NEW FINDINGS ON THE STRATIGRAPHY AND PALEONTOLOGY OF THE NORTHWESTERN TALKEETNA MOUNTAINS: IMPLICATIONS FOR REGIONAL STRATIGRAPHIC CORRELATION AND COLLISION OF WRANGELLIA

New stratigraphic data and geologic mapping have recognized a three-part stratigraphy for the northwestern Talkeetna Mountains. 1) Upper Triassic Strata - The lower part of this section consists of quartz-rich sandstone, siltstone and argillite that grades upsection into siliceous mudstone and chert. The siliceous strata are overlain and interfinger with limestone and basalt. The occurrence of the hydrozoan Heterastridium and the bivalve Monotis subcircularis indicate a middle to late Norian age for the upper part of the section. 2) Upper Jurassic-Lower Cretaceous Strata - The Kahiltna assemblage disconformably overlies the Triassic section and consists of an upward-coarsening package of marine argillite, siltstone, and sandstone that is ~ 3 km thick. Documentation of minor interbedded Buchia limestone hash confirms previously reported Late Jurassic-Early Cretaceous ages for these strata. 3) Upper Cretaceous-Miocene Strata? - An angular unconformity defines the contact between the Kahiltna assemblage and overlying nonmarine strata. Strata above the unconformity consists of quartz- and chert-rich sandstone, siltstone, and mudstone. Analysis of plant megafossils and palynomorphs are currently being completed on samples collected this summer to determine the age of these strata.

We interpret the Upper Triassic section as correlative with the lower member of the McCarthy Formation and representing a distal part of Wrangellia. The Kahiltna assemblage is interpreted as backarc basin strata that formed during initial collision of Wrangellia. The as-yet-undated nonmarine strata overlying the Kahiltna assemblage may be related to post-collisional suturing and regional shortening of Wrangellia.