Abstract The Canning River delta on the Beaufort Sea coast of Alaska has been modeled in three dimensions as a typical example of a polar fan-delta complex. Polar deltas result from severely inhibited sedimentary processes. Free water is abundant only during the short summer when the carrying capacity of the stream is greatly exceeded so that most deposition occurs subaerially in braided-stream channels. Consequently, polar fan-deltas prograde seaward only minimally. Three-dimensional geologic block modeling with the computer was highly effective in assimilating the large volumes of data available from shallow borings and has provided a new and unexpected insight into the structure and origin of this complex sedimentological body. The model shows that at least five depositional sequences are present beneath the delta surface. Each sequence began with the subaerial deposition of a massive, poorly sorted gravel unit in braided-stream channels. The massive gravels are generally followed by thick units of dark-colored, poorly sorted silt or silty sand. These reflect a fresh or brackish water swamp environment in the lower section and suggest shallow-water nearshore marine deposition in the upper sections. The shallow-water marine sediments show signs of ice scouring and some sequences are not complete because erosion has occurred during extreme sea-level lows. Thus some apparently massive gravel units are complexes representing more than one sea level cycle. Barrier islands are common along the Beaufort Sea coast and their origin has been frequently discussed in the literature. The Canning River delta model shows the islands to be ephemeral structures built around cores of earlier gravels or marine sediments left behind from the Woronzofian transgression. These islands and their associated lagoons may not be typical of polar fan-deltas but may be a fortuitous occurrence resulting from the fact that the present sea level is slightly lower than during the Woronzofian sea-level maximum. The main effect of the lack of a lagoon would be reduction of the width of the zone of shore-fast sea ice, which would result in more intense scouring of the shallow-water marine sediments by sea ice.

ABSTRACT Large river systems deliver significant quantities of fine-grained sediment to continental shelf regions. In specific areas off deltas, deposition rates are rapid and the sediment may be involved in a variety of mass-movement processes on the subaqueous slopes (slumps and slides, debris flows, and mudflows), causing rapid sediment accumulation at shelfbreak depths and resulting in active progradation of the shelfedge. Seismically, the deposits appear as large-scale foresets and are commonly composed of in situ deep-water deposits alternating with shallow-water sediments transported by mass movement. On electric logs, sands within these units are sporadic and display sharp basal planes and blocky shapes. Progradation of the shelfedge deposits is generally accompanied by oversteepening and large-scale instability of the upper shelfbreak slopes. Deep-seated and shallow rotational slides move large volumes of sediments and deposit them on the adjacent slopes and upper rise. Extensive contemporaneous faults commonly form at the shelfedge. Continuous addition of sediment to the fault scarps, particularly by mass movement from nearby delta-front instability, causes large volumes of shallow-water sediment to accumulate on the downthrown sides of the faults, mostly forming large-scale rollover structures. Continued movement along the concave-upward shear planes commonly results in compressional folds and diapiric structures. Contemporaneous accumulation of shallow-water mass-movement deposits may occur in association with these structures. Massive retrogressive, arcuate-shaped landslide scars and canyons or trenches can also form at the shelfedge owing to slumping and other mass-movement processes. Such canyons and trenches can attain widths of 10–20 kilometers, depths of 800 meters, and lengths of 80–100 kilometers. The Mississippi Canyon probably originated in this manner. The creation of such features by shelfedge instability results in the yielding of exceptionally large volumes of shallow-water sediment to the deep basins in the form of massive submarine fans. The infilling of depressions by deltaic progradation is rapid, forming large foresets near the canyon heads. The low strength of the rapidly infilled, under-consolidated sediments causes downslope creep or reactivation of failure mechanisms, resulting in multiple episodes of filling and evacuation.