Facies characteristics and patterns in mid-estuary intertidal flat deposits, Willapa Bay, Washington

The purpose of this study is to characterize the deposits of midestuary intertidal flats in Willapa Bay, Washington, based on texture, composition, and sedimentary structures (both physical and biogenic).The average textural characteristics of mid-estuary intertidal flat sediments are best described as poorly sorted, fine-grained sand with strongly-fine skewed leptokurtic distributions.Distribution patterns of textural parameters show consistent trends of sediments fining upslope and up-estuary reflecting a response to decreasing hydraulic energy and increasing distance from the main sand source (the tidal inlet).Compositionally, mid-estuary intertidal flat sediments are mainly (greater than 90%) light minerals, mostly quartz with small amounts of lithic fragments, pumice, vegetation, and biogenic shell fragments.Heavy minerals make up about 4% of the sediment.The most common heavy minerals are clinopyroxene, orthopyroxene, hornblende, epidote, and opaque.Clay minerals comprise less than 5% of the sediment.The principal clay minerals are montmorillonite, illite, and chlorite; two clay mineral suites are present: (1) muddy flat suite with 44% montmorillonite, and (2) sandy flat suite with 88% montmorillonite.Fossils make up less than one percent of the sediment.The most common microfossils are forams separated into two assemblages -one modern shallow-water and the second a relic deep-water assemblage.Macrofossils consist of shells of the modern tidal flat mollusc assemblage.Where sand occurs, bedforms ranging from large sandwaves to small scale ripples exist* Due to low sedimentation rates on the flats, the sediment experiences extensive biogenic reworking except where local conditions preclude faunal activity.Overall, therefore, bioturbate textured sediments are characteristic of mid-estuary intertidal deposits.The most common preservable trace in sandy sediments are the large subvertical, branching burrows of the ghost shrimp Callianassa.Large Upogebia borings are common in exposed, consolidated Pleistocene mud deposits which floor some parts of the intertidal flats.Mid-estuary intertidal flats are similar in texture to outer flats (intertidal flats near the bay inlet); structurally, however, midestuary intertidal flats are similar to inner flats (intertidal flats near the river).Compositionally, mid-estuary intertidal flats are transitional between outer and inner flats reflecting a mixture of components derived from main sand sources at the bay mouth and mud supplied by the rivers. ORGANIC NITROGEN (ppm)