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 Smith, T.E., 1981, Geology of the Clearwater Mountains, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Geologic Report 60, 73 p., 1 sheet, scale 1:63,360.
Symbol Kp
Unit name Spotted phyllite
Description Medium- to dark-gray foliated rocks with planar compositional laminations. Spotting due to porphyroblastic biotite in random orientations, or less commonly to knots of amphibole or garnet. Numerous rotational microtextures
Lithology Metamorphic

Correlated geologic units

Label KJgm
Description Undifferentiated greenstone, metapelitic rocks, and metasandstone of the Gravina belt
Geologic age Jurassic to Early-Cretaceous
Geologic setting Metamorphic, undivided
Lithology Form Importance
Metagraywacke < Metasandstone < Metaclastic < Metasedimentary < Metamorphic Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Metasiltstone < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Slate < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Calcareous Minor
Chert < Chemical < Sedimentary Bed Minor
Plutonic < Igneous Incidental
Volcanic < Igneous Incidental