Abstract

Quarry walls in Pleistocene marginal-marine coarse-grained deposits adjacent to Willapa Bay, Washington, expose a contact from which unusual sedimentary structures originate. These structures have two distinct occurrences: (1) vertical-to-subvertical columns where laminae and bedding deflect downward, and (2) normally graded beds with symmetric or asymmetric U-shaped structures with flared limbs. The scale, morphology, and distribution of the features suggest these are not physical sedimentary structures. Rather, they are more akin to biogenic sedimentary structures generated by the predatory action of marine animals on deep-burrowing bivalves. Several animals are known to forage sediment: elasmobranch fishes, fish, crabs, sea stars, sea otters, whales, and walruses. In particular, walruses generate distinctive excavations on the sea floor as they root for prey with their snouts and emit a jet of water that liquefies the bottom sediments where a bivalve has burrowed. The trace fossils reported likely represent the first examples of walrus feeding from the geologic record. Documentation in recent years of sea-floor furrows and pits on the Bering Shelf and Chukchi Sea produced by the Pacific Walrus (Odobenus rosmarus Linnaeus) provides modern analogues for the ancient trace fossils described from Willapa Bay. We present three significant implications from this comparison: (1) The method of hydraulic jetting employed by walruses for extraction of their prey leaves a distinctive trace fossil that can be used to identify the presence and activities of foraging walruses. (2) These predation structures are temporally significant in that they provide a minimum time of exposure and corresponding rate of accretion for the ancient estuary inlet. (3) Feeding excavations in paleo-Willapa Bay, Washington, were produced by walrus herds that wandered from the northern Pacific ice front during the Pleistocene after becoming barricaded from their present habitat in the Bering Shelf and Chukchi Sea.

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