical data from Kc lavas document ~121-117 Ma arc volcanism. New stratigraphic analyses document that the Kc is ~1770-m-thick at its Bonanza Creek type section. The base of the section conformably overlies shallow marine strata of the Nutzotin Mts. Sequence (NMS) and lies unconformably below the terrestrial Beaver Lake Formation, which hosts dinosaur footprints and plant fossils. We divide the Kc at Bonanza Creek into three units. The lower Kc includes basalt and andesite lavas, with local pillows, mudstone and volcaniclastic conglomerates, which include Inoceramus and belemnite fossils. The middle Kc includes lavas, some with autobrecciated bases/tops, volcaniclastic conglomerates with Inoceramus fossils, breccia, and block and ash deposits that record effusive eruptions, lahars, and pyroclastic eruptions. The upper Kc consists of lavas with oxidized, autobrecciated tops/bases and volcaniclastic conglomerates; these rocks are consistent with subaerial eruptions and deposition, in contrast to the subaqueous and transitional characteristics of the lower and middle Kc, respectively. We interpret these units and their facies to represent a progradational upsection transition from subaqueous to subaerial processes. In contrast, ~70 km outboard in the Jacksina Creek drainage, subaerially erupted Kc lavas conformably overlie a fluvial-deltaic NMS facies. Kc lavas typically contain plagioclase, pyroxene, and amphibole phenocrysts. They range in composition from basalt through andesite, with some higher-Si compositions, are subalkaline to transitional on a total alkali vs. SiO₂ diagram, and tholeiitic to calc-alkaline on an AFM diagram. Our data imply more significant terrestrial eruptions and sedimentation than previously recognized for this part of the mid-Cretaceous North American Cordillera and adds additional constraints on the timing and evolution of the accretion of the Wrangellia terrane.