Volcanic rocks of the Gravina-Nutzotin belt

Unit symbol: KJgv
Age range Lower Cretaceous or Upper Jurassic? (163.5 to 100.5 Ma)
Lithology: Sedimentary
Group name: Gravina-Nuzotin unit
Included here are the Chisana Formation, Douglas Island Volcanics, and Brothers Volcanics, as well as other volcanic rocks in the Gravina-Nutzotin belt that can not be explicitly assigned to the Lower Cretaceous Douglas Island or Brothers Volcanics. Primarily exposed in southeast Alaska but rocks also occur in the Healy quadrangle of central Alaska. Unit consists of andesitic to basaltic flows, flow breccia, agglomerate, and tuff (generally containing conspicuous clinopyroxene phenocrysts), subordinate graywacke and mudstone, and regionally metamorphosed and deformed equivalents of these strata. Chisana Formation is exposed in east central Alaska; Douglas Island and Brothers Volcanics (part of Stephens Passage Group) are exposed in southeast Alaska on and near Admiralty Island (Lathram and others, 1965) and in the Juneau area (Ford and Brew, 1973, 1977; Brew and Ford, 1985). Unit is a thick volcanic arc assemblage of marine and subaerial volcanic and volcaniclastic rocks. Rocks of its older part consist primarily of volcaniclastic rocks, such as would be included in the informal middle member of Gravina Island Formation, which consists chiefly of distinctly foliated, but locally massive, greenish-hued metavolcanic rocks derived from andesitic and subordinate basaltic tuff and agglomerate (Berg, 1973) as well as other volcanic rock units included in the belt for which explicit age control is not available. Age constrained by the Cretaceous and Jurassic sedimentary rocks of the Gravina-Nutzotin unit (KJgn), which generally overlie, but also intertongue, with the volcanic rocks

Source map information

Source map Berg, H.C., 1973, Geology of Gravina Island: U.S. Geological Survey Bulletin 1373, 41 p., 1 plate, scale 1:63,360.
Symbol Jgv
Unit name Andesitic to basaltic metavolcanic rocks of the Gravina Island Formation
Description Middle member of the Gravina Island Formation consists of metavolcanic rocks that gradationally overlie detrital sedimentary rocks and gradationally underlie conglomerate. Rocks are foliated, greenish metavolcanic rocks derived from andesite, tuff, and agglomerate, with subordinate gray, green, and black phyllite and phyllitic siltstone that increase in proportion at upper and lower contacts. Rocks are typically porphyritic, thickly bedded,, crudely foliated to coarsely fragmental, with clasts ranging up to 2 feet in dimension. Phenocrysts, up to 15 percent of the rock, include saussuritized plagioclase, augite, and hornblende. Matrix contains greenschist facies metamorphic minerals, including augite, hornblende, green amphibole, chlorite, epidote-clinozoisite, albite, quartz, muscovite, prehnite, calcite, and leucoxene. Amygdules are rare. Clasts are locally flattened in excess of 10:1. Estimated thickness is 2500 feet.
Lithology Metamorphic

Correlated geologic units

Label KJgb
Description Gravina Belt volcanic rocks in southeast Alaska, Early Cretaceous to Late Jurassic in age. Includes Seymour Canal Formation, Stephens Passage Group, Douglas Island and Brothers Volcanics (see also 2822)
Geologic age Late-Jurassic to Early-Cretaceous
Geologic setting Extrusive, undivided
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Flow Major
Mafic-volcanic < Volcanic < Igneous Volcaniclastic Major
Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Mafic-volcanic < Metaigneous < Metamorphic Greenschist Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Incidental
Slate < Metaclastic < Metasedimentary < Metamorphic Incidental