Graded storm sand layers offshore from the Yukon Delta, Alaska

Abstract

The northern Bering Sea has a history of severe storm surges. The most recent, and perhaps the worst in historical times, occurred in November, 1974 (Fathauer, 1975). Evidence of storm surge events is exhibited in sea-floor stratigraphy as well as shoreline flooding and indicates that significant widespread changes in sea-floor sedimentation take place (Nelson and Creager, 1977). These changes have implications for installations on the sea floor and for mass transport of pollutants. This paper describes the interbedded sand layers found in southern Norton Sound off the modern Yukon Delta that are deposited by the storm surge events. Such deposits are evident in both modern and ancient deposits of epicontinental shelves (Hays, 1967; Howard and Reineck, in press; Anderton, 1976.). These graded sand layers in very shallow water mimic many of the features of thin-bedded turbidite sands, although the shallow water deposits are thought to have a very different mechanism of deposition related to storm surge processes. Two factors in the oceanographic setting of northern Bering Sea magnify the effects of storm surge. The sea floor is very shallow (less than 20 m deep over wide areas) particularly in Norton Sound. Consequently there is intensive wave reworking which causes extensive sea-floor erosion, mass movement, displacement, and offshore progradation of significant amounts of sediment during storm surges; The second factor is a system of strong dynamic_ bottom currents that can move large amounts of sediment northward to Chukchi Sea during normal weather. Much more sediment is moved when the current is reinforced by relaxing of the sea surface set-up caused by storm surge (Fig. 1) (Flemming and Heggarty, 1966; Coachman et al., 1976; Cacchione and Drake, 1979; Schumacher and Tripp, 1979). Another important influence on the sedimentation in southern Norton Sound is the effect of seasonal processes on the Yukon River delta, During the winter months from November to May the Yukon River averages 40,000 cfs when the ice-covered river is fed mainly by base flow (Dupre, 1976). Within less than a week of river breakup, peak discharges may reach 1,000,000 cfs or more and then decline throughout the summer.