GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 216-4
Presentation Time: 2:30 PM


GARVER, John I., Geology Department, Union College, 807 Union ST, Schenectady, NY 12308-2311 and DAVIDSON, Cameron, Department of Geology, Carleton College, 1 N College St., Northfield, MN 55057

Outboard accreted units that flank Wrangellia (aka the Baja BC block) in Alaska are dominated by Upper Cretaceous to Eocene turbidites of the Chugach-Prince William (CPW) terrane and the accreting Yakutat block, which includes the Maastrichtian to Paleocene Yakutat Group. The stratigraphy and provenance of these clastic rocks is important in understanding their accretion and translation history. U/Pb analyses of detrital zircon from nearly 100 sites allow a refined understanding of their stratigraphy and provenance. The Yakutat Group (YAK) consists of flysch and mélange: the flysch includes Maastrichtian to Paleocene turbidites and delta-front sandstone, shale, and conglomerate; the sandstone-rich mélange is dominated by sandstone phacoids of the same age (and older phacoids and inclusions of sandstone, greenstone, marble, metaplutonic rocks, and chert). The maximum depositional age of the sandstones, distinctive U/Pb zircon grain-age distribution, and distinctive phacoids of the mélange allows correlation of the Yakutat Group to rocks of the Western Mélange Belt (WMB) and Nanaimo Group (NAN) in the Pacific NW. The original depositional setting of the Yakutat Group and at least some CPW rocks may have been at a latitude of southern California. The Yakutat Group, WMB, and NAN have a provenance link to the Orocopia Schist and allied schists in the Mojave area. Thus initial deposition in the Maastrichtian-Paleocene may have been in proximity to the distinctive segment of the southern California-Mojave arc built on SW Laurentian Proterozoic basement with a distinctive bimodal U/Pb zircon grain age distribution with peaks at ~1380 and ~1710 Ma. CPW rocks are less distinctive in terms of provenance, but published paleomagnetic studies require restoration far to the south of their current position, and we envision them in a position outboard of the Baja BC block. The YAK, WMB, and Nanaimo Group on the Insular terrane originally formed adjacent to the Mojave tectonic block and were then translated to the Pacific NW together at the southern end of the Baja BC block in the Paleocene and prior to the collision of Siletzia. YAK rocks were then excised and translated northward to Alaska after initial collision of older units of Siletzia in the Early Eocene.