Metamorphic rocks, southeast Alaska

Unit symbol: TrPvs
Age range Triassic to Permian (541 to 201.3 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks, southeast Alaska
Recrystallized, relatively low-grade, andesitic volcanic and volcaniclastic rocks, flysch, and minor limestone in the central and eastern parts of the Coast Plutonic Complex of Brew and Morrell (1979b). Slightly metamorphosed basalt of the Stuhini group of Brew and others (2009) in the Atlin quadrangle is included; it has relict pillows in a 30-m-thick bed within a sequence of metamorphosed tuff, mudstone, and minor pebbly mudstone, as well as several hundred meters of semischistose graywacke (Brew and others, 2009). Brew and others (2009) also mapped 90 m of massive, locally laminated gray dolostone, dark-gray metamudstone, light-gray dolostone, and lesser dark-gray limestone that was locally fossiliferous. To the south, the dominant rocks are interbedded andesitic tuff breccia, volcanic graywacke, siltstone, and argillite. Coarse volcanic conglomerate, possible broken-pillow breccia, and dark-blue-gray marble occur as lesser constituents (Koch and Berg, 1996). Also included here is light- to dark-gray and medium-brown limestone, layers enclosed within metasedimentary and metavolcaniclastic rocks. Locally, the carbonate layers exceed 10 m thick (Koch and Berg, 1996). The sequence extends eastward into British Colombia and crops out discontinuously northwestward along the Alaska-Canada boundary. In the Atlin quadrangle, Brew and others (2009) report middle Early Triassic conodonts and Middle Triassic ammonites and conodonts, whereas in the Bradfield Canal quadrangle, Koch and Berg (1996) report fossils of Permian age including the conodonts Adetognathus lautus and Adetognathus(?), a brachiopod, Yakovlevia(?), fusulinids, Pseudofusulinella sp.(?), Pseudoschwagerina sp., Schwagerina sp., and the gastropod Omphalotrochus(?)

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet, 23 p., 2 sheets, scale 1:250,000.
Symbol Tehv
Unit name Hornblende and albite-epidote-chlorite hornfels
Description MAINLAND BELT: TULSEQUAH SUB-BELT: Protolith interpreted to be relatively undeformed intermediate to mafic volcanic rocks of "map-unit 4" of Souther (1971); relict breccias, tuff breccias, and tuffs locally recognizable; exposed quite extensively in the upper Hades Highway Devils Paw part of the Juneau Icefield.
Lithology Metamorphic

Correlated geologic units

Label TrPzvs
Description Stikine, recrystallized volcanic and sedimentary rocks
Geologic age Paleozoic to Triassic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Greenschist Major
Metasedimentary < Metamorphic Greenschist Minor
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Incidental