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


Wrangellia superterrane (island arc)Ñ



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Wrangellia superterrane (island arc)ÑConsists of middle Paleozoic and older Alexander sequence, late Paleozoic and early Mesozoic Wrangellia sequence, and Jurassic Peninsular sequence. Overlain by younger late Mesozoic and Cenozoic sedimentary and volcanic rock assemblages. Equivalent to combined Alexander, Peninsular, and Wrangellia terranes of Jones and others (1987) and Monger and others (1987). Corresponds to Wrangellia composite terrane of Plafker and others (1989b) and Nokleberg and others (in press). Alexander and Wrangellia sequences occur throughout southern and southeastern Alaska. Peninsular sequence occurs mainly on Alaska Peninsula and adjacent areas to northeast. Chief differences between regions underlain by superterrane are notable contrasts in the relative abundances of coeval strata. From east to west, the Alexander, Wrangellia, and Peninsular sequences form successively higher levels of a structural-stratigraphic succession. The dominantly early and middle Paleozoic Alexander sequence, the oldest part of the succession, occurs mainly to the east and southeast in southeastern Alaska; the late Paleozoic and early Mesozoic Wrangellia sequence, the middle part of the succession, occurs mainly to the north and west in southern Alaska; and the middle Mesozoic Peninsular sequence, the youngest part of the succession occurs mainly to the west on the Alaska Peninsula.

Substantial tectonic shortening occurs within the Wrangellia superterrane (Plafker and others, 1989b; Nokleberg and others, in press). The faulted juxtaposition of the Jurassic Talkeetna of the Peninsular sequence in eastern-southern Alaska against the Wrangellia sequence in the Wrangell Mountains requires extensive post-Middle Jurassic tectonic displacement of this part of the Peninsular sequence relative to the Wrangellia sequence. This post-Jurassic displacement resulted in juxtaposition of different facies and, therefore, in the original designation of the Peninsular and Wrangellia terranes as separate units.


WRA Alexander sequence (island arc) (southeastern Alaska)ÑOccurs along length of southeastern Alaska and consists of Late Proterozoic to middle Paleozoic volcaniclastic turbidites, siliceous shale, chert, limestone, intermediate to mafic volcanic rocks, local polymictic conglomerate, and early to middle Paleozoic plutonic rocks that are overlain by late Paleozoic and early Mesozoic units of the Wrangellia sequence, and by younger overlap assemblages. Age of exposed units varies from north to south.

In the southern part of southeastern Alaska, late Proterozoic and Early Cambrian Wales Group consists of metamorphosed volcanic, turbidite, and carbonate rocks that are overlain by Ordovician through Mississippian, interlayered volcaniclastic turbidites, carbonate rocks, and volcanic rocks, shallow-water clastic and carbonate rocks, and sparse volcanic rocks. These lithologies occur in the Port Refugio and Karheen Formations, Heceta Limestone, and Descon Formations. In the northern part of southeastern Alaska, the equivalent units, as depicted on stratigraphic column, equivalent units are the Ordovician through Devonian Gambier Bay, Freshwater Bay, and Cedar Cove Formations, Kennel Creek Limestone, and Bay of Pillars, Point Augusta, Hood Bay, Cannery, Saginaw Bay, and Iyoukeen Formations.

Alexander sequence includes abundant early to middle Paleozoic granitic to gabbroic, and minor ultramafic plutons. In southeastern Alaska, the Alexander sequence and overlying Wrangellia sequence are locally overlain by parts of Gravina-Nutzotin overlap assemblage, including Douglas Island Volcanics and Brothers Volcanics, and Seymour Canal and Gravina Island Formations that are intruded by coeval plutonic rocks of the Muir-Chichagof belt and Gravina arc. Locally unconformably overlain by Tertiary and Quaternary basalt, dacite, rhyolite, sandstone, and minor conglomerate.

In southeastern Alaska, Alexander sequence (and overlying Wrangellia sequence) faulted against Chugach terrane to west and against Behm Canal terrane to east. Alexander terrane interpreted as a displaced fragment of an early through mid Paleozoic island arc. REFERENCES: Loney and others, 1975; Turner and others, 1977; Eberlein and others, 1983; Brew and others, 1984; Gehrels and Saleeby, 1986, 1987; Gehrels and others, 1987; Monger and Berg, 1987; Berg and others, 1988; Brew and others, 1991; Gehrels, 1990, 1991; McClelland and Gehrels, 1992; McClelland and others, 1991; D.A. Brew, written commun., 1991


WRA Alexander sequence - (island arc) (southern Alaska)ÑOccurs in eastern Wrangell Mountains, eastern Alaska Range, and northern part of Chugach Mountains. In Wrangell Mountains consists chiefly of early to middle Paleozoic Kaskawulsh metamorphic rocks of Gardner and others (1988), which are equivalent to the Kaskawulsh Group in adjacent parts of Canada (Figure 3) (MacKevett, 1978; Campbell and Dodds, 1982a, b; Gardner and others, 1988). Kaskawulsh metamorphic rocks are mainly a highly deformed sequence of early Paleozoic marble, greenstone, metagreywacke, phyllite, slate, schist, argillite, siltstone, mafic volcanic rocks, and volcaniclastic rocks that are about a few thousand meters thick. Multiply folded and strongly schistose. Metamorphosed from upper greenschist to amphibolite facies. Devonian and older megafossils occur widely in unit in Alaska and in the Yukon Territory to the east. Intruded by Middle Pennsylvanian Bernard Creek pluton composed of granite, syenite, and alkali granite, and by Jurassic and Cretaceous tonalite and granodiorite of Chitina and Chisana arcs. Intrusion of the Middle Pennsylvanian Bernard Creek pluton into both Alexander and Wrangellia sequences indicates either stratigraphic continuity or structural juxtaposition by about the Early Pennsylvanian.

In the eastern Alaska Range in eastern-southern Alaska, Alexander sequence is interpreted to consist of pre-Pennsylvanian units of mica and quartz schist, and relatively younger metagranitic rocks occur as roof pendants in Pennsylvanian granitic rocks that also intrude upper Paleozoic volcanic and related rocks (Nokleberg and others, in press). In northern Chugach Mountains, north of the Border Ranges fault, the Alexander sequence is interpreted to consist of sparse roof pendants of quartz and pelitic schist, and marble (Knik River terrane) that occur in, or are faulted against the adjacent Border Ranges ultramafic-mafic assemblage of the Peninsular sequence, described below (Plafker and others, 1989b; Nokleberg and others, in press). Alexander sequence in southern Alaska interpreted as a displaced fragment of an early and middle Paleozoic island arc. REFERENCES: MacKevett, 1978; Campbell and Dodds, 1982a, b; Gardner and others, 1988; Plafker and others, 1989b; Nokleberg and others, in press


WRP Peninsular sequence (island arc) (southern Alaska)ÑOccurs mainly on Alaska Peninsula and adjacent area to northeast, and Kachemak Bay area of the Kenai Peninsula. Consists mainly of an areal extensive Jurassic island arc volcanic and plutonic sequence (Talkeetna arc) and strata. Volcanic and plutonic sequence consists mainly of: (1) Upper Triassic (Norian) and Lower Jurassic andesitic flows, breccias, and volcaniclastic siltstone and sandstone (Talkeetna Formation) (volcanic part of Talkeetna arc); (2) Jurassic batholithic granitic rocks (plutonic part of Talkeetna arc); and (3) Middle Jurassic to Cretaceous arc-derived clastic rocks (Shelikof and Chinitna Formations, Tuxedni Group, Kialagvik Formation, Naknek Formation, Staniukovich and Herendeen Formations), and bioclastic limestone (Nelchina Limestone). Unconformably overlying units are Cretaceous to lower Tertiary, progradational marine and non-marine sandstone, shale, and minor conglomerate (Matanuska, Hoodoo, Chignik, and Kaguyak Formations).

Southern part of Peninsular sequence, adjacent to Denali fault, consists of Early Jurassic Border Ranges ultramafic-mafic assemblage (BRUMA) interpreted as roots of Talkeetna arc (Plafker and others, 1989b; Nokleberg and others, in press). Peninsular sequence probably originally overlain by Kahiltna assemblage along southern part of northwestern margin, but is thrust over Kahiltna overlap along Talkeetna thrust. Peninsular sequence is bounded to southeast by Border Ranges fault and Chugach terrane, and is locally extensively overlain by Cenozoic volcanic rocks of Aleutian arc. Peninsular sequence interpreted mainly as a Upper Triassic and Jurassic oceanic island arc. REFERENCES: Jones and others, 1981; Pavlis, 1983; Burns, 1985; Plafker and others, 1989b; Detterman and others, in press; Wilson and others, in press


WRW Wrangellia sequence (island arc) (Southern and southeastern Alaska)ÑOccurs mainly in southern and southeastern Alaska and is inferred to occur to a minor extent in Puale Bay region of Alaska Peninsula. Consists of late Paleozoic and Mesozoic sedimentary, volcanic, and plutonic rocks. In southern Alaska, Wrangellia sequence consists of: (1) Pennsylvanian and Permian island-arc-related volcanic breccias, flows, and volcaniclastic rocks, mainly andesite and dacite, and associated marine sedimentary rocks (Tetelna Volcanics, and Station Creek and Slana Spur Formations). (2) Permian limestone, pelitic rocks, and chert (Hasen Creek and Eagle Creek Formations). (3) Triassic (Ladinian) black cherty argillite. (4) A thick section of Upper Triassic (Norian) subaerial and marine tholeiitic pillow basalt and lesser argillite (Nikolai Greenstone, up to thousand meters thick). (5) Upper (Norian) Triassic platform and basinal limestone (Chitistone and Nizina Limestones); and (6) Late Triassic and Jurassic basinal, siliceous, argillaceous and calcareous rocks (McCarthy Formation). On northern flank of Wrangell Mountains, unconformably overlying, Upper Jurassic and Lower Cretaceous flysch and andesite volcanic rocks (Gravina-Nutzotin overlap assemblage and Chisana Formation) form part of a major tectonically-collapsed flysch basin that is intruded by coeval granitic plutons. On southern flank of Wrangell Mountains, unconformably overlying, mainly Middle Jurassic to Maastrichtian progradational, volcaniclastic sequences (Root Glacier, Nizina Mountain, Chititu, Moonshine Creek, Schulze, and Kennicott Formations). Overlain by Upper Cenozoic to Holocene volcanic rocks (Wrangell Lava).

Upper Paleozoic sedimentary and volcanic rocks in Wrangellia sequence in eastern-southern Alaska are intruded by coeval metaplutonic rocks of the Skolai island arc. Late Triassic basalt and older rocks are intruded by mafic and ultramafic dikes, sills, and small plutons. Late Triassic basalts and associated plutonic rocks interpreted as products of rifting and (or) oceanic plume activity. Wrangellia sequence locally intruded by extensive Late Jurassic and Early Cretaceous granitic plutons of the Chisana, Chitina, and Glacier Bay-Chichagof arcs. Pervasive weak greenschist facies metamorphism in Early Cretaceous and older rocks. In eastern Wrangell Mountains, Wrangellia sequence welded to northern Alexander sequence by Middle Pennsylvanian granitic plutons of Skolai arc. In southern Alaska, Wrangellia sequence faulted against Yukon-Tanana and Maclaren terranes to north along Denali fault and Broxson Gulch thrust, and to south faulted against southern Wrangellia terrane along Chitina Valley and Paxson Lake faults.

On Alaska Peninsula, Wrangellia sequence consists of upper Paleozoic and lower Mesozoic island arc and oceanic sequences that occur in Puale Bay area and areas to north. Major units are: (1) Pennsylvanian(?) andesite flows and agglomerate; (2) Permian limestone; and (3) Upper Triassic (Norian) basalt, mafic tuff, limestone, and argillite (Kamishak Formation), and correlative units of Cottonwood Bay and Chilikadrotna Greenstones, and informally-named Tlikalika complex of Wallace (1983).

In southeastern Alaska, Wrangellia sequence consists of: (1) late Paleozoic clastic and carbonate rocks; and (2) Late Triassic mafic and lesser siliceous volcanic rocks. Major units are Mississippian, Pennsylvanian, and Early Permian clastic, volcaniclastic, and carbonate rocks of Cannery, Pybus, Ladrones Limestone, and Saginaw Bay, and Klawak Formations, and Late Triassic mafic and lesser siliceous volcanic, and carbonate rocks including the Hyd Group, Keku Volcanics, Goondip Greenstone, Retreat Group (Duttweiler-Kelly, 1990), White Stripe Marble, Chapin Peak Formation, and an unnamed sequence on the Chilkat Peninsula. Overlying Upper Jurassic and Lower Cretaceous flysch of Gravina-Nutzotin overlap assemblage. REFERENCES: Richter and others, 1975; Richter, 1976; Jones and others, 1977; MacKevett, 1978; Silberling and others, 1981; Johnson and Karl, 1985; Nokleberg and others, 1985, 1992, in press; Gardner and others, 1988; and Plafker and others, 1989a, b; Barker and others, 1989; Richards and others, 1991


WS Wickersham terrane (passive continental margin)ÑOccurs as one of several narrow east-northeast-trending terranes in east-central Alaska and consists of turbidites comprising two major lithofacies: (1) a structurally lower unit of mainly gray, maroon, and green slate with local quartzose sandstone and granule to pebble conglomerate, with dolomite and dark limestone; (2) an upper unit of mainly thick-bedded quartzose sandstone and pebble conglomerate with local layers of gray slate with local Oldhamia trace fossils of probable Cambrian age. Former Beaver terrane of Churkin and others (1982). Southern margin of Wickersham terrane exhibits penetrative metamorphic fabric and lower greenschist metamorphism that parallels fault bounding southern margin. Singly deformed and weakly metamorphosed. Structurally overlies White Mountains terrane to northwest and Fairbanks schist unit of Nokleberg and others (1989a) of Yukon-Tanana terrane to southeast. Wickersham terrane interpreted as displaced fragment of the early Paleozoic continental margin of North America Cordillera. REFERENCES: Churkin and others, 1982; Weber and others, 1985, 1988; Pessel and others, 1987; Moore and Nokleberg, 1988; Dover, 1990; Grantz and others, 1992
WY Windy terrane (metamorphic)ÑOccurs as narrow, discontinuous lenses along Denali fault in central and eastern Alaska Range and consists of a structural melange of: (1) small to large fault-bounded lenses of Silurian or Devonian limestone and marl; (2) Upper Triassic limestone; (3) Jurassic(?) basalt and chert; (4) Cretaceous flysch and volcanic rocks, mainly argillite, quartz-pebble siltstone, quartz sandstone, metagraywacke, and metaconglomerate with lesser andesite and dacite; and (5) Cretaceous(?) gabbro and diabase dikes and sills. Flysch contains sparse Cretaceous ammonites. Intensely faulted and sheared with a weak to intense schistosity formed at lower greenschist facies; local phyllonite and protomylonite. Minimum structural thickness of a few thousand meters. Windy terrane interpreted as a structural melange formed during tectonic mixing during Cenozoic dextral-slip movement along the Denali fault. Mesozoic flysch and associated volcanic rocks interpreted as tectonic remnants of the Kahiltna overlap assemblage and Chisana arc. Fragments of Silurian(?) and Devonian limestone and marl may be derived from Dillinger, Mystic, and (or) Nixon Fork terranes. REFERENCES: Jones and others, 1981; Nokleberg and others, 1985, 1989a, 1992; Stanley and others, 1990; Csejtey and others, 1992
YA Yakutat terrane (accretionary wedge - dominantly turbidites)ÑOccurs in southern Alaska and northern margin of Gulf of Alaska and consists of an allochthonous composite terrane. Mesozoic Yakutat Group forms eastern part, and Eocene oceanic crust forms western part. Both eastern and western parts overlain by younger Cenozoic sedimentary and volcanic rocks. Yakutat Group divided into: (1) melange facies, chiefly basalt, chert, argillite, tuff, and sandstone of Late Jurassic(?) and Early Cretaceous age, with exotic blocks of Permian and Upper Triassic carbonate rocks, and Middle and Late Jurassic tonalite; and (2) flysch facies, chiefly Upper Cretaceous volcanic sandstone, siltstone, and minor conglomerate with structurally interleaved lenses of disrupted chert, argillite, and volcanic rocks. Yakutat Group metamorphosed from zeolite to lower greenschist facies and intruded by Eocene granitoid plutons. Cenozoic sedimentary and volcanic rocks divided into: (1) Lower and middle Tertiary oceanic basalt, sandstone, siltstone, shale, and conglomerate (Poul Creek, Redwood, Kulthieth, and Tokun Formations) that includes Eocene basalt and shale rich in organic material; and (2) an overlap assemblage of Miocene and younger, shallow marine sandstone, siltstone, diamictite, and conglomerate (Yakataga and Redwood Formations). Terrane thrust under Prince William terrane to north along gently north-dipping Chugach-Saint Elias fault, and to west along subhorizontal Kayak Island fault zone; underthrust by Pacific plate to south along Transition fault. Yakutat terrane interpreted as a fragment of Chugach terrane (Yakutat Group) and Eocene oceanic crust that, together with overlying pre-Miocene sequence, was displaced at least 600 km northward along the Fairweather transform fault since the early Miocene. REFERENCES: Jones and others, 1981; Plafker, 1987
YO York terrane (passive continental margin)ÑOccurs on western margin of Seward Peninsula and consists of a weakly metamorphosed, structurally complex assemblage of Ordovician through Mississippian limestone, argillaceous limestone, dolostone, and fine-grained clastic rocks deposited in shallow-marine environment. Ordovician carbonate rocks comprise most of unit and may range up to 3,000 m thick. Late Proterozoic rocks may also occur. Terrane intruded by small, Late Cretaceous granitic stocks, and may correlate with passive continental margin rocks of Hammond terrane to northwest. York terrane interpreted as a displaced fragment of the Paleozoic and early Mesozoic Arctic continental margin.




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