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GEOREF RECORD

The significance of Pleistocene Psilonichnus at Willapa Bay, Washington

Murray K. Gingras, Stephen M. Hubbard, S. George Pemberton and Tom Saunders
The significance of Pleistocene Psilonichnus at Willapa Bay, Washington
Palaios (April 2000) 15 (2): 142-151

Abstract

Two types of ichnofossils from Pleistocene outcrop at Willapa Bay are described. Because both trace fossils are characterized by an inclined to horizontal tunnel, are unlined, have an exaggerated J-shaped morphology, rarely branch, and have an unconstricted apertural opening, they have been classified as Psilonichnus upsilon Type A and B. Psilonichnus upsilon Type A is generally 1 to 3 cm in diameter and is infilled with laminated sediment. In general, P. upsilon Type A is observed in ancient point-bar deposits. It has an extremely simple architecture that is almost identical to that produced by the crab Hemigrapsus oregonensis in modern tidal flats at Willapa Bay. Psilonichnus upsilon Type B normally exceeds 10 cm diameter and is infilled with laminated sediment. The passive infill commonly is deposited in couplets and may be delivered to the burrow network by tide-generated currents. Psilonichnus upsilon Type B is observed in intertidal flat deposits. The overall morphology of this trace fossil is most similar to burrows generated by large crustaceans such as crabs, stomatopods, and lobsters. The occurrence of these traces leads to four findings: (1) Psilonichnus upsilon has a more variable architecture than discussed in the literature. The size and angle of the tunnel are variable, and Psilonichnus may aggrade, forming Teichichnus-like structures. (2) In the modern bay, burrowing shrimp dominate subtidal, point-bar, and intertidal deposits. The Pleistocene record indicates that burrowing crabs sometimes occupied similar niches in the ancient bay. (3) Laminated, heterolithic burrow fills provide evidence of rhythmic sedimentation. These laminae represent tidal or episodic sedimentation and provide the only evidence of such processes in otherwise muddy deposits. (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.


ISSN: 0883-1351
Serial Title: Palaios
Serial Volume: 15
Serial Issue: 2
Title: The significance of Pleistocene Psilonichnus at Willapa Bay, Washington
Affiliation: University of Alberta, Ichnology Research Group, Edmonton, AB, Canada
Pages: 142-151
Published: 200004
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 56
Accession Number: 2000-059136
Categories: Invertebrate paleontologyQuaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
N46°15'00" - N46°45'00", W124°04'60" - W123°19'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200019
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