NEW STRATIGRAPHIC REVELATIONS IN THE SUBSURFACE SUSITNA BASIN, SOUTH-CENTRAL ALASKA, FROM RECENT ISOTOPIC AND BIOSTRATIGRAPHIC RESULTS

New isotopic and palynological data from wells in the Susitna basin of south-central Alaska indicate the presence of Paleogene nonmarine sedimentary and volcanic strata that provide a record of the tectonic history of the area. The Susitna basin is located north of the Castle Mountain fault and the petroliferous Cook Inlet basin. Seven exploratory wells drilled in the Susitna basin during 1964-2005 found no commercial amounts of oil or gas.

The deepest wells in the Susitna basin are the Trail Ridge Unit 1 (latitude 61.843˚, longitude -151.084˚) and Pure Kahiltna Unit 1 (62.041˚, -150.756˚), which reached measured total depths of 4,178 m and 2,214 m, respectively, and are 27 km apart. Both wells bottomed in a package of interstratified sedimentary and volcanic rocks of late Paleocene to early Eocene age. The ages are based on late Paleocene palynomorphs and ⁴⁰Ar/³⁹Ar step-heating ages on andesite and basalt of 57.3 ± 0.2 Ma, 56.9 ± 0.4 Ma, and 54.3 ± 0.4 Ma. This package is about the same age as the Arkose Ridge Formation in the Talkeetna Mountains and volcanic rocks on the eastern flank of the Tordrillo Mountains.

The volcanic-bearing package is overlain by a nonmarine sequence of sandstone, siltstone, and coal that has an apparent thickness of about 1,400 m and contains early to middle Eocene palynomorphs. This sequence, in turn, is unconformably overlain by a nonmarine interval of primarily conglomerate and sandstone with apparent thicknesses of about 2,500 m in the Trail Ridge well and 150 m in the Pure Kahiltna well; this interval contains early to middle Miocene palynomorphs in its lower part and Quaternary palynomorphs in the upper part.

We infer that late Paleocene and Eocene strata in the Susitna basin record volcanism, subsidence, and sedimentation that accompanied eastward passage of a slab window related to subduction of the hypothesized Resurrection-Kula spreading ridge. The Miocene-on-Eocene unconformity is not precisely dated but may represent uplift and erosion that accompanied the initiation of Yakutat microplate subduction beneath south-central Alaska. The mechanisms of subsidence that accommodated the thick Miocene to Quaternary deposits are unclear but may have included sediment loading, faulting, and lithospheric flexure associated with subduction of the Yakutat microplate.