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Source map Karl, S.M., 2001, Unpublished data.
Symbol KJgc
Unit name Conglomerate of the Gravina Formation
Description In Juneau area, Massive to thick bedded; pebble to cobble size well rounded clasts; poorly sorted; according to Lathram and others (1959) the matrix is graywacke and the most common clasts are dark gray to black argillite, bedded graywacke, light to medium gray limestone, marble, felsite, quartz, chert, granite, and quartz diorite; exposed on Shelter Island (as the Shelter Formation of Barker (1957) and on northeastern Admiralty Island. On southern Admiralty Island (Lathram and others, 1965) thick bedded to massive, pebble to cobble polymictic conglomerate, moderately to poorly sorted, dominantly clast supported, subrounded to rounded clasts. Clasts include argillite, siltstone, graywacke, limestone, marble, chert, felsic to mafic volcanic rock, granitic rock, foliated metamorphic rock. Deposits up to 1000 m thick may represent submarine debris flows or canyon fill west of Olive Inlet. Meter to 100 m-thick deposits at Pybus Bay may represent channel facies. East of Cape Fanshaw (Gehrels and others, 1992), clasts in conglomerate are dominated by intraformational shale, mudstone, graywacke, and limestone, and also include chert, feldspar porphyritic dacite(?), and granitoid rocks. The conglomerate may range up to 10 m in thickness. Sedimentary breccia occurs as several-meter-thick layers, consisting of randomly oriented blocks of intraformational sandstone, limestone, and shale enclosed in a chaotic mudstone-sandstone matrix. These layers probably formed as olistostromes. On Gravina Island (Berg, 1973) Light gray and brownish-gray pebble to cobble conglomerate interbedded with grit, sandstone, and calcareous siltstone. Clasts range up to 4 feet in maximum dimension. Clasts include light gray metarhyolite, leucocratic trondhjemite, amygdaloidal intermediate and basaltic volcanic rocks, dark gray to black siltstone, argillite, and metachert, and dark brownish-gray aphanitic, dioritic, metavolcanic, and very fine-grained detrital rocks. Matrix is siltstone and mudstone. Strong cleavage: clasts are flattened, locally exceeding 10:1. Thickness of unit about 350 feet. Conglomerate and grit overlies Upper Triassic rocks and underlies siltstone and argillite. This is the lowest Gravina Formation conglomerate. The uppermost member of the Gravina Island Formation consists of conglomerate gradationally overlies metavolcanic rocks. Clasts include phyllitic detrital rocks, intraformational aphanitic and porphyritic metavolcanic rocks, and rounded clasts up to a foot in diameter of light gray altered quartz diorite in as section up to 30 feet thick. The matrix of the conglomerate is gray phyllite and phyllitic grit, and the unit includes thin lenses of dark gray limestone that grades laterally to greenish gray sandstone, grit, and phyllite. Unit is a few hundred feet thick In Ketchikan area (Berg and others, 1988), polymictic metaconglomerate with flattened clasts in pelitic phyllite and semischist matrix. Clasts are rounded and include dark flysch, leucocratic plutonic rocks up to 20 cm in diameter, quartzite, marble, metavolcanic rocks. Characterized by prominent spheroidal to ellipsoidal relict clasts of fine to coarse grained leucocratic plutonic rocks as much as 20 cm in diameter. Other relict clasts are more strongly deformed and include quartzite as well as phyllite and semischist derived from fine grained sedimentary and volcanic rocks. Thin sections show that the matrix is fine grained schist containing quartz, plagioclase, biotite, epidote clinozoisite, garnet, calcite, muscovite, and pyrite. Some of the biotite and garnet crystals are partly altered to chlorite. At Gnat Cove, unit includes lenses of brown-weathering marble; at Back Island the metaconglomerate is interbedded with flysch. Metaconglomerate occurs in layers as thick as 2 meters . Jurassic radiometric ages of granitic cobbles indicates the metaconglomerates are correlative with the Gravina Island Formation
Lithology Sedimentary

Correlated geologic units

Label KJcg
Description Conglomerate, and minor sandstone, siltstone, shale, and volcanic rocks of the Gravina-Nuzotin belt
Geologic age Late-Jurassic to Early-Cretaceous
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Latite < Felsic-volcanic < Volcanic < Igneous Dike or sill Indeterminate, major
Latite < Mafic-volcanic < Volcanic < Igneous Dike or sill Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major