Wales Group, undivided

Unit symbol: CPxwg
Age range Cambrian to Proterozoic (1000 to 485.4 Ma)
Lithology: Metamorphic
Group name: Wales Group, undivided
Complex assemblage of intercalated metabasite, metafelsite, metaclastic rocks, marble, and siliceous and carbonaceous black phyllite derived predominantly from andesitic to basaltic marine fragmental volcanic rocks and flows, graywacke, mudstone, and shale that contains locally interlayered marble (unit _<wgm). All rocks of the unit have been regionally deformed and metamorphosed to greenschist and, locally, amphibolite facies. The most abundant and widely distributed lithology is greenish-gray, thinly foliated, commonly crenulated albite-epidote ± quartz ± actinolite schist that is compositionally layered parallel to schistosity and is probably derived from tuffaceous mudstone, siltstone, and graywacke (Eberlein and others, 1983; S.M. Karl, unpub. data). Pillows and centimeter-scale pyroclastic rock fragments are locally preserved. Metasedimentary rocks show relict rhythmic and graded bedding, but other protolith features are mostly obscured by metamorphic recrystallization, penetrative foliation, high degree of flattening, and moderate elongation. Unit includes subordinate black phyllite and schist, meter-thick layers of silicic metavolcanic rocks, and light-colored, coarsely recrystallized marble. Metakeratophyre layers up to 3 m thick are common and typically contain rounded blue quartz eyes and phenocrysts or glomeroporphyritic clots of twinned albite set in a chert-like microscopic groundmass of quartz and albite (Herreid and others, 1978, Eberlein and others, 1983). Unit is folded, crenulated, and lineated (showing preferred orientation of minerals such as actinolite), and has quartz and carbonate boudins up to 5 m thick and 20 m long. Quartz segregation layers parallel and crosscut the foliation, and are concentrated in the crest regions of folds. There is evidence for at least two, and as many as four deformation events in these rocks. Unit believed to be at least several thousand meters thick (Eberlein and others, 1983). K/Ar hornblende age of 483 Ma (Turner and others, 1977) suggests deformation and metamorphism happened prior to the end of the Early Ordovician. Available constraints indicate that rocks in the Wales Group were deposited prior to Late Cambrian time and regionally metamorphosed and deformed before or during Early Ordovician time (Gehrels, 1992)

Source map information

Source map Karl, S.M., 1992, Map and table of mineral deposits on Annette Island, Alaska: U.S. Geological Survey Open-File Report 92-690, 57 p., 1 map sheet, scale 1:63,360.
Symbol SOua
Unit name Amphibolite facies volcanic, sedimentary, and intrusive rocks, undivided
Description Hornfels, schist, and gneiss containing plagioclase, blue-green hornblende, brown biotite, epidote-clinozoisite, chlorite, actinolite, sericite, calcite, almandine garnet, staurolite(?), and potassium feldspar.
Lithology Metamorphic

Correlated geologic units

Label PzZsv
Description Undivided Wales Group
Geologic age Neoproterozoic to Cambrian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Amphibolite Major
Metaclastic < Metasedimentary < Metamorphic Minor
Phyllite < Metaclastic < Metasedimentary < Metamorphic Minor
Marble < Metacarbonate < Metasedimentary < Metamorphic Minor
Metadacite < Metavolcanic < Metaigneous < Metamorphic Amphibolite Incidental
Metarhyolite < Metavolcanic < Metaigneous < Metamorphic Amphibolite Incidental