The significance of Pleistocene Psilonichnus at Willapa Bay, Washington

Two types of ichnofossils from Pleistocene outcrop at Willapa Bay are descr ibed. Because both trace fossils are characterized by an inclined to horizo ntal tunnel, are unlined, have an exaggerated J-shaped morphology: rarely b ranch, and have an unconstricted apertural opening, they have been classifi ed as Psilonichnus upsilon Type A and B. Psilonichnus upsilon Type A is generally 1 to 3 cm in diameter and is infil led with. laminated sediment In general, P. upsilon Type A is observed in a ncient point-bar deposits. It has an extremely simple architecture that is almost identical to that produced by the crab Hemigrapsus oregonensis in mo dern tidal flats at Willapa: Bay. Psilonichnus upsilon Type B normally exce eds 10 cm diameter and is infilled with laminated sediment, The passive inf ill commonly is deposited in couplets and may be delivered to the burrow ne twork by tide-generated currents. Psilonichnus upsilon Type B is observed i n intertidal flat deposits. The overall morphology of this trace fossil is most similar to burrows generated by large crustaceans such as crabs, stoma topods, and lobsters. The occurrence of these traces leads to four findings: (1) Psilonichnus ups ilon has a more variable architecture than discussed in the literature. The size and angle of the tunnel are variable, and Psilonichnus may aggrade, f orming Teichichnus-like structures. (2) In the modern bay, burrowing shrimp dominate subtidal point-bar, and intertidal deposits. The Pleistocene reco rd indicates that burrowing crabs sometimes occupied similar niches in the ancient bay. (3) Laminated, heterolithic burrow fills provide evidence of r hythmic sedimentation. These laminae represent tidal or episodic sedimentat ion and provide the only evidence of such. processes in otherwise muddy dep osits. (4) A large burrowing crab that might make P. upsilon Type B may not be present in the modern bay. However, such a trace maker was present when these Pleistocene deposits accumulated.