U. S. Geological survey pamphlet introduction, tectonic definitions, acknowledgments



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TECTONO-STRATIGRAPHIC TERRANES

[Arranged alphabetically by map symbol; inferred tectonic environment in parentheses]
Arctic Alaska superterraneÑConsists of Endicott Mountains, De Long Mountains, Hammond, North Slope, and Tigara terranes. North Slope terrane occurs along northern margin of Arctic Alaska, Endicott Mountains, De Long Mountains, and Hammond terranes occur generally successively to south. Tigara terrane occurs in western Arctic Alaska. Coldfoot terrane (Moore, 1992; Moore and others, 1992) designated as separate terrane in this study. Arctic Alaska superterrane generally overlain by Jurassic and Cretaceous marine sedimentary rocks, and by Cenozoic marine and continental sedimentary rocks. Various terranes of Arctic Alaska terrane are interpreted as originally forming in a passive continental margin environment that subsequently was substantially tectonically imbricated during major Mesozoic thrusting followed by local to substantial extension along southern margin (Moore, 1992; Moore and others, 1993)
AAD De Long Mountains terrane (passive continental margin)ÑOccurs along length of Brooks Range and consists of at least four allochthonous sequences that are composed of Upper Devonian or Lower Mississippian to Lower Cretaceous sedimentary rocks and that are distinguished by various stratigraphic characteristics. Terrane typically consists of: (1) Upper Devonian carbonate rocks (Baird Group, part); (2) Lower Mississippian shallow- to deep-marine clastic rocks and black shale (Kayak Shale of Endicott Group); (3) Lower Pennsylvanian through Triassic chert and argillite (Etivluk Group), and (or) Mississippian carbonate platform rocks (Lisburne Group); and (4) Upper Jurassic and Lower Cretaceous (Neocomian) flysch (Okpikruak Formation). Individual allochthonous sequences distinguished by: (1) presence or absence of Upper Devonian and Mississippian arkosic debris (Nuka Formation) containing 2.06 Ga clastic zircons; (2) Mississippian and Lower Pennsylvanian shallow-marine carbonate platform deposits (Lisburne Group); (3) abundant Permian(?) diabase sills intruding upper Paleozoic strata; and (4) proportion of clastic material within siliceous Permian, Triassic, and Lower Jurassic deposits (Etivluk Group). De Long Mountains terrane structurally overlies Endicott Mountains terrane and is interpreted as a displaced fragment of the late Paleozoic and early Mesozoic Arctic continental margin of North America. REFERENCES: Jones and others, 1981, 1987; Dumoulin and Harris, 1987; Mayfield and others, 1988; Karl and others, 1989; Moore and Mull, 1989; Moore and others, 1992
AAE Endicott Mountains terrane (passive continental margin)ÑOccurs along length of northern Brooks Range and consists of: (1) Upper Devonian marine shale and sandstone (Hunt Fork Shale, Noatak Sandstone, part) grading upwards into thick coarse-grained uppermost Devonian and lowermost Mississippian(?) fluvial deposits (Kanayut Conglomerate), and capped by transgressive marine shale (Kayak Shale) that comprise Endicott Group and represent a fluvial-deltaic clastic wedge shed from a northern and eastern source area using present-day coordinates; (2) Mississippian and Pennsylvanian continental-margin carbonate platform deposits (Lisburne Group), and Mississippian and Pennsylvanian shale, chert, dolomite, and sparse marine keratophyre flow and tuff (Kuna Formation); (3) Permian to Jurassic siliceous argillite, chert, and silicified limestone (Siksikpuk and Otuk Formations); and (4) Jurassic to Lower Cretaceous (Neocomian) shale (Ipewik unit) containing a thin, but extensive Buchia coquina unit. Overlap units are Neocomian (Okpikruak Formation) and Albian (Fortress Mountain and Torok Formations) flysch and lutite. Terrane displays chlorite-grade greenschist facies metamorphism of Mesozoic age at base. Terrane structurally overlies Hammond and North Slope terranes and underlies De Long Mountains terrane. Endicott Mountains terrane interpreted as displaced fragment of the Paleozoic and early Mesozoic Arctic continental margin of North America. Includes Kagvik terrane of Churkin and others (1979). REFERENCES: Dutro and others (1976); Churkin and others, 1979; Jones and others, 1981, 1987; Mull and others, 1982; Nokleberg and Winkler, 1982; Mayfield and others, 1978, 1988; Nilsen, 1981; Karl and others, 1989; Moore and Mull, 1989; Moore and others, 1992
AAH Hammond terrane (passive continental margin)ÑOccurs along length of central and southern Brooks Range in four complexly imbricated assemblages: (1) Late(?) Proterozoic quartz-mica schist, quartzite, marble, calc-schist, metabasalt, and phyllite metamorphosed generally at lower greenschist facies; (2) metamorphosed Cambrian to Silurian and older carbonate rocks (Skajit Limestone and undifferentiated Baird Group); (3) Devonian phyllite, metasandstone, metaconglomerate, metatuff, metabasalt, and limestone (Beaucoup Formation) and upper Paleozoic (Mississippian) conglomerate and black shale (Kekiktuk and Kayak Formations); (4) sparse Mississippian and Pennsylvanian(?) carbonate platform rocks (Lisburne Group); and (5) sparse siltstone of Permian(?) and Triassic(?) age (Saddlerochit(?) Group). Older two assemblages locally intruded by Late Proterozoic and Devonian gneissic granitic rocks. Hammond terrane characterized by widespread presence of the second assemblage and a strong structural fabric overprinting relict primary sedimentary structures. Oldest assemblage exposed in local structural highs. Upper Paleozoic sequence correlative with coeval stable shelf deposits of the North Slope terrane. Hammond terrane displays mainly chlorite-grade greenschist facies metamorphism and relatively older blueschist and amphibolite facies metamorphism. Hammond terrane interpreted as a composite of several terranes displaced from unknown early Paleozoic carbonate platforms and clastic basins along the North American(?) and (or) Siberian(?) continental margins. Hammond terrane structurally underlies Endicott Mountains and Coldfoot terranes. REFERENCES: Tailleur and others, 1977; Dillon and others, 1980, 1987; Jones and others, 1981, 1987; Dumoulin and Harris, 1987; Dillon, 1989; Karl and others, 1989; Moore and Mull, 1989; Moore and others, 1992
AAN North Slope terrane (passive continental margin)ÑOccurs in North Slope subsurface, northeastern Brooks Range, and Doonerak structural window and consists of three major sequences: Franklinian sequence (pre-Mississippian), Ellesmerian sequence (Mississippian to Early Cretaceous), and Brookian sequence (mid-Cretaceous and Cenozoic).

Franklinian assemblage is complexly deformed, stratigraphically complicated, and may be a composite of several terranes. The best studied part (illustrated in stratigraphic column) occurs in Sadlerochit Mountains, on the north flank of the northeastern Brooks Range, and consists of a carbonate platform to basin assemblage composed of: (1) Katakturuk Dolomite (Proterozoic), Nanook Limestone (Proterozoic(?) or lower Cambrian to Ordovician); and (2) Mount Copleston Limestone (Lower Devonian). Pre-Mississippian rocks elsewhere in terrane consist of Late Proterozoic through Silurian slate, phyllite, chert, graywacke, carbonate rocks, mafic and intermediate volcanic and volcaniclastic rocks, and the Neruokpuk Quartzite. Franklinian sequence contains sparse Devonian metagranitic plutons.

Ellesmerian sequence unconformably overlies Franklinian sequence and consists of: (1) transgressive unit of Mississippian nonmarine conglomerate and marine shale (Kayak Shale and Kekiktuk Conglomerate of Endicott Group); (2) Mississippian to Permian carbonate platform deposits (Lisburne Group); (3) Permian and Triassic marine shale, sandstone and conglomerate (Echooka Formation and Ivishak Formation of Sadlerochit Group); (4) Triassic black shale, limestone and quartz-rich sandstone (Shublik Formation) and Jurassic and Lower Cretaceous black shale and sparse fine-grained sandstone (Kingak Shale).

Brookian sequence disconformably to unconformably overlies the Ellesmerian sequence and consists of: (1) mid- to Upper Cretaceous condensed basinal deposits (pebble shale unit and the Hue Shale); and (2) northeast-prograding fluvial deltaic deposits of mid-Cretaceous age (Fortress Mountain Formation, Torok Formation, Nanushuk Group, and of Early Cretaceous and Tertiary age (Canning Formation, Colville Group, Sagavanirktok Formation). Prograding deltaic deposits separated by regional Cenomanian disconformity.



North Slope terrane structurally overlain by Endicott Mountains and De Long Mountains terranes. Base not exposed. North Slope terrane interpreted as a displaced fragment of the Paleozoic and early Mesozoic Arctic continental margin of North America. REFERENCES: BrosgŽ and others, 1962; BrosgŽ and Tailleur, 1970; Jones and others, 1981, 1987; Mull and others, 1982; Bird, 1985, 1988; Blodgett and others (1986, 1988, 1992; Mayfield and others, 1988; Moore and Mull, 1989; Grantz and others, 1990; Moore and others, 1992
AAT Tigara terrane (passive continental margin)ÑOccurs on Lisburne Peninsula in northwestern Alaska, and on western tip of Seward Peninsula in western Alaska and consists of orogenic deposits unconformably overlain by stable shelf deposits. These deposits are: (1) Ordovician and Silurian slaty argillite, graptolitic shale, and graywacke turbidites of the Iviagik Group (Martin (1970) that is more than 1,500 m thick; (2) unconformably overlying, Lower Mississippian, intercalated marine and nonmarine argillite, lutite, quartzite, and coal of the Kapaloak sequence that is more than 600 m thick; (3) Upper(?) Mississippian marine shale (similar to Kayak Shale) and Lower Mississippian to Pennsylvanian carbonate rocks of the Nasorak, Kogruk, and Tupik Formations of Lisburne Group that is about 2,000 m thick; (4) Pennsylvanian to Jurassic argillite, siliceous shale, and argillaceous chert of the Etivluk Group that is about 200 m thick; and (5) Upper Jurassic(?) to Neocomian marine graywacke turbidite and marine mudstone of the Ogotoruk, Telavirak, and Kismilok Formations that together are more than 5,100 m thick. Tigara terrane inferred to be overlapped by Colville Basin deposits, including marine turbidite sandstone and mudstone that may be part of the Albian Fortress Mountain Formation. Tigara terrane resembles in part North Slope terrane, and in part Endicott Mountains terrane (Jones and others, 1987). Tigara terrane may extend offshore to at least part of the adjacent Herald Arch of the central Chukchi Sea. In western Brooks Range, terrane may be thrust over Cretaceous sedimentary rocks to east; in western Seward Peninsula, thrust over York terrane to east. Tigara terrane interpreted as a fragment of the displaced Paleozoic and early Mesozoic Arctic continental margin of North America. REFERENCES: Campbell, 1967; Martin, 1970; Jones and others, 1981, 1987; Murchey and others, 1988; Grantz and others, 1983, 1991; Moore and others, 1992
AM Angayucham terrane (subduction zone - dominantly oceanic rocks)ÑOccurs in east- and west-central, and southwestern Alaska, along southern flank of Brooks Range, and in large klippen in western Brooks Range. Divided into lower (Slate Creek), middle (Narvak), and upper (Kanuti) thrust panels.



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