Six folds that we suspect have diapiric cores were located by seismic reflection profiling beneath the edges of Umnak Plateau, a broad platform at a depth near 1,860 m in the southeastern corner of the Bering Sea. The plateau is underlain by 2,000–3,000 m of generally flat-lying strata believed to be largely of Neogene age. These overlie an “acoustic basement” that probably consists of more lithified rocks of Paleogene or possibly Cretaceous age. All the diapiric (or diapirlike) folds found are along major structural trends that delineate the principal physiographic elements of the southeastern region of the Bering Sea.

The piercement cores rise as much as 1,500 m above the acoustic basement and to within 700 m of the sea floor. Folding above the cores decreases upward, although the surface of the plateau is gently domed above some of them. Intrusion of the diapiric cores probably formed the folds in late Tertiary time; however, the very slight folding in the upper 200–400 m of the plateau may simply be the result of differential compaction and/or draping that followed intrusion.

The cores of the folds do not produce a clearly recognizable magnetic anomaly. This may mean that the intrusive plugs are not of igneous origin, although such an origin is suggested strongly by their proximity to volcanic and intrusive rocks of late Cenozoic age on the Aleutian Ridge and at Bogoslof Island. The apparent nonmagnetic character of the diapirs and the probable presence of a thick sequence of shale beds at the base of the stratified sequence forming the plateau also strongly imply that the intrusive plugs consist of shale. The plugs could be salt diapirs; parent salt beds probably would be of early Mesozoic or Paleozoic age.

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