Abstract: Transgressive Barriers of Beaufort Sea Coast

Dag Nummedal, Jeffrey S. Knoth, Ian A. Fischer, Robert K. Fahnestock

The Beaufort Sea coast of Alaska is characterized by a thermally eroding mainland shore, partly protected by transgressive, gravel barrier-island chains. The barrier islands are best developed along northeast-facing coastal segments where longshore transport is most significant because of the dominant easterly waves.

Net-sediment transport toward the northwest is demonstrated by the recent development of large recurved spit systems at the last downdrift member of each island chain, the increased spacing of the islands toward the northwest, the presence of migrating giant cusps, a generally fining sediment trend, and the northwestward dispersal of natural tracers (diagnostic lithologies) from barrier and mainland point sources.

A rapid landward migration of most of the islands is demonstrated by extensive ice-push features and washover terraces commonly extending the entire width of the barrier, and the absence of Holocene dunes and vegetation. Air photo mapping of selected barriers demonstrates a landward and westward island migration for the last few decades.

The unusual crescentic shape of the Beaufort barriers is a direct consequence of sediment-transport processes in this virtually tideless environment. Gravel washed across the island by storm waves or moving into the lagoon via the recurved spits at either end cannot be reintroduced into the littoral circulation system because of the absence of competent tidal currents. The islands can only move landward and downdrift and will assume, with time, an increasingly crescentic shape.

This pattern of barrier migration and development is in sharp contrast with that of common sandy mesotidal barriers. For example, in the southeastern United States, barriers are relatively stable, have developed multiple beach ridges, and characteristically assume a drumstick shape with the wide updrift end formed in response to a balance between the landward and downdrift movement of sediment owing to wave action and the seaward transport by ebb-tidal currents and river effluents.

Crescentic barriers indicate rapid transgression and the absence of a currently active pathway for sediment supply to the beach. They appear to represent an "end-member" in terms of barrier-island development, forming in extremely microtidal wave-dominated environments. Invariably, they are separated from the mainland by wide, open lagoons.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma