FORMATION OF THE KAHILTNA BASIN, ALASKA, BY TIME-TRANSGRESSIVE EARLY TO LATE CRETACEOUS DEXTRAL-OBLIQUE JUXTAPOSITION OF TWO INDEPENDENT DEPOSITIONAL SYSTEMS

Sandstone petrography and detrital zircon (DZ) U-Pb ages across the Kahiltna basin in the western Alaska Range identify distinct sediment sources from opposite sides of the basin. Sandstones at the NW edge of the basin, the Dalzell petrofacies, contain dominantly chert, argillite, quartz, subordinate feldspar, mica, carbonate, metamorphic (Lm) and volcanic (Lv) grains, with Qm₂₅F₂₁Lt₅₄, Lm₁₂Lv₁₉Ls₆₉, and Precambrian and Paleozoic DZ. The youngest DZ ages suggest a maximum depositional age (MDA) ~102 Ma. The overlying Whistler petrofacies has Qm₃₃F₂₂Lt₄₅, Lm₂₀Lv₉Ls₇₁, abundant Precambrian DZ, and a MDA ~88 Ma. These sandstones contain grains and DZ ages similar to those of the Farewell and Yukon Tanana terranes, supporting continental margin sources.

These sandstones were not observed in depositional contact with sandstones on the SE side of the Kahiltna basin, where the older Trimble petrofacies contains abundant Lv, feldspar, and clinopyroxene (cpx), minor argillite and quartz, Qm₁₂F₅₂Lt₃₆, Lm₂Lv₄₈Ls₅₀, mainly Mesozoic DZ, and MDA ~121 Ma. The younger Old Man petrofacies contains little or no quartz, with Qm₅F₄₉Lt₄₆, Lm₁Lv₇₆Ls₂₃, youngest DZ at ~91 Ma, and interbedded tuffs that have U-Pb zircon ages of ~97-93 Ma. Abundant Lv and cpx support sources in volcanic rocks of the Peninsular-Wrangellia (P-W) terrane SE of the basin.

An inferred fault system in the basin axis juxtaposes Whistler and Old Man petrofacies rocks that lack compositional overlap, indicating separate depositional systems prior to ~88 Ma. This is supported by contrasting U and Th concentrations in DZ <115 Ma from Whistler compared to Old Man samples.

In all 4 petrofacies K-feldspar is lacking. A newly identified sandstone, the Hayes petrofacies, overlaps the basin axis and contains Precambrian and Paleozoic DZ with MDA ~81 Ma, 4-11% K-feldspar suggesting a previously unavailable source, and mica, Lm, cpx, and Lv indicating contributions from both the continental margin and the P-W arc for the first time. Previous work at the NE apex of the Kahiltna basin has shown similar mixed sandstone compositions and DZ ages indicating juxtaposition in the earliest Cretaceous. Our data suggest dextral-oblique time-transgressive juxtaposition of the continental margin and P-W volcanic arc from Early Cretaceous in the NE to Late Cretaceous in the SW.