Siletzia terrane 

(accretionary wedge - predominantly oceanic rocks) 

(Western part of U.S.A. Pacific Northwest)

Consists chiefly of basalt and greenstone of the early and middle Eocene Crescent and Metchosin 
Formations and the Siletz River Volcanics. The Siletzia terrane includes parts of the Crescent and Siletz 
terranes of Silberling and others (1987). The greenstone consists of metamorphosed tholeiitic pillow basalt, 
massive basalt, diabase, gabbro, tuff, and breccia. Intercalated with the greenstone are minor clastic rocks, 
ferruginous coccolithic limestone, and argillite. Total structural thickness of the terrane is over 16,000 km 
(Babcock and Engebretson, 1991). On the Olympic Peninsula, the greenstone and basalt sequence appears 
to be stratigraphically underlain by continent-derived graywacke, argillite and minor conglomerate (Tabor 
and Cady, 1978).

The origin of these rocks is controversial (Snavely and others, 1968; Cady, 1975; Glassley, 1974; Lyttle 
and Clarke, 1975; Mueller, 1980; and Wells and others, 1984). Many workers now interpret the thick 
sequences of basaltic rocks to have been rapidly erupted in a continental-margin rift basin between about 
56 and 45 Ma (Wells and others, 1984; Babcock and Engebretson, 1991).

Deep seismic-reflection studies on Vancouver Island suggest that the Siletzia terrane was partly subducted 
beneath the Wrangellia superterrane (Clowes and others, 1987). The basaltic rocks are stratigraphically 
overlain by middle and late Eocene turbiditic sedimentary rocks of the Northern Olympic forearc basin. 
REFERENCES: Snavely and others, 1968, Glassley, 1974; Cady, 1975; Lyttle and Clarke, 1975; Tabor 
and Cady, 1978; Mueller, 1980; Wells and others, 1984; Clowes and others, 1987; Silberling and others, 
1987; Babcock and Engebretson, 1991.