Gravina-Nutzotin-Gambier volcanic-plutonic-sedimentary belt 

(Late Jurassic and Early Cretaceous) 

(Eastern-southern Alaska, southeastern Alaska, and Canadian Cordillera, units KJv, KJs, KJpf, 
Jpf)

Forms a major middle Mesozoic sequence of volcanic, sedimentary, and plutonic rocks deposited on and 
intruded into the Wrangellia superterrane. Interpreted as an elongate island arc. Consists of three major 
volcanic and sedimentary assemblages and coeval plutonic rocks.

In eastern-southern Alaska, the Nutzotin assemblage consists chiefly of argillite, graywacke, and 
conglomerate, with lesser andesitic and basaltic volcanic and volcaniclastic rocks of the Chisana 
Formation, Douglas Island Volcanics, and similar unnamed volcanic units. Sedimentary rocks range from 
deep-marine turbidite deposits to shallow-marine and nonmarine deposits. In eastern-southern Alaska, 
coarse clastic and volcaniclastic rocks in the Nutzotin part of assemblage were derived from volcanic 
sources, as well as locally from the underlying Wrangellia superterrane and from unknown metamorphic 
source terranes.

In southeastern Alaska and the Canadian Cordillera, the belt consists chiefly of intercalated volcanic rocks 
and sedimentary rocks (Gravina, Dezadeash, and Gambier units) that range in age from Late Jurassic 
(Oxfordian) through Early Cretaceous (Albian) (Berg and others, 1972; Monger and Berg, 1987; 
McClelland and others, 1992). Coarse clastic rocks in the Gravina part of assemblage derived from the 
stratigraphically underlying Wrangellia superterrane mainly to west, but may also have been derived in part 
from the Stikinia and Yukon Tanana terranes to the east. The Nutzotin part of the unit in eastern-southern 
Alaska is correlated with the Gravina part of the unit in southeastern Alaska and the Canadian Cordillera. 
Apparently both units were deposited in separate coeval basins because of a lack of continuity of the units. 
The Gravina-Nutzotin assemblage is correlated with the Kahiltna assemblage (unit kh) that occurs in 
western-southern Alaska. The Gravina-Nutzotin unit is locally intensely faulted and folded, and is intruded 
by mid- and Late Cretaceous granitic, gabbroic, and ultramafic rocks, that form parts of the Chisana, 
Chitina, and Glacier Bay-Chichagof arcs, and by younger granitic rocks. In southeastern Alaska, the 
Gravina part of the unit forms an extensive assemblage that overlies the Wrangellia superterrane, and is 
locally overthrust by the Yukon-Tanana and Taku terranes to the east.

The belt also contains coeval and slightly older (165-94 Ma) quartz diorite and granodiorite plutons. North 
of 54  N latitude and extending into eastern-southern Alaska, the plutonic rocks of the coeval Nutzotin-
Chichagof and Tonsina-Chichagof plutonic belts of Hudson (1983) and coeval stratified units of the 
Gravina-Nutzotin-Gambier belt intrude or overlie the Wrangellia and Alexander sequences of the 
Wrangellia superterrane (McClelland and others, 1992; Nokleberg and others, 1994b). South of latitude 
54  N, the central Coast Belt contains abundant plutonic rocks of Early Cretaceous age (Wheeler and 
McFeely, 1991; Woodsworth and others, 1991; Monger, 1991b; Monger, 1993). The Gambier part of the 
belt in the Canadian Cordillera contains mostly Early Cretaceous strata that is intruded by coeval and 
slightly younger plutonic rocks (130-94 Ma). These Early Cretaceous strata stratigraphically overlie 
Middle to Late Jurassic (165-145 Ma) plutons, as well as the Wrangellia superterrane and the Harrison 
Lake terrane. Coeval Jurassic and Early Cretaceous granitic plutons occur extensively in the southern part 
of the Canadian Cordillera, with local marked nonconformity between Jurassic granitic rocks and the 
overlying Early Cretaceous strata (Friedman and Armstrong, 1990; Monger, 1991b, 1993).

In the U.S.A. Pacific Northwest, the Gambier part of the belt consists of the Wells Creek Volcanics of 
Misch (1966) and the Nooksack Group of Misch (1966) which range in age from Middle Jurassic to Early 
Cretaceous. The strata are relatively unmetamorphosed arc-derived dacite volcanic rocks of the Wells 
Creek Volcanics that dip shallowly. These volcanic rocks grade upward into predominantly fine-grained 
clastic and fossiliferous, but locally coarse-grained conglomeratic arc deposits of the Nooksack Group 
(Sondergaard, 1979). REFERENCES: Misch, 1966; Berg and others, 1972; Richter, 1976, Sondergaard, 
1979; Hudson, 1983; Barker, 1987; Brew and Karl, 1988a, b, c; Ford and Brew, 1988; Friedman and 
Armstrong, 1990; Gehrels and others, 1990; Wheeler and McFeely, 1991; Woodsworth and others, 1991; 
Monger, 1991b; McClelland and others, 1992; Haeussler, 1992; Cohen and Lundberg, 1993; Monger, 
1993.