INDEPENDENT JURASSIC TO LATE CRETACEOUS DEPOSITIONAL SYSTEMS DEFINED BY SANDSTONE PETROGRAPHY AND DETRITAL ZIRCONS CONSTRAIN CLOSURE OF THE KAHILTNA-KUSKOKWIM BASIN IN SOUTHERN AND SOUTHWESTERN ALASKA

Sandstone petrography and detrital zircon (DZ) U-Pb ages in the Kahiltna basin, western Alaska Range, identify distinct provenance from opposite sides of the NE-trending basin, and link the two sandstone units to rocks in the Kuskokwim and Koksetna River units in SW Alaska. Sandstones on the NW flank of the Kahiltna basin contain dominant chert and argillite (Ls), quartz, subordinate feldspar, mica, carbonate, metamorphic (Lm) and volcanic (Lv) grains, with Qm₂₅F₂₁Lt₅₄, Lm₁₂Lv₁₉Ls₆₉, and Precambrian and Paleozoic DZ. Valanginian fossils and youngest DZ ages suggest maximum depositional ages (MDA) ranging from ~125 to ~102 Ma. These grade to sandstones richer in quartz and mica that have Qm₃₃F₂₂Lt₄₅, Lm₂₀Lv₉Ls₇₁, abundant Precambrian DZ, and MDA of ~88 Ma, very similar to coeval sandstones in the Kuskokwim Group. Sandstone clast compositions and DZ ages suggest derivation from the Farewell and Yukon-Tanana terranes (F-YTT).

Sandstones on the SE side of the Kahiltna basin contain abundant Lv, feldspar, and clinopyroxene (cpx), minor argillite and quartz, Qm₁₂F₅₂Lt₃₆, Lm₂Lv₄₈Ls₅₀, mainly Mesozoic DZ, sparse older DZ that differ in age from the F-YTT suite, and MDA of ~121 Ma. Overlying sandstones contain little quartz, with Qm₅F₄₉Lt₄₆, Lm₁Lv₇₆Ls₂₃, MDA of ~91 Ma, and interbedded tuffs that have U-Pb zircon ages of ~95 Ma. Lv and cpx imply volcanic sources in the Peninsular (PE) or Wrangellia (WR) terranes SE of the basin. Along strike to the SW, sandstones of the Koksetna River unit that have similar compositions and DZ ages depositionally overlie Chilikadrotna basalts which have distinctive trace element chemistry closely resembling WR basalts, suggesting Koksetna sandstones were deposited on WR basement.

The NW and SE sandstones lack compositional overlap, indicating separate depositional systems prior to ~88 Ma, supported by contrasting U/Th in DZ <115 Ma. A newly identified sandstone overlaps the basin axis, containing Precambrian DZ, K-feldspar, mica, Lm, cpx, Lv, and MDA of ~81 Ma, indicating the first contributions from both F-YTT and WR. At the NE apex of the basin, similar mixed sandstone compositions and DZ ages suggest Early Cretaceous juxtaposition. Our data are consistent with dextral-oblique time-transgressive juxtaposition of F-YTT and WR-PE from Early Cretaceous in the NE to Late Cretaceous in the SW.