Large incised channels on the Chukchi Shelf provide new constraints on onshore drainage: Implications for tectonic and climatic evolution of NW Alaska

High resolution Chirp seismic data acquired onboard the USCGC Healy in September 2002 imaged large, incised channels across the Chukchi Shelf, offshore NW Alaska. The channels range from several meters to tens of kilometers in width and trend roughly NNW across the margin. Subbottom profiles across many of the channels display evidence for a marine transgression, with stratal geometries that suggest fluvial infill as well as marine. Piston cores taken within the channels show relatively coarse, well-sorted sand overlain by open marine muds. The level of discharge required to excavate the observed offshore channels is not consistent with the discharge observed from modern streams and rivers on the NW Alaskan margin. This appears to indicate that the offshore channels are misfit with the present day drainage. There are several ways to reconcile the very low, modern discharge and the huge offshore drainage networks observed in the Chirp data. One possibility is that there has been much greater climate variability than previously recognized, such that past discharge levels were much higher than the modern. Higher discharge levels require an increased moisture supply, either in the form of increased precipitation rates or released from storage in regional ice cover. Alternatively, several tectonic explanations may be invoked. Previous researchers have indicated that any channels on the Chukchi shelf should be related to modern drainage from the Hope Valley region. This would require the formation of Herald Bank, a bathymetric high that effectively acts as a modern drainage divide between Hope Valley and the outer Chukchi shelf, to post-date the channel incision. However, the stratal geometry observed in the Chirp subbottom profiles indicates that the underlying, Cretaceous, northward dipping strata were tilted prior to channel incision. Our preferred explanation is that tectonic deformation has altered onshore drainage patterns rather than offshore. In this scenario, tectonic deformation and erosional uplift of the Brooks Range in NW Alaska may have resulted in headward stream capture and diverted much of the drainage to the east. The modern drainage shows an axial parallel pattern, flowing east along the trend of the Brooks Range. Examination of the topography and drainage divides indicates that relatively minor uplift in the region could result in a reorganization of the stream networks. This hypothesis does not exclude the possibility of increased climatic variability. Rather, isostatic rebound derived from increased mechanical erosion and/or glacial unloading could also result in the uplift, stream capture, and changes in drainage discharge.