Kuskokwim Group 

(Cretaceous) 

(Southwestern Alaska, unit Ks)

Consists chiefly of Cretaceous (Albian to Coniacian) marine turbidite deposits, and subordinate shallow-
marine and fluvial strata, mainly quartzose lithic conglomerate, and sandstone and siltstone turbidite 
deposits. The Kuskokwim Group was deposited in a faulted, elongate southwest-trending sedimentary 
basin. The group is thickest (greater than 10,000 m thick) and displays the deepest water facies in the 
central part of the basin. The mixed marine and nonmarine sections are relatively thin (less than 3,000 m 
thick) and are mainly restricted to the margins of the basin. Sandstone and conglomerate clast compositions 
from through the basin suggest derivation from local sources. The Kuskokwim Group is generally 
deformed into broad open folds, but near the southeast margin, is deformed into overturned or isoclinal 
folds and thrust faults. For paleomagnetic determinations, mid-Cretaceous sedimentary rocks of the 
Yukon-Koyukuk basin have been extensively sampled and yield four grade A data sets. These indicate 
southward displacements between 10o and 17o with respect to North America. Five volcanic rock 
sequences sampled range in age from 65 to 43 Ma, and yield grade A to D data. All of these data indicate 
no displacement within their error limits. The Kuskokwim Group overlaps the Angayucham, Dillinger, 
Goodnews, Kilbuck-Idono, Nixon Fork, Ruby, Togiak, and Tikchik terranes, and the Late Jurassic and 
Cretaceous Kahiltna sedimentary and volcanic assemblage (unit kh). In poorly mapped areas, the Kahiltna 
assemblage seems to grade upward into the Cretaceous Kuskokwim Group. REFERENCES: Harris, 1985; 
Harris and others, 1986; Wallace and others, 1989; Box and Elder, 1992; Decker and others, 1994; 
Kirschner, 1994.