Variation in sand body types of the eastern Bering sea epicontinental shelf

Abstract

The eastern epicontinental shelf of the Bering Sea is characterized by variations in river and glacial sediment supply, wave energy, tidal range (microtidal to mesotidal). and tidal, geostrophic, and storm-induced currents. These factors, combined with the effect of the Holocene rise in sea level, have resulted in the formation of a complex assemblage of generally linear sand bodies of similar morphology and lithology. but different origins. The sand bodies are large features > 10 km long. found from the present shoreline to tens of kilometers offshore, in water depths up to 50 m. They include modern sand bodies formed by present-day processes. relict sand bodies formed during lower stands of sea level, and palimpsest sand bodies formed under past conditions but modified by modern day processes. Linear tidal sand ridges (5-35 by 1-3 km) which form at the present time in the macrotidal, funnel-shaped Kuskokwim Bay, are oriented perpendicular to the shoreline. enclosed by tidal flat and shelf mud, and sometimes sigmoidal in shape. The modern shore parallel shoals (including barrier islands, 5-10 by 0.5-1 km) form in mesotidal environments, are the smallest of the shelf sand bodies, and typically are bounded bij tidal flat mud inshore and shelf mud offshore. Delta front channels (20-30 by 2-4 km) extend seaward from the modern river distributaries and form sand bodies perpendicular to the shoreline; they are enclosed by graded overbank sandbeds and muds and are characterized by large to small-scale trough-cross lamination. Lee side shoals. (25-100 by 5-25 km) form behind obstructions to unidirectional shelf currents, are the longest, possess the finest grain size, and exhibit the most consistent rhythmic flat lamination of any sand bodies encountered on the Bering shelf. Ancient shoreline shoals are remnant shoreline features paralleling strand lines of lower sea levels; they contain cycles of ripple and trough-cross lamination, alternating with high angle foreset beds formed by modern sand waves that cover crests of these sand bodies. Relict sand and gravel bodies modified from moraines are distinguished by their coarse grain size and irregular size and shape.