Geologic Map of Alaska: Gravina-Nuzotin unit

Gravina-Nuzotin unit

Unit symbol:	KJgn
Age range	Lower Cretaceous and Upper Jurassic (163.5 to 100.5 Ma)
Lithology:	Sedimentary
Group name:	Gravina-Nuzotin unit

Marine graywacke and mudstone, subordinate conglomerate and andesitic to basaltic volcanic rocks, minor limestone, and regionally metamorphosed and deformed equivalents of these strata. Unit is exposed in southeast Alaska and in eastern Alaska near the Denali Fault System. Metamorphic grade varies regionally; in southeast Alaska, it generally increases from nonmetamorphosed or subgreenschist facies in the southwest to greenschist and locally amphibolite facies in the northeast (Berg and others, 1972; Gehrels and Berg, 1992). Higher grade parts of unit are primarily phyllite, schist, and gneiss. Unit consists of the Gravina Island and Seymour Canal Formations and unnamed strata in southeast Alaska (Berg and others, 1978, 1988; Eberlein and others, 1983; Lathram and others, 1965; Muffler, 1967). In south-central Alaska, includes the Nutzotin Mountain sequence of Lowe and others (1982) and other unnamed graywacke and mudstone rock units south of the Denali Fault System. In southwest Alaska, the Koksetna River sequence of Wallace and others (1989) is also included here. In southeast Alaska, the Gravina-Nutzotin unit is locally mapped as an undivided unit that includes several volcanic-rich parts. Where possible these volcanic-rich rocks are included here in unit KJgv. Gehrels and Berg (1992) suggested that geologic and geochemical considerations indicate that some volcanic rocks in this unit are genetically related to Early Cretaceous ultramafic bodies of unit Kum here (Irvine, 1973, 1974) and possibly to Early Cretaceous and (or) Jurassic diorite and gabbro of unit KJdg here (Berg and others, 1978, 1988). Fossils in the widespread Gravina-Nutzotin unit range in age from Cenomanian to Late and possibly Middle Jurassic (Berg and others, 1972; Brew and others, 1984; Buddington and Chapin, 1929). Wallace and others (1989) reported four fossil localities in the Koksetna River sequence, which contain Buchia mosquensis of Late Jurassic (late Kimmeridgian) age and Buchia sublaevis of Early Cretaceous (Valanginian) age. Unit represents a thick deep-ocean trench, fossil-poor marine flysch sequence. Unit locally subdivided into unit KJgv

Source map information

Source map	Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol	KJss
Unit name	Gravina belt, Stephens Passage Group, Seymour Canal Formation
Description	Graywacke, slate, and minor conglomerate; composed largely of volcanic debris, except for the conglomerates, which are polymictic and contain granitic clasts; most graywacke and slate were turbidites, but nothing more is known of the depositonal environment; weathers dark greenish gray, brownish gray, and very dark gray, graywacke and slate/argillite are locally calcareous and lighter colored; sedimentary structures common, although few directional features have been noted. On Douglas Island this unit encloses the Treadwell "albite diorite"sill which is the host rock for the Treadwell gold deposits.
Lithology	Sedimentary

Correlated geologic units

Label	KJfn
Description	Marine sedimentary rocks of Gravina-Nuzotin unit
Geologic age	Late-Jurassic to Berriasian
Geologic setting	Sedimentary, slope-and-deep-water

Lithology	Form	Importance
Argillite < Sedimentary	Bed	Indeterminate, major
Chert < Chemical < Sedimentary	Bed	Indeterminate, major
Sandstone < Clastic < Sedimentary	Bed	Indeterminate, major
Limestone < Carbonate < Sedimentary	Bed	Incidental