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 Saleeby, J.B., 2000, Geochronologic investigations along the Alexander-Taku terrane boundary, southern Revillagigedo Island to Cape Fox areas, southeast Alaska, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, southeastern Alaska and British Columbia: Boulder, Colorado, Geological Society of America Special Paper 343, p. 107-143.
Symbol coarse stipple
Unit name Alava sequence
Description Crinoidal and argillaceous marble, mafic flows and hypabyssal rock, pillow breccia and aquagene tuff, carbonaceous argillite and phyllite, with exceedingly thin orthoquartzite layers in the lower Alava sequence that contain Precambrian detrital zircons. The depositional base of the Alava sequence is not preserved.
Lithology Metamorphic

Correlated geologic units

Label TrPzms
Description Alava sequence; correlated with the Perserverance Group (Middle and Late Triassic metased), Gastineau Group (Permian metavolcanic), and unnamed Permian phyllite of Gehrels and others (1992)
Geologic age Pennsylvanian to Triassic
Geologic setting Undivided
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Amphibolite Major
Metasedimentary < Metamorphic Amphibolite Major