REVISITING THE ICHNOLOGY AND NEOICHNOLOGY OF WILLAPA BAY, WASHINGTON, USA

Willapa Bay has been an excellent source of ichnological, neoichnological and sedimentological data that have aided in understanding and identifying bays and estuaries in the fossil record. Using observations from modern deposits and well exposed Pleistocene terraces, key depositional elements with characteristic ichnological and sedimentological features were presented in 1999. In the intervening time, increased data and observations have led to a better understanding of the sedimentology and ichnology of Willapa Bay. The environments that this paper revisits include: (1) intertidal flat deposits, which are typically heavily bioturbated by various shafts and burrows (such as Skolithos, Planolites, Thalassinoides and Siphonichnus) and can be reliably differentiated from less burrowed subtidal deposits; (2) unburrowed to bioturbated tidal creek point-bar deposits, which are sporadically burrowed by minute Skolithos and Planolites; and (3) tidal-channel associated point-bar deposits, which are characterized by Inclined Heterolithic Stratification and are unburrowed to sporadically bioturbated with small Skolithos, Siphonichnus and Psilonichnus. The typical facies association observed comprises subtidal point-bar deposits overlain by intertidal flat and intertidal channel deposits. Stacked and recurring facies associations are separated by sharp discontinuities that commonly display robust Thalassinoides in the context of the Glossifungites Ichnofacies denoting the presence of a firmground.

Crucially, the proximity of the subtidal bar deposits to the estuary head can be easily interpreted from the ichnological data. In the modern, burrow size and diversity decrease landwards before reaching a “no bioturbation” limit in the fresh-water zone of the estuary. The same is interpretable from the Pleistocene terraces wherein the ichnological content decreases as evidence for fluvial influence (i.e. mud content, plant roots, plant detritus) increase. Importantly other features, such as Teredolites-bored wood clasts, are only seen in those deposits that are otherwise bioturbated; this suggests that Teredolites are at least very rare in the fresh-water reaches.