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TitleHitchin' a ride - a tale of a well-traveled Buchia
AuthorHaggart, J W
Source13th British Columbia Paleontological Symposium, Vancouver 2021, virtual symposium abstracts; 2021 p. 19
Alt SeriesNatural Resources Canada, Contribution Series 20210441
PublisherBritish Columbia Paleontological Alliance
PublisherVancouver Paleontological Society
PublisherPacific Museum of Earth
Meeting13th British Columbia Paleontological Symposium; Vancouver, BC; CA; July 9-10, 2021
Mediaon-line; digital
File formatpdf
AreaWashington State; City of Mukilteo; United States of America
Lat/Long WENS-122.3500 -122.1500 48.0000 47.8667
Subjectspaleontology; surficial geology/geomorphology; Science and Technology; Nature and Environment; Lower Cretaceous; Upper Jurassic; fossils; provenance; source rocks; bedrock geology; lithology; sedimentary rocks; sandstones; depositional environment; glacial history; glaciation; ice flow; sediment transport; Buchia; Bivalves; Clams; Buchia keyserlingi; Buchia pacifica; Peninsula Formation; Cordilleran Ice Sheet; Puget Lobe; marine beach sediments; Phanerozoic; Mesozoic; Cretaceous; Jurassic
ProgramGSC Pacific Division
Released2021 07 01
AbstractThe beaches of the City of Mukilteo, Washington State, are composed predominantly of pebbles and cobbles of volcanic rock derived from the North Cascades crystalline core and the more recent Cascade magmatic arc, mixed in with common shell fragments. Locals commonly walk the beach for exercise and to locate pieces of elusive beach glass. However, in May, 2018, an unusual new object was found on the beach - a large cobble of silty fine-grained sandstone containing fossils!
The fossils are preserved as moulds and casts of bivalves (clam shells) assignable to the genus Buchia. The Buchia fossils are densely packed in the cobble with several scores to hundreds of individuals likely present. They are oriented in all positions, and represent a chaotic assemblage of shells that likely accumulated in a high-energy environment on the sea-bottom.
The genus Buchia existed during Late Jurassic to Early Cretaceous time and species of the genus form an extensive lineage of relatively short-lived forms which are useful for biostratigraphic age dating. There appear to be several different Buchia species morphologies present in the cobble, including Buchia keyserlingi and Buchia pacifica, which together suggest a mid-Valanginian age (ca. 135 million years ago) for the cobble.
Buchia fossils are found in outcrop at several places in northern Washington State, including in the Mount Baker region and in the San Juan Islands. As the Buchia species that are present in San Juan Islands are different from those found in the Mukilteo Beach cobble, we can rule out that locality as the original source of the cobble. As well, the exposures of Jurassic-Cretaceous strata in the Mount Baker foothills are limited in extent and the packed accumulations of Buchia that characterize the Mukilteo cobble have so far not been identified there, only isolated, less numerous examples being found. Interestingly, one of the closest localities to Mukilteo where such an assemblage of Buchia fossils is known is in the Peninsula Formation of the Harrison Lake region of British Columbia. The accumulations of Buchia fossils found in the Peninsula Formation are also densely packed, with tens of thousands of individuals forming thick shell beds. It is hypothesized that the Mukilteo Buchia cobble was transported from its point of origin in the Harrison Lake area to Mukilteo via the Puget Lobe of the Cordilleran Ice Sheet.
Summary(Plain Language Summary, not published)
The author describes a cobble containing fossils found on the beach at Mukilteo, Washington State, USA. The fossils are bivalves or Early Cretaceous age, and the closest known locality where the fossils are known in place is at Harrison Lake, British Columbia. The author posits that the cobble was transported to its present location by the Cordilleran Ice Sheet.

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