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 Richter, D.H., 1976, Geologic map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-932, 1 sheet, scale 1:250,000.
Symbol Kc
Unit name Chisana Formation
Description Subaqueous and subaerial volcanic and volcaniclastic rocks; Upper Part: primarily andesite, basaltic andesite flows, fragmental volcanic rocks, subordinate porphyritic andesite, thin-bedded conglomerate, grit, sandstone, and tuff. Lower Part: fragmental volcanic rocks, dacite to andesite porphyritic flows, volcanic sandstone and siltstone, argillite, graywacke, conglomerate, and a tuffaceous mudstone
Lithology Igneous

Correlated geologic units

Label Kcve
Description Chisana Formation, andesitic flows and volcaniclastic rocks. Also includes Douglas Island and Brothers Volcanics where separately mapped from other Gravina Belt volcanic rocks in southeast Alaska
Geologic age Early-Cretaceous
Geologic setting Intrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Andesite < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major