Petrography of the Nanushuk Group and Torok Formation

Abstract

The Nanushuk Group, of Early and Late Cretaceous/yage, is a transitional unit containing deltaic, interdeltaic, and shallow marine sandstone and shale sequences that crop out in the western and central foothills belt and is widespread in the subsurface of the Arctic North Slope. The Lower Cretaceous Torok Formation, conformably underlying and laterally equivalent to the Nanushuk Group, is predominantly a shale sequence of deeper water origin than the Nanushuk Group. Sandstones collected from scattered outcrops, from eight measured sections, and from twenty wells drilled by the Navy and the U.S. Geological Survey in the Nanushuk Group and Torok Formation were examined for compositional differences. Results of the modal analyses show the suite of sandstones to be be lithic and quartz-rich when plotted on a Q-F-L ternary diagram. Two depositional lobes (western and central) can be distinguished by variations in the lithic component of the sandstone modal analyses as well as paleontologic data.