SEQUENCE STRATIGRAPHIC AND STRUCTURAL INTERPRETATION OF THE TOROK-NANUSHUK FORMATIONS IN THE NATIONAL PETROLEUM RESERVE IN ALASKA

The Jurassic – Lower Cretaceous Kingak Shale in the National Petroleum Reserve in Alaska (NPRA) comprises a series of southward offlapping depositional sequences that culminate in a relict, lobate-southward shelf margin with more than 1 km of relief. The relict shelf margin subsequently was draped by condensed shale comprising the Hauterivian – Aptian pebble shale unit and gamma-ray zone, and then buried by onlap/downlap by an Aptian – Cenomanian clinothem (Torok and Nanushuk Formations) deposited during eastward-progradation in the Colville foreland basin. Along the southern, orogenic side of the basin the Torok-Nanushuk clinothem onlaps a tectonic wedge comprising mainly sandstone and conglomerate of the Aptian Mt. Kelly Formation. The complex stratal geometry of this Jurassic – Cretaceous, and particularly the presence of mechanically weak condensed shale draped on sequence flooding surfaces, significantly influenced the distribution and geometry of detachments during Tertiary contractional deformation associated with the Brooks Range orogeny.

We present a regional interpretation of 2-D seismic reflection data, augmented by surface and well data in NPRA that refines this understanding of Jurassic – Cretaceous stratigraphy and subsequent Tertiary deformation. In the southern foothills, outcrop of the oldest, proximal facies of the Nanushuk Fm. and underlying tectonic wedge of Lower Cretaceous Mt. Kelly and Fortress Mtn Fms. have been tied to distinctive seismic events in the subsurface. Correlated sequences of the oldest topset Nanushuk Fm. and foreset/bottomset Torok Fm., show that the distal facies of these sequences onlap to the north the relict Kingak ultimate shelf margin. Subsequent Torok-Nanushuk sequences are present throughout the NPRA as the inherited accommodation is filled.

Late Cretaceous and Tertiary contraction exploited this stratigraphic complexity. Detachments in the NPRA southern foothills consist of several condensed organic shales, resulting in pervasively imbricated thrust sheets. In contrast, detachments in the northern foothills are less well-developed, always within Torok slope facies, and result in localized imbrication and fault bend folding. Our observations are supported by structural balancing and restoration.