PETROGRAPHIC AND DETRITAL ZIRCON DATA FROM THE KAHILTNA BASIN SUGGEST EARLY CRETACEOUS JUXTAPOSITION OF THE CONTINENTAL MARGIN AND THE PENINSULAR TERRANE IN THE WESTERN ALASKA RANGE

Sandstone petrography, fossils, and detrital zircon (DZ) U-Pb ages across the Kahiltna basin in the western Alaska Range identify sediment sources and depositional patterns in space and time. Modal sandstone data (n=68) for the Kahiltna assemblage indicate distinct sequences on the northwest (NW) and southeast (SE) sides of the basin. Sandstones from the base of the NW basin margin, the Dalzell petrofacies, contain chert, argillite, detrital carbonate, quartz, subordinate feldspar, white mica, metamorphic (Lm) and volcanic (Lv) clasts, with Qm₂₅F₂₁Lt₅₄ and Lm₁₂Lv₁₉Ls₆₉, Early Cretaceous fossils, Precambrian DZ, and youngest DZ age peaks at ~110 Ma. The top of the NW section, the Whistler petrofacies, contains Qm₃₃F₂₂Lt₄₅, Lm₂₀Lv₉Ls_71, Precambrian DZ, and youngest DZ age peaks at ~85 Ma. Sandstones from the base of the SE Kahiltna basin margin, the Trimble petrofacies, contain abundant Lv, feldspar, and clinopyroxene (cpx), minor argillite and quartz, with Qm₁₂F₅₂Lt₃₆ and Lm₂Lv₄₈Ls₅₀, only Mesozoic DZ, and youngest DZ age peaks at ~150 Ma. The top of the SE section, the Old Man petrofacies, contains Qm₅F₄₉Lt₄₆ and Lm₁Lv₇₆Ls₂₃, sparse Precambrian DZ, and youngest DZ age peaks at ~95 Ma.

Sandstone petrofacies indicate different sources for the opposite sides of the Kahiltna basin. Clasts in the NW section, including black radiolarian argillite cut by quartz veins, resemble rocks in the adjacent Farewell terrane; sparse Lv reflect sparse volcanic rocks near the NW basin margin. NW section DZ age peaks resemble those of the Farewell and Yukon Tanana terranes, and support continental margin sources for the NW section. Abundant Lv and cpx in SE petrofacies support sources in volcanic arc rocks of the Peninsular terrane SE of the basin. Sparse Precambrian DZ ages in the Old Man petrofacies near the basin center suggest limited sediment input in the Late Cretaceous from multiple sources.

Sandstone compositions and ages suggest deposition dominated by local basin margin sources and initial deposition in the Early Cretaceous for the NW section, and local margin sources and initial deposition in the Late Jurassic for the SE section. Our data support Early Cretaceous uplift and erosion of the NW flank of the Kahiltna basin in the western Alaska Range, and juxtaposition of the Peninsular terrane and continental margin at this time.