Shirshov Ridge volcanic belt 

(early Tertiary?) 

(Western Bering Sea, unit Czv)

Consists chiefly of two units: (1) a relatively older oceanic assemblage of amphibolite, gabbro, diabase, 
basalt, and chert. Chert contains Late Cretaceous (Campanian to Maastrichtian) to early Paleogene 
microfauna; and (2) a relatively younger volcanic arc assemblage of altered andesite, volcaniclastic 
sedimentary rocks, and shale of Miocene and younger age. Lithologic data for both units from sparse 
dredge samples. The western slope and the summit of the Shirshov Ridge comprise a series of normal 
scarps that face the Komandorsky Basin. The scarps divide the ridge into several north-striking blocks that 
descend step-by-step toward the basin floor. The ridge is interpreted either as: (1) the rifted remnant of an 
early Tertiary volcanic arc that formed approximately in place (Scholl and others, 1992); (2) a thrust-
thickened stack of oceanic crust that formed in the Neogene (Bogdanov and Neprochnov, 1984); or (3) a 
mass of igneous rock constituting an oceanic plateau that was accreted to the margin of the northeastern 
Kamchatka Peninsula (Koryak Highland) (Ben-Avraham and Cooper, 1981). REFERENCES: Ben-
Avraham and Cooper, 1981; Baranov and others, 1984; Bogdanov and Neprochnov, 1984; Baranov and 
others, 1991; Scholl and others, 1992.