Metagraywacke and phyllite

Unit symbol: TrPzgp
Age range Triassic? and late Paleozoic (427.4 to 235 Ma)
Lithology: Metamorphic
Group name: Metagraywacke and phyllite
“Dark-gray to black phyllite and brown-weathering lithic sandstone, sandstone, and mudstone are exposed along the southern boundary of the schist belt. The unit varies in breadth along strike. In the Wiseman and Chandalar quadrangles, two subunits are recognized * * *: a northern dark, fine-grained phyllite or phyllonite, and a southern metamorphosed lithic sandstone-rich unit. * * * The northern unit is lithologically homogeneous, locally well foliated, and locally contains small bodies of mafic schist similar in composition to those in the underlying schist belt (Dsq; Moore and others, 1997b; Gottschalk and others, 1998). * * * Rocks in the southern subunit retain relict sedimentary features that may reflect contrasting depositional settings. Sedimentary structures, such as Bouma sequences, have been recognized and are thought to indicate deep-water turbidite fan deposition (Murphy and Patton, 1988); others, such as hummocky cross-stratification and oscillation ripple marks, were seen elsewhere and are indicators of shallow-water deposition (Gottshalk and others, 1998). Palynflora from sandstones with shallow-water structures are Early Devonian (Gottschalk, 1998)” (Till and others, 2008a). Correlative phyllite and metagraywacke is described by Patton and others (2009) in the Yukon-Koyukuk Basin. There, the “phyllite and metagraywacke are overprinted by a low-grade penetrative metamorphic fabric, but turbidite features, such as sole marks and graded bedding, are locally discernible. * * * Unit locally contains slices of little deformed shallow-water Devonian carbonate rocks that are enveloped in basalt flows and debris-flow(?) breccias composed of blocks of vesicular basalt in a matrix of volcanic and carbonate debris” (Patton and others, 2009). As shown here, unit also includes rocks mapped by Till and others (2008a) as “Phyllite, fine-grained schist, and phyllonite of the Central belt that underlie areas of poor exposure in the northeastern Baird Mountains quadrangle, western Ambler River quadrangle, and northwestern Chandalar quadrangle. Locally contains minor lenses of metalimestone and metaconglomerate.” Also includes in the Ruby quadrangle “phyllite and subordinate fine-grained metagraywacke cut by abundant vein quartz. The phyllite and metagraywacke are overprinted by a low-grade penetrative metamorphic fabric, but turbidite features, such as sole marks and graded bedding, are locally discernible. Age of unit is uncertain, probably Devonian or late Paleozoic” (Patton and others, 2009)

Source map information

Source map Brosge, W.P., and Reiser, H.N., 1964, Geologic map and section of the Chandalar quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-375, scale 1:250,000.
Symbol Dgw
Unit name Graywacke and phyllite -- graywacke
Description Gray, fine- to medium-grained micaceous graywacke with 50 to 60 percent quartz and chert fragments, no volcanic rocks fragments; interbedded black slate. Grade into Dp (Phyllite) below.
Lithology Sedimentary

Correlated geologic units

Label Pzg
Description Metagraywacke of Arctic Alaska and Ruby terranes
Geologic age Devonian to Middle-Triassic
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Carbonaceous Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Minor