SANDSTONE PETROGRAPHY, FOSSILS, AND DETRITAL ZIRCON AGES DISTINGUISH PROVENANCE IN THE TIKCHIK LAKES AREA, SOUTHWEST ALASKA

Petrography, fossils, and detrital zircon (DZ) ages provide insights on provenance, depositional environments, and ages of sandstones in the Tikchik Lakes area. Sandstone, limestone, chert, and volcanic rocks in older units, referred to as the Tikchik assemblage, are structurally imbricated beneath volcanic and sedimentary rocks of the Togiak arc. Late Cretaceous Kuskokwim Group (Kk) sandstones unconformably overlie the Tikchik and Togiak rocks.

The Tikchik assemblage contains calcareous to siliceous immature sandstones that have 40%-70% angular monocrystalline quartz (Qm), and traces of feldspar, mica, phyllite, veined black argillite, and veined chert; and chert pebble conglomerate containing sandstones having up to 90% rounded chert clasts, with volcanic and fossil debris, and interstitial angular Qm. DZ have major peaks at 560, 765, 890-925, and 1665 Ma, which are distinctly non-North American ages, and lesser peaks at 1065, 1150, 1480, 1790-1965, 2500, and 2700-3025 Ma, which are less diagnostic of craton source. A sample from the conglomerate unit contains 280 Ma DZ, equivalent to local mid-Permian fossil ages. Clasts and fossils indicate warm shallow to deep marine depositional environments that received debris from local carbonate platforms, deformed marine basin deposits, and cratonic granitic sources. A general lack of DZ younger than 500 Ma suggests disappearance of the granitic source, perhaps by rifting of a passive margin, in the Early Paleozoic. Late Paleozoic arc volcanic rocks in the Tikchik assemblage may be the source of DZ younger than 500 Ma.

The Togiak arc assemblage includes fossiliferous Triassic limestone interlayered with volcanic wacke that contains locally derived Permian to Triassic DZ, and turbidites that contain Triassic to Jurassic fossils and dominantly Triassic to Early Cretaceous DZ, similar to Gemuk unit DZ populations.

Kk sandstone in the Tikchik area contains plutonic, metamorphic, volcanic, and sedimentary clasts, and a DZ population extending from 82 Ma to 2 Ga that shows age peaks similar to Kk sandstones throughout southwest Alaska. Slate Lake sandstone contains Early Cretaceous bivalves and DZ 103 Ma and older, with peaks matching those of the underlying Tikchik assemblage. Kk is Late Cretaceous; the sandstone of Slate Lake may be a locally-sourced precursor to Kk.