AGE AND PROVENANCE OF THE RESTRICTED GEMUK GROUP (TRIASSIC TO LOWER CRETACEOUS), SOUTHWEST ALASKA

The restricted Gemuk Group is an Upper Triassic to Lower Cretaceous deep marine succession in southwest Alaska. First defined by Cady et al. in 1955, the Gemuk Group quickly became a regional catch-all unit that ended up as part of four different terranes. We restrict the Gemuk Group to rocks in the original type area (in Taylor Mts. quadrangle), plus the contiguous rocks to the north (in Sleetmute quadrangle). Discontinuous exposure, hints of complex structure, the reconnaissance level of our mapping, and spotty age constraints, together permit definition of only a rough stratigraphy. The age range of the restricted Gemuk Group is tightened on the basis of nine new fossil ages, two new U/Pb zircon ages on tuff, and U/Pb ages of 110 detrital zircons from two sandstones. The Triassic part of the Gemuk, which consists of intermediate pillow lavas interbedded with siltstone, chert, and rare limestone, produced radiolarians, bivalves, and conodonts of Carnian and Norian ages. The Jurassic part of the Gemuk appears to be mostly siltstone and chert, and yielded radiolarians of Hettangian-Sinemurian, Pliensbachian-Toarcian, and Oxfordian Jurassic ages. Two tuffs near the Jurassic-Cretaceous boundary record nearby arc volcanism; one at 146 Ma is interbedded with red and green siltstone and a second at about 137 Ma is interbedded with graywacke turbidites. Graywacke appears to be the dominant rock type in the Lower Cretaceous part of the Gemuk. The detrital zircon population of one of these sandstones is dominated by zircons of 133-180 Ma (oldest grain is 316 Ma); another sample is dominated by zircons of 130-154 Ma (oldest grain is 292 Ma). The youngest zircons are not much older than the sandstone. The restricted Gemuk Group represents at least 80 m.y. of sedimentation, probably in a back-arc, intra-arc, or forearc basin associated with the Togiak terrane volcanic arc.