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Quaternary Stratigraphy of Northern Chukchi Sea, Alaska

R. Lawrence Phillips, Peter W. Barnes, Mitchell W. Colgan, Joyce M. Miley

A widespread ash deposit within Quaternary marine sediments of the northern Chukchi Sea, Alaska, provides a unique time-stratigraphic horizon that defines both regions of sediment erosion and regions of deposition on a shallow, ice-covered epeiric sea. High-resolution seismic profiles show a thin, usually less than 5 m thick, Quaternary sediment cover overlying folded bed rock, which is incised by filled fluvial channels. Vibracores, to 6 m depth, record the stratigraphy and depositional history preserved within the thin blanket deposit. The basal units cored consist of overconsolidated silt, sand, and pebbly mudstone. They represent the underlying bed rock, channel-fill deposits, or Quaternary shallow-marine sequences containing ice-rafted cobbles. The ash deposit, rangi g in thickness from 2 m to over 3 m, covers the basal stratigraphic units. It lies from 1 to 4 m beneath the sea floor and forms a distinctive reflector on 3.5 khz seismic profiles. The ash extends laterally at least 100 km east-west and 200 km north-south. The well-sorted, massive to laminated ash was deposited in a marine environment based on the occurrence of diatoms, foraminifera, and sponge spicules. Overlying the ash is a thin, less than 1 m thick, pebbly mudstone or marine sand. An abrupt textural break separates the uppermost depositional sequence from the strata overlying the ash. The uppermost sequence grades vertically from a gravel-shell lag, granules, or coarse sand to bioturbated mud and represents marine sediments deposited since the Holocene transgression. Recognition of he stratigraphic sequences bounding the ash deposit will aid in correlating and interpreting the depositional events and processes on this shallow sea.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.