Discrimination between Subtidal and Intertidal Facies in Pleistocene Deposits, Willapa Bay, Washington

Abstract

ABSTRACT Sea cliffs on the north, west, and south sides of Willapa Bay, Washington, present excellent exposures of Pleistocene estuary deposits. Analysis of these deposits and comparison of them with modern facies in Willapa Bay indicate that a primary key to their interpretation is the delineation of stillstand units--that is, shallowing-upwards bodies of sediment deposited at a particular stand of the sea. Once identified, these units provide a basis for studying both vertical sequences and lateral trends as well as identifying the effects of tectonic deformation. To recognize a stillstand unit, one must infer the relative water depth represented by successive facies or deposits. This inference in turn requires a distinction between intertidal and subtidal facies. In the analysis of the Willapa Bay deposits, a set of criteria for this distinction, derived largely from characteristics of modern facies in the bay, proved useful. Criteria for identifying deposits as subtidal include the presence of 1) abundant Ostrea lurida in growth position; 2) units of inclined strata more than 2 m thick; 3) laterally persistent lag deposits; 4) laterally persistent thin layers of mud; 5) medium- to large-scale crossbedding; 6) directionally uniform crossbedding and ripple lamination; and 7) assemblages of predominantly concave-up shells and shell fragments. Of these criteria the first is diagnostic of subtidal facies; the others are characteristic but not necessarily diagnostic of this environment. In association, however, the characteristic features provide generally adequate evidence for the indicated facies. Criteria for recognizing intertidal deposits include the presence of 1) root or rhyzome structures; 2) evidence of runoff channels; 3) regular lamination characteristic of supratidal deposits; 4) supratidal bluff breccia; and 5) vertical sequences in which upper accretionary bank deposits underlie those of the tide flat, which in turn underlie supratidal deposits. Some of these (root or rhyzome structures) are probably diagnostic of an intertidal facies; the others are characteristic of such a facies.