Metasedimentary and minor metavolcanic rocks along the west side of the Coast plutonic complex of Brew and Morrell (1979b)

Unit symbol: MzPzss
Age range Triassic to Paleozoic (485.4 to 201.3 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks associated with the Coast plutonic complex of Brew and Morrell (1979b)
Predominately metasedimentary rocks, but locally includes metavolcanic rocks of unit MzPzsv where the occurrences are too small to map separately. This map unit includes a number of informally named rocks; these include (1) the Perseverance group of Gehrels and others (1992); (2) the Alava sequence of Rubin and Saleeby (1991); and (3) a number of unnamed units of Brew and others (1984) and Karl and others (1999). The Perseverance group of Gehrels and others (1992) consists of dark-gray carbonaceous shale and carbonaceous metalimestone. Unit MzPzss is interbedded with metavolcanic rocks (shown here as unit MzPzsv where possible). Three less abundant but locally conspicuous lithologies are also included with this unit: silvery gray pyritic phyllite; rusty-weathering phyllite or schist (possibly derived from felsic or intermediate tuff); and outcrops of massive to laminated gray marble similar to that of the Permian marble subunit (unit Pm here). Age control comes from fossils, including the Late Triassic Halobia cf. H. superba Mojsisovics, Arcestes or Paraganides, Trachyceras (Prototrachyceras) lecontei Hyatt and Smith, Atractites cf. A. phillipi Hyatt and Smith, (S.M. Karl, unpub. data), and indeterminate ammonites in the Sheep Creek area near Juneau (Martin, 1926, p. 95). The Perseverance group of Gehrels and others (1992) also contains ammonites of late Ladinian (Middle Triassic) age in the Ketchikan area, (Berg and others, 1988; S.M. Karl, unpub. data). Gehrels and others (1992) suggested that the unit is as much as several kilometers thick but thins to the north. The Perseverance group overlies Permian volcanic rocks that were informally named the Gastineau group by Gehrels and others (1992); the Gastineau is included here in unit MzPzsv. Underlying the Gastineau group is a map unit we include here that Gehrels and others (1992, p. 570) described as pre-Gastineau strata that consists of “a heterogeneous sequence of dark gray to black phyllite derived from black shale and mudstone; silver to gray phyllite that was originally a tuffaceous mudstone; green to gray chloritic phyllite derived from basalt flows, breccia, and tuff; light green to buff siliceous phyllite probably derived from dacitic to rhyolitic tuff and breccia; and coarse-grained muscovite-actinolite-garnet schist that was probably intrusive in origin.” The unit also includes the Alava sequence of Rubin and Saleeby (1991) mapped east of Ketchikan, which is equivalent to the combined Perseverance group, Gastineau group, and pre-Gastineau sequence of Gehrels and others (1992). The Alava sequence consists of an upper Paleozoic section of metamorphosed pillow basalt, mafic tuff, and crinoidal marble that show “* * * penetrative foliation, ductile folding and middle-greenschist- to amphibolite-facies metamorphic assemblages * * * divided into four groups” whose stratigraphic sequence is obscured by younger deformation: (1) marble, volcanic rocks, and argillite; (2) waterlain breccia and tuff; (3) interlayered marble and quartzite; and (4) crinoidal marble (Rubin and Saleeby, 1991, p. 884). These rocks contain late Early Pennsylvanian to Late Permian conodonts, and Early Permian brachiopods. Rubin and Saleeby (1991, p. 885) also describe a lower Mesozoic section of “carbonaceous phyllite, fine-grained argillaceous and siliceous limestone, metabasalt, mafic breccia, and tuff.” The lower Mesozoic rocks contain fragments of halobiid bivalves (Daonella) as well as Pentacrinites, conodonts, and ammonites, which indicate a Triassic age (Rubin and Saleeby, 1991). Between the Sumdum and Ketchikan areas, Brew and others (1984) and Karl and others (1999) mapped various unnamed metamorphic units that include biotite schist, phyllite, and biotite gneiss, which are undivided equivalents of the rocks above. Unit locally subdivided into unit Pm

Source map information

Source map Karl, S.M., and Baichtal, James, 2013, Unpublished data.
Symbol Pzgm
Unit name Gneiss and marble
Description Gray, green, black, and tan mafic and quartzofeldspathic to siliceous schist and gneiss interlayered with subordinate quartzite and marble. Unit consists mainly of metavolcanic rocks. Rock types include amphibolite, actinolite schist, garnet-plagioclase-hornblende semischist and gneiss, pelitic and semi-pelitic kyanite-garnet-biotite-feldspar-quartz schist, fine-grained quartz-feldspar-biotite schist, and marble; characterized by compositional layers a few cm to a meter in thickness. In the Juneau area, unit includes foliated and layered medium- to coarse-grained hornblende gneiss with lesser amounts of hornblende and biotite schist (Brew and Ford, 1985; Himmelberg and others, 1991). Protoliths are inferred to be basaltic, rhyolitic, and dacitic volcanic rocks, pelite, quartz-rich clastic strata, and limestone. Quartz-porphyritic metarhyolite with large blue quartz phenocrysts in a centimeter scale fragmental texture is a diagnostic rock type (Gehrels and others, 1992). Metarhyolite is commonly interlayered with metabasalt that sometimes displays relict fragmental and pillow structures. In Leconte and Thomas Bays, the unit includes amphibolite, muscovite-biotite pelitic schist, siliceous semischist, quartzite, and marble. Marble layers range from 10 cm to 20 m in thickness. Siliceous semischist has layers that contain up to 75 per cent quartz and alternate on a cm-scale with finer-grained biotite-rich layers, inferred to represent sandstone turbidites (McClelland others, 1991). Quartzite forms cm- to meter-scale layers, contains less than 20 per cent plagioclase, K-feldspar, biotite, muscovite, amphibole, epidote, and opaques, locally alternating with sub-millimeter laminae of biotite and white mica, and is inferred to represent both quartzose sandstone and felsic tuff (McClelland others, 1991). Felsic metatuff occurs as meter-scale layers and as 1-10 cm-thick layers in marble (McClelland others, 1991). Amphibolite layers up to 30 m in thickness are inferred to represent mafic extrusive rocks; massive and homogenous plagioclase-garnet-hornblende gneiss may represent mafic flows or intrusive rocks (McClelland others, 1991). In general the proportion of metavolcanic rocks increases southwestward. Metaclastic quartzite and quartz-cobble conglomerate are more common to the northeast (Gehrels and others, 1992). The rocks are strongly foliated and lineated, showing evidence for more than one deformation, and are locally proto mylonitic or phyllonitic. In the Ketchikan area, unit includes quartzofeldspathic biotite schist and gneiss, quartz-muscovite semischist, calc-schist, marble, and quartzofeldspathic biotite-amphibole gneiss. Metamorphic minerals include quartz, plagioclase, chlorite, epidote-clinozoisite, biotite, muscovite, actinolite, hornblende, garnet, kyanite, sillimanite, staurolite, calcite, and pyrite, indicating amphibolite grade regional metamorphism (Stowell, 1989; McClelland and others, 1991). These rocks are structurally interlayered with rocks of units Pzsg and PzZy
Lithology Metamorphic

Correlated geologic units

Label Pzgm
Description Portland Peninsula gneiss and marble
Geologic age Paleozoic to Mesozoic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Amphibole-schist < Schist < Metamorphic Major
Paragneiss < Metasedimentary < Metamorphic Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Minor