Modern shallow-water graded sand layers from storm surges, Bering Shelf: a mimic of Bouma sequences and turbidite systems

ABSTRACT: A sequence of graded sand layers, interbedded with mud, extends offshore over 100 km from the Yukon Delta shoreline across the flat, shallow (<20 m depth) epicontinental shelf of the northern Bering Sea, Alaska. Proximal graded sand beds on the delta-front platform near the shoreline are coarser (2-34), thicker (10 to 20 cm), and contain more complete vertical sequences of sedimentary structures than distal beds. The inshore graded vertical sequence of structures from the base to the top of individual sand layers includes plane-parallel lamination (Sb), cross lamination (So), plane-parallel lamination (Sd), and mud (So) analogous to the Bouma T, ~ turbidite sequence. Structures vary between interchannel p atform deposits with complete Sb-S ~ sequences and channel-floor sands that are all trough cross-laminated. Distally, storm-sand layers change to thin (1 to 5 cm) silt beds that contain fiat and tipple-drift lamination (S,,.~), are commonly bioturbated, and are associated with shell and pebble lags from storm-wave reworking. The sequence of graded sands appears to be related to the major storm surges that occur every several years. The major storms increase the average 10-m water depth in southern Norton Sound as much as 5 m and cause fluctuations in pore pressure from wave cyclic loading that may liquefy the upper 2 to 3 m of sediment. Storm-associated bottom currents, possibly dominated by rapidly waning ebb flow, transport the liquefied inshore sand far offshore (> 100 kin). Such shallow-water g aded layers off lobate deltas may be distinguished from similar deep-water turhidites by: 1) the predominance of trough cross-lamination, perhaps resulting from wave oscillation effects, m the proximal part of the system, and 2) gradation to common shallow marine fossils, bioturbation, and storm lag layers in distal areas