Geology of Norton Basin and Continental Shelf Beneath Northwestern Bering Sea, Alaska

Abstract

The rocks that floor the Norton basin and the northwestern Bering Sea are most likely of Precambrian and Paleozoic age, like those rocks that crop out around the basin. A maximum of 6.5 km of mainly Cenozoic strata lie over basement in the basin. On the basis of the geometry of reflections in seismic data, we believe alluvial fans to be present deep in the basin and to border major basement fault blocks. These fans are the lowest units of the basin fill in many areas and consist of uppermost Cretaceous or lower Paleogene, possibly coal- and volcanic-rich rocks. Mainly clastic nonmarine sedimentary rocks overlie the fan deposits. The Neogene and Quaternary basin rocks apparently were deposited in a marine environment. The basement of the northwestern Bering Sea, west of Norton basin, is 1 km below sea level for the most part, but local structural lows are filled with at most 2 km of strata of probable late Neogene and Quaternary age. The continental shelf in this area is separated structurally from the Seward Peninsula by a zone of small basement antiforms, over which relatively high gravity values have been measured. The Norton basin comprises two structural areas that are separated by a major northwest-striking horst. The first structural area lies west of this horst, where major normal faults strike northwest to form local areas where the basin is as deep as 5 km. The second area, east of the horst, includes major normal faults that strike east and northeast. The deepest part of the Norton basin (6.5 km) lies there. During the Late Jurassic and Early Cretaceous, basement rocks now beneath the Norton basin were affected by the orogeny that formed the Brooks Range. These basement rocks were metamorphosed and thrust, and then eroded deeply nearly 50 m.y. before the basin began to form. In the middle Late Cretaceous, the area of the eastern Seward Peninsula and eastern Norton Sound was subjected to east-west compression and consequent eastward thrusting. The strike-slip Kaltag fault probably formed at this time. If Norton basin formed as a pull-apart basin along this fault, then the basin may be as old as middle Late Cretaceous. However, during the latest Cretaceous and early Paleogene, the compression gave way to regional extension that formed northeast-trending grabens onshore, east of Norton basin We believe that the extensional Norton basin most likely formed in response to this regional extension. Initial deepening of the basin was controlled by major normal faults and occurred rapidly, whereas subsidence was slower and more regional in scale.