Stratigraphic, Structural, and Basin Modeling of Kuskokwim Group, Southwestern Alaska

I (Jay Kalbas) recently returned from a 72 day field season in central and southwestern Alaska. My objectives were to gain a detailed understanding of the stratigraphic and structural characteristics of the Kuskokwim and Kahiltna basins and better understand their relationships to adjacent Paleozoic and Mesozoic basement terranes. I measured detailed stratigraphic sections along multiple traverses through the Kuskokwim Group and Kahiltna assemblage and produced high resolution geologic maps and cross sections. I worked mostly in the Sleetmute quadrangle (Kuskokwim Group crops out in spectacular exposure along the Kuskokwim River) and in the eastern half of the McGrath and western half of the Talkeetna Quadrangles (Kahiltna assemblage is well exposed in the Alaska Range). Ken Ridgway and I measured 16 stratigraphic sections (8.9 km of high resolution section) and worked on the relationship between the Kahiltna assemblage and Farewell Terrane basement rocks.

Southwestern Alaska accommodates the juxtaposition in time and space of contractional and strike-slip deformation. Here, the Aleutian megathrust, an active oblique-subduction plate boundary, sits in close proximity to the western segments of the Denali strike-slip fault zone, one of the largest active strike-slip faults on Earth. Current uplift and exhumation from regional deformation produce a significant westward longitudinal sediment flux from the Alaska range to Bristol Bay and the Yukon River-Kuskokwim River delta system (areas of active petroleum exploration). Our basin analysis indicates the current depositional and tectonic setting is comparable to Cretaceous and early Tertiary southwestern Alaska during suturing of the Wrangell composite terrane to southern Alaska. Progradational submarine fan and fluvial-deltaic sequences that record that collision are exposed in the Kuskokwim mountains and in the western Alaska Range. Despite this detailed stratigraphic record, numerical simulations of the Cretaceous tectonic setting have not been achieved because reliable constraints for models are difficult to establish. To solve this problem, I am developing finite element experiments that simulate the neotectonic setting in southwestern Alaska. I am primarily utilizing a thin-shell finite element program, SHELLS, developed by P. Bird at UCLA. This code maintains isostatic equilibrium in tectonic simulations while accounting for dynamic heat flow and differential loading of the lithosphere. In this way, and because traction on all boundary surfaces can be employed, three-dimensional numerical experiments are possible. Model results can be compared to current velocity distributions (from regional GPS studies) and will yield insight into stress state and strain accumulation in this complex, oblique contractional setting. I am using results from models of neotectonic southwestern Alaska to calibrate paleotectonic models of the analogous Cretaceous system with reasonable strain rates, slip distributions, and sedimentary processes. Cretaceous models can be validated by scoring experimental results against know (from this study) basin geometry, subsidence rate, source area location (i.e., regions uplifted and eroded). The goal is to eventually produce numerical model results that have a reduced degree of non-uniqueness, and therefore provide insight into physics of the ancient system.

Synchronously, I am working to define the regional stratigraphy and structural configuration of the Kuskokwim Group and Kahiltna assemblage in southwestern Alaska. Several mechanisms for basin formation have been proposed: 1) transtensional pull-apart subsidence; 2) foreland basin subsidence from flexural loading of the crust; and 3) passive margin extension. My stratigraphic analysis of these basins suggests a long-lived axial drainage system transported clastic detritus westward from the suture zone in central Alaska and deposited them into proximal deltas and distal, deep submarine fans. Rapid subsidence and clastic sedimentation continued into the early Tertiary as the basins themselves were deformed and uplifted during the terminal stages of suturing.