GRAVITY AND MAGNETIC INVESTIGATION OF SUSITNA BASIN STRUCTURE, COOK INLET, ALASKA

The Susitna sedimentary basin is located northwest of Anchorage in south-central Alaska and spans an area of about 12,000 km². The basin is bounded on the north and west by the Alaska Range, on the east by the Talkeetna Mountains, and on the southeast by the Castle Mountain fault and Cook Inlet Basin. The Susitna and Cook Inlet basins share similarities in Cenozoic and Mesozoic stratigraphy and geologic history, notwithstanding published estimates of right-lateral displacement along the Castle Mountain fault of 26 km since 34-30 Ma and 110-130 since about 159-144 Ma. Like the Cook Inlet basin, the Susitna basin is of interest for energy resources (oil, gas, and coal) and seismic hazards.

A sparse network of vintage seismic data does not reveal the structural margins of the basin. Regional gravity data show a prominent 40-mgal low, but are too widely spaced to assist with detailed interpretation. Twenty closely spaced gravity measurements, collected in 2011 along a twenty km traverse across the western margin of the basin, indicate a gradual negative gravity gradient from the exposed igneous rocks of Beluga Mountain to the gravity-identified minimum. This broad gradient precludes the possibility that the basin is bounded by steeply dipping faults with significant displacement, as might be inferred from the abrupt topographic margin of the basin. Two end-member models for a shallow-dipping margin are (1) overthrusting of Jurassic to early Tertiary volcanic and plutonic rocks to the northeast over low-density Tertiary sedimentary strata, or (2) a northeast-dipping subsurface continuation of volcanic and plutonic rocks into the basin combined with an unknown lower density unit to the southwest beneath the igneous exposures. Geophysically our preferred solution is (1) the overthrust of igneous rocks over the Tertiary strata. However, one can make a plausible geologic argument for the presence of low-density Kahiltna assemblage rocks in the subsurface as support for model (2). Further data collection and integration of our potential field models with seismic data in the basin will improve our model constraints.