SEDIMENTOLOGICAL, ICHNOLOGICAL, AND SHALLOW SEISMIC CHARACTERISTICS OF INCLINED HETEROLITHICALLY STRATIFIED (IHS) TIDAL POINT BAR DEPOSITS, WILLAPA BAY, WASHINGTON

Pleistocene-age outcrops of the Willapa Bay region have been previously interpreted to represent tidally influenced bay/estuary deposits. The Pleistocene strata comprise sand- and mud-dominated inclined heterolithically stratified(IHS) beds that are interpreted to represent avulsing tidal channels. Within these deposits, sedimentological and ichnological characteristics are appreciably diverse, with sand-dominated IHS displaying larger, more robust traces and abundant thin mud flasers. On the contrary, mud-dominated IHS display a low-diversity suite of simple trace fossils and wavy through lenticular bedded sedimentary facies.

Distinguishing landward versus bayward IHS facies is extremely complicated when dealing with shallow seismic data, as ichnological and sedimentological characteristics are unavailable. However, significant differences in geobody shape and morphology exist between landward and bayward point bars, allowing for identification using seismic. For instance, bayward point bars tend to be thicker, extend further laterally, and dip at a shallower angles (3 to 5 degrees), whereas landward point bars tend to be shorter, display more noticable vertical accretion packages, and dip steeper (5 to 7 degrees).

This presentation attempts to, (i) identify characteristic ichnological and sedimentological signatures of IHS deposits in landward and bayward depositional locations by using examples from older Pleistocene outcrops, and x-rays from the modern Palix, Niawakium, and Willapa Rivers; and, (ii) compare IHS geobody shapes and morphologies of Pleistocene outcrops with those observed in seismic data obtained from the modern Palix River.