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Source map Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol KJgs
Unit name Sedimentary rocks of the Gravina belt
Description On the Chilkat Peninsula, includes 1700m of greenish gray volcaniclastic turbidites. The turbidites consist of thin-bedded, ripple- to plane-laminated upper Bouma sequence deposits, with local sand-filled channels and lenticular calcareous concretions. At one locality, 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 Juneau area, 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 hosts the Treadwell "albite diorite"sill, which is the host rock for the Treadwell gold deposits. On Admiralty Island, Graywacke turbidites, dark gray argillite, and polymictic conglomerate, with lenses of tuff and limestone. Unit is dominantly dark gray to black slatey argillite. Graywacke is commonly calcareous. Limestones nodules and lenses to 50 cm thick are common in the argillite. Graywacke contains dominantly feldspar, argillite, and volcanic rock fragments, but also contains quartz, detrital biotite, chert, epidote, and pyrite. Conglomerate clasts include argillite, graywacke, limestone, marble, chert, quartz, felsite, mafic volcanic rocks, granitic rocks, and foliated metamorphic rocks. Unit at least 2500 m thick. At Cape Fanshaw (Gehrels and others, 1992), green to gray sandstone to siltstone graywacke layers ranging from several centimeters to several meters in thickness. These layers are commonly separated by several centimeter thick layers of gray mudstone. Graded beds and channels are common. Many graywacke beds grade into conglomerate at their bases. Clasts in conglomerate are dominated by intraformational shale, mudstone, graywacke, and limestone, and also include chert, feldspar porphyritic dacite(?), and granitoid rocks. Detrital zircon ages correlate with Yukon Tanana terrane zircon ages (Kapp and Gehrels, 1998). The graywacke grades up-section into thin-bedded siltstone-mudstone-shale turbidites. Locally finely laminated limestone forms lenses up to 30 cm in thickness. In Cape Fanshaw/Stikine River/Zimovia Strait area, fine grained, rhythmically bedded graywacke turbidites are regionally recrystallized to subgreenschist sericitic and slate and subphyllite or greenschist facies sericitic and chloritic phyllites, exhibiting isoclinal folding and kink bands. On Mitkof Island, lithic sandstone and mudstone turbidites. Calcareous lithic wacke and arenite; light gray, thin-bedded, fine- to very fine-grained, with interbeds of dark gray mudstone. On Marsh Island, Volcaniclastic sandstone and siltstone turbidites, mudstone, clast-supported conglomerate, and matrix-supported debris flows. Conglomerate clasts are dominantly sedimentary lithic fragments, including chert, sandstone, mudstone, and limestone. In Keku Strait, Lithic sandstone and mudstone turbidites. Calcareous lithic wacke and arenite; light gray, thin-bedded, fine- to very fine-grained, with interbeds of dark gray mudstone; interpreted here to be a facies of the turbidite complex of the Gravina Belt. In Ketchikan area, unit consists of basal conglomerate and grit that grade upward and laterally into slaty, dark gray, thin-bedded graywacke turbidites and minor limestone. The conglomerate consists of poorly sorted angular to subrounded clasts, as much as a meter in maximum dimension, derived from stratigraphically underlying units, with a matrix of graywacke and grit. On Annette Island and eastern Gravina Island consists of regionally metamorphosed greenschist facies flysch and minor intertonguing andesitic metatuff. Unit is gradationally more recrystallized and more deformed from west to east. On Gravina Island (Berg, 1973), top of section is dark gray and brownish gray calcareous siltstone and argillite, and subordinate silty limestone, grit, pebbly to cobbly mudstone, and minor fine-grained intermediate volcanic rocks. Pebbly mudstone contains scattered clasts of altered quartz diorite. Satey cleavage; phyllitic. Minimum 500 feet thick. Greenschist facies phyllite grades eastward to biotite schist. Regional metamorphic fabric overprinted by low pressure metamorphism to pyroxene hornfels facies that occur in aureoles of Cretaceous and Tertiary plutons. Late Cretaceous pluton aureoles contain quartz, feldspar, biotite, sericite, staurolite, andalusite, sillimanite, and garnet. On the Chilkat Peninsula, prismatic shell fragments in flysch are inferred to represent the bivalve Inoceramus, and the rocks were assigned a tentative Jurassic and/or Cretaceous age (Plafker and others, 1989). The flysch conformably overlies carbonate and chert of Upper Carnian to Upper Norian age. On Admiralty Island Late Jurassic pelecypods, brachiopods, buchia, foraminifera, and dinoflagellate cysts. Cretaceous brachiopods and ammonites. 40Ar/39Ar ages for detrital biotite in graywacke range from 80 to 160 Ma; and for detrital hornblendes in graywacke range from 88 to 120 Ma (Cohen and Lundberg, 1993). Detrital zircons analyzed by Kapp and Gehrels (1998) yielded ages of 105-120 Ma, 140-165 Ma, 310-380 Ma, 400-450 Ma, 520-560 Ma, 920-1310 Ma, 1755-1955 Ma (Kapp and Gehrels. 1998). Island north of Cape Fanshaw contains Buchia rugosa and Buchia concentrica of latest Oxfordian to early Kimmeridgian age. Locality on south shore of Port Houghton contains an ammonite of probable Albian age. (D.L. Jones written commun. 1987, in Gehrels and others, 1992). On Marsh Island and Etolin Island, Late Kimmeridgian to Late Tithonian (Late Jurassic) buchia, early (Early Cretaceous) ammonites, Berriasian (Early Cretaceous) buchia; belemnites and pectinids and pelecypods of Middle Jurassic to Early Cretaceous age (Karl and others, 1999). Near Gravina Island, Entolium?, Camptonectes?,and belemnites Cylindroteuthis of Middle or Late Jurassic age from Blank Islands (Berg, 1973); Buchia, cf. B. Rugosa of Late Jurassic (Kimmeridgian) age, Buchia Aucella and pectens of Upper Jurassic or Lower Cretaceous age (Berg and Cruz, 1982). The youngest fossils reported for the Gravina belt sedimentary rocks are Albian to Cenomanian Terebratulid brachiopods from Pybus Bay on Admiralty Island, as well as biotite as young as 80 Ma and hornblende as young as 88 Ma, (Cohen and Lundberg, 1993) and the cephalopod Anagaudryceras of probable Albian to Cenomanian age from Etolin Island (Karl and others 1999). Intruded by quartz diorite plutons ranging from 87 to 93 Ma.
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