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 disconformably overlying Lower Cretaceous (Valanginian to Hauterivian and possibly younger) Sentinel Island Formation.

The 100-m thick Spieden Bluff Formation is subdivided into two members: (1) a lower member consisting of 5 m of massive sandstone and tuff overlain by 75 m of volcanic breccia-conglomerate largely of debris flow (laharic?) origin; and (2) an upper 20-m thick fossiliferous sandstone and siltstone member deposited on a shallow-marine slope. Sediments of the Spieden Bluff Formation were derived from an active volcanic source to the north consisting of 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 of shallow-marine origin; and (2) an unconformably overlying 600-m thick upper member consisting of volcanic conglomerate deposited by debris-flow and stream-flow processes on an alluvial fan. The source terrane for the Sentinel Island Formation lay to the northeast and was also primarily composed of Upper Jurassic volcanic rocks.

Spieden Group strata are broadly folded and unmetamorphosed. They contrast sharply with highly deformed, metamorphosed, flysch-like rocks of the same age to the south in the San Juan Islands and in neighboring geologic provinces. The Spieden Group and these latter age equivalent units might have been juxtaposed by fragmentation of a once coherent regional convergent margin, by large-scale tectonic transport of allochthonous blocks, or by some combination of the two mechanisms.

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