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