Spieden and Sentinel Islands, San Juan Islands, Washington, are underlain by the only known occurrence of the Spieden Group, composed of the Upper Jurassic (Oxfordian or Kimmeridgian) Spieden Bluff Formation and the Lower Cretaceous (Valanginian and Hauterivian and possibly younger) Sentinel Island Formation, separated by a disconformity.The 100 m thick Spieden Bluff Formation is subdivided into two members: (1) an 80 m thick lower member, consisting of 5 m of sandstone, siltstone, and tuff overlain by 75 m of volcanic breccia–conglomerate largely of debris flow (laharic?) origin; and (2) a 20 m thick upper member consisting of fossiliferous sandstone and siltstone deposited on a shallow marine slope. Sedimentologic, petrologic, and geochronologic data suggest that sediments of the Spieden Bluff Formation accumulated near an active volcanic source to the north contributing primarily andesite, dacite, and basaltic andesite.The 740 m thick Sentinel Island Formation is also subdivided into two members: (1) a 140 m thick lower member consisting of fossiliferous sandstone and siltstone deposited in a shallow marine environment; and (2) an unconformably overlying 600 m thick upper member consisting of volcanic conglomerate deposited as an alluvial fan. The source terrane for the Sentinel Island Formation was also primarily Upper Jurassic volcanic rocks and lay to the northeast.Rocks of equivalent age occur in the southern part of the San Juan Islands and in neighboring geologic provinces, but most of these correlative rocks differ from the Spieden in sedimentology, structural style, and metamorphism. Juxtaposition of the Spieden Group and these correlative rocks might have been accomplished by shortening and fragmentation of a regional convergent margin, by large-scale transport of allochthonous blocks, or by some combination of the two mechanisms.

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