SEDIMENTARY RECORD OF THE MESOZOIC AND CENOZOIC TECTONIC DEVELOPMENT OF SOUTHERN ALASKA: EXPLORING SOME OF DWIGHT BRADLEY’S GEOLOGIC “BLACK HOLES”

The Mesozoic and Cenozoic tectonic development of southern Alaska is defined by fundamental changes in sedimentary basin types, structural styles, and magmatism and is marked by at least four major tectonic stages. Stage I includes mainly submarine fan deposition in Late Jurassic to Early Cretaceous retroarc foreland and remnant ocean basins located between the colliding Wrangellia composite terrane (WCT) and Mesozoic continental margin rocks. The final stages of this collisional event marked a change from an oceanic arc system to a Late Cretaceous continental arc system that marks the beginning of Stage II. Establishment of this continental arc system resulted in the development of a marine forearc and a nonmarine to marginal marine retroarc foreland basin system. New radiometric ages from bentonite layers and maximum depositional ages based on U-Pb detrital zircon ages indicate nonmarine and marginal marine deposition from ~ 75 – 65 Ma. Stage III is characterized by a change from Cretaceous deposition in the foreland basin to localized Paleocene-middle Eocene deposition in nonmarine strike-slip basins, intraplate magmatism, and exhumation. During this stage, volcanic and plutonic rocks of the central Alaska Range and Talkeetna Mountains were erupted and intruded along the Mesozoic suture zone between the WCT and the former continental margin rocks. Magmatism was coincident with and in some cases related to Paleocene-Eocene shallow subduction of a spreading ridge. Ridge subduction in the forearc region resulted in surface uplift, unconformity development, and a change from marine to nonmarine deposition. Stage IV is defined by mid-Eocene oblique subduction (arc-like plutons in central Alaska Range) and the Oligocene to Recent collision of the Yakutat terrane with the southern margin of Alaska. This collision has prompted growth of the largest coastal mountain range on Earth, exhumation of older forearc basinal strata, renewed exhumation of the Alaska Range, construction of the Wrangell volcanic belt and displacement on the Denali fault.