2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 15
Presentation Time: 8:00 AM-12:00 PM


HAMPTON, Brian A., Geology Department, University of California, Davis, One Shields Avenue, Davis, CA 95616, RIDGWAY, Kenneth D., Dept. Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907 and GEHRELS, George E., Department of Geosciences, University of Arizona, Tucson, AZ 85721, bhampton@purdue.edu

Collision of a major island arc complex, the Wrangellia composite terrane, with the Mesozoic continental margin of North America marks one of the largest additions of juvenile crust to the Cordillera. New U-Pb detrital zircon ages from Upper Triassic-Upper Cretaceous strata exposed in the northwestern Talkeetna Mountains and Alaska Range define four stages in the development of this suture zone.

Stage 1 is marked by deposition of Upper Paleozoic-Lower Triassic strata. U-Pb detrital zircon ages from the redbeds of the Chulitna terrane show a maximum depositional age of Norian near the top of this unit with prominent age clusters between 300-500 Ma, 1100-1300 Ma, and 1700-2000 Ma (Mz-Pz-Pc - 5%/65%/30%). The oldest grains fall between 2500-3000 Ma. We interpret Stage 1 deposition to represent pre-collision continental margin sedimentation and volcanism adjacent to a continental source during the Upper Paleozoic-Late Triassic time.

Stage 2 is defined by deposition of the lower part of the Upper Jurassic-Lower Cretaceous Kahiltna assemblage. U-Pb detrital zircon ages indicate a maximum depositional ages of Valanginian and show detrital age clusters between 145-165 Ma and 179-199 Ma. The oldest grains in these samples are Mesozoic (212 + 8 Ma). The lower part of the Kahiltna is interpreted as representing deposition associated with exhumation of the Wrangellia island arc (lower Talkeetna arc) during the initial stages of Late Jurassic-Early Cretaceous collision.

Stage 3 is characterized by deposition of the upper part of the Kahiltna assemblage. Maximum depositional ages for these strata are Aptian-Albian. Prominent age clusters occur between 100-140 Ma, 160-240 Ma, and 300-400 Ma with the oldest grains being >2500 Ma (Mz-Pz-Pc - 70%-13%-17%). The upper part of the Kahiltna assemblage is interpreted as syncollisional strata that record erosion of both island arc and contintenal margin sources during Cretaceous time.

Stage 4 is represented by a nonmarine succession of mainly fluvial cross-stratified sandstone, interbedded with fossil leaf-bearing siltstone that overlie the Kahiltna assemblage (locally defined by an angular unconformity). Palynomorphs from this unit indicate a latest Cretaceous to younger age for these strata. Stage 4 is interpreted to represent the latest stages of collision.