The Shumagin continental margin lies between Kodiak Island and Unimak Pass. The oldest rocks known to underlie the margin are highly deformed, deep-water turbidites of Late Cretaceous age (Shumagin Formation). These turbidites were intruded by Paleocene and early Eocene granodiorites. Paleogene sedimentary rocks of the Kodiak region may extend southwest to and underlie at least parts of the Shumagin margin but are not known in outcrop on shelf islands. The Cretaceous and Paleogene(?) rocks form acoustic basement on multichannel seismic reflection data, and are overlain by a basin fill of probable late Miocene and younger age. Mesozoic rocks of the Alaska Peninsula extend seaward only to the inferred location of the Border Ranges fault.

The Shumagin margin is characterized by five major structural features or trends: (1) Shumagin basin, containing about 2.5 km of late Miocene and younger strata above acoustic basement; (2) Sanak basin, containing as much as 8 km of dominantly late Cenozoic strata in two subbasins separated by a basement high; (3) Cenozoic shelf-edge and upper-slope sedimentary wedges that are 3-4 km thick and possibly as thick as 6 km; (4) a midslope structural trend, Unimak ridge, that is characterized by numerous surface and subsurface structural highs; and (5) a 30-km wide accretionary complex at the base of the slope. A thin (less than 1-2 km) sediment cover of Miocene and younger age covers the continental shelf areas outside of Shumagin and Sanak basins.

The tectonic history of the margin includes: (1) Late Cretaceous or early Tertiary removal of the seaward part of the Cretaceous Alaska Peninsula margin along the Border Ranges fault and accretion of the Shumagin Formation against the truncated margin; (2) Miocene uplift and erosion of the shelf; (3) middle or late Miocene uplift of Unimak ridge; and (4) late Miocene and younger subsidence and infilling of Sanak and Shumagin basins, and subduction-accretion along the Aleutian Trench.

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