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Source map Karl, S.M., 2012, Unpublished data.
Symbol KJg
Unit name Sedimentary and volcanic rocks of the Gravina Belt, undivided
Description Mixed sedimentary and volcanic and undifferentiated rocks, including massive graywacke, graywacke turbidites, dark gray slate and argillite, polymictic conglomerate, fine- to coarse-grained mafic pyroclastic and volcaniclastic rocks and breccia, distal volcaniclastic turbidites, augite porphyritic mafic sills, dikes, pillowed flows, and mafic pyroclastic and volcaniclastic strata. The turbidites consist of partial to full Bouma sequences, with local sand-filled channels and lenticular calcareous concretions. At one locality on the Chilkat Peninsula, the unit includes up to 20 m of maroon and green shale, with minor intercalated gray to black siliceous shale, and fine-grained calcareous sandstone and shale that locally contain inoceramus fragments. In correlative units in Southeast Alaska, graywacke and argillite are locally calcareous and contain limestone nodules and lenses up to 50 cm thick. Unit is locally hornfelsed in km scale aureoles around mid-Cretaceous plutons. On the Cleveland Peninsula Revillagigedo Island, and the Portland Peninsula, the unit is metamorphosed to dark grey phyllite, fine-grained semischist, and greenschist facies andesite and basalt flows that contain epidote, albite, and chlorite. Primary depositional features are preserved. Unit includes the Seymour Canal Formation , the Douglas Island Volcanics, and the Brothers Volcanics of the Stephens Passage Group and the Gravina Island Formation. The rocks in this unit are collectively referred to as the Gravina belt; the depositional package is at least 3000 m thick, on an unconformable base
Lithology Sedimentary

Correlated geologic units

Label KJgb
Description Sedimentary and volcanic rocks of the Gravina-Nutzotin belt, undivided
Geologic age Late-Jurassic to Early-Cretaceous
Geologic setting Undivided
Lithology Form Importance
Volcanic < Igneous Pyroclastic Indeterminate, major
Volcanic < Igneous Flow Indeterminate, major
Clastic < Sedimentary Bed Indeterminate, major