Stratigraphy, Provenance, and Tectonic Setting of Mesozoic Strata in the Northwestern Talkeetna Mountains, Alaska

Late Triassic through Cretaceous strata in the northwestern Talkeetna Mountains are part of a regional suture zone between the Mesozoic continental margin and allochthonous oceanic terranes. Previous studies suggest that the northwestern Talkeetna Mountains consist of deep marine "flysch" deposits of the Kahiltna Assemblage that are underlain by oceanic crust. Our new data show a three-part stratigraphy for this region that suggests that the Kahiltna Assemblage was deposited on strata derived from continental sources. Data also show that marine deposits of the Kahiltna were deformed by the Cenomanian and are overlain by nonmarine strata. Newly-defined stratigraphy include the following:

(1) The lowermost unit consists of Upper Triassic marine strata defined by quartz-rich sandstone, siltstone, and mudstone that grade upsection into siliceous mudstone and chert. Siliceous deposits "interfinger" along-strike with pillow basalt and limestone. A middle to late Norian age is assigned to the upper portion of this section based on occurrences of the bivalve Monotis subcircularis and the hydrozoan Heterastridium. (2) The middle unit consists of the Upper Jurassic-Lower Cretaceous (Kimmeridgian-Valanginian) Kahiltna Assemblage that is defined by a ~3-km-thick marine succession of siltstone, mudstone, and sandstone. Overall these strata coarsen upward and contain volcanic detritus. The Kahiltna overlies Triassic strata above a disconformity marked by a chert-pebble conglomerate. (3) The upper unit consists of Upper Cretaceous (Campanian) nonmarine siliciclastic deposits made up of quartz-rich sandstone, siltstone, and conglomerate. Palynomorphs indicate a Cenomanian and younger age for this unit. The upper contact between the Kahiltna and overlying Cenomanian strata is characterized by an angular unconformity.

Our new data suggest that during the late Triassic, quartz-rich sediment may have been derived from continental sources and deposited in what is now the northwestern Talkeetna Mountains. This depositional phase was followed by rifting and emplacement of pillow basalts coeval with deposition of siliceous mudstone, chert, and limestone. The Kahiltna Assemblage may represent deposition in a back-arc basin that was deformed by the Cenomanian and overlain by Late Cretaceous nonmarine strata.