Abstract: Bed Forms and Internal Structures in Deep Estuary Channel System, Willapa Bay, Washington

R. L. Phillips, H. E. Clifton, R. J. Anima

A broad channel, 1 to 2 km wide and 10 to 22 m deep, occupies the axis of Willapa Bay, Washington. The channel morphology is complex, with internal ridges rising 7 m from the channel floor. The sand that underlies the channel is shaped into bed forms that differ in morphology, orientation, and internal structure depending on their position within the channel system. Side-scanning sonar surveys, diving observations, and coring delineate the nature of these bed forms and their relation to tidal currents within the channel system.

The side-scanning sonar recorded bed forms oriented normal to the flow of the tidal currents in nearly all parts of the channel. Straight-crested bed forms (sand-waves), up to 1 m high, dominate the channel bottom, whereas dunes as much as 2 m high form on the sides of the channels. The dunes may move as much as 3 m during one tidal cycle. Fields of ebb- and flood-oriented bed forms may coexist simultaneously within the channel. A tidal-current ridge covered by bed forms that reverse with the tide locally separates the two fields. During the maximum tidal exchanges, most of the bed forms within the channel system reverse their orientation. At some places, the bed forms are active only during these spring tides.

Oriented cores, as much as 2 m long, through the channel-bottom sediments show sets of medium-scale cross-strata containing reactivation surfaces interbedded with ripple lamination (commonly climbing). Shell concentrations, wood fragments, and bioturbation are common in the channel bottom. Sediment on the tidal-current ridges contains medium-scale cross-strata and ripple bedding, whereas at similar depths, sediment on the channel sides is mostly in large-scale cross-strata. The cross-strata are bimodally opposed on the tidal ridges and in some channel-bottom areas, but are nearly unidirectional on the sides of the channel and in other channel-bottom areas. Ripple bedding tends to be bimodally opposed throughout the channel system.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA