Four diapiric folds and two folds that are probably diapiric have been located by seismic-reflection profiling beneath the edges of Umnak Plateau, a broad platform at a depth of about 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 thought to be of Neogene age. These strata overlie an "acoustic basement" that probably consists of lithified rocks of Paleogene age or older.

The piercement cores appear to rise from, or through, the acoustic basement, or possibly from the basal beds of the strata that form the plateau. The 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, but the plateau is gently domed in places. Intrusion of the diapiric cores probably formed these folds in late Tertiary time; however,

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the very slight folding of the upper 200-400 m of strata may simply be due to differential compaction that followed the intrusion.

The cores of the folds do not noticeably affect the magnetic field: hence, we doubt that they are igneous bodies. Nonetheless, an igneous origin cannot be discounted; the cores are close to late Tertiary and Quaternary volcanic and intrusive structures along the adjacent Aleutian Ridge, and near the historically active volcano, Bogoslov, which forms an island nearby. Shale or even salt cores may be present beneath the folds; however, salt diapirs probably would have to be derived from a deeply buried parent body of Paleozoic or early Mesozoic age. All the diapiric and diapirlike folds that have been found are along major structural trends; these trends delineate the principal physiographic elements of the southeastern region of the Bering Sea.

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