Sedimentary rocks

Unit symbol: TrDtz
Age range Triassic to Devonian (419.2 to 201.3 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks and chert (Angayucham)
Includes thinly and rhythmically interbedded dark-gray argillite and platy, laminated, gray siltstone; very fine- to fine-grained, gray, chert-rich, turbiditic sandstone; quartz- and chert-bearing granule-to-pebble conglomerate. Locally calcareous and fossiliferous (Weber and others, 1992). Unit also includes chert, which is dominantly medium-dark- to dark-gray, thin-banded, bedded, and radiolarian-bearing, and contains thin interbeds of slaty argillaceous rocks including argillite, slate, and phyllite. In addition, locally includes fine- to coarse-grained sandstone, limy sandstone, sandy limestone, siltstone, and shale. Sandstone ranges from clean quartz arenite to a lithic arenite that contains as much as 25 percent muscovite and metamorphic rock fragments. Also includes subordinate intermediate to basaltic volcanic and volcaniclastic rocks including lithic and water-laid tuff; fossiliferous shallow-water limestone; grit; and arkosic sandstone lenses. Where it can be determined from source maps that rock packages are predominantly igneous, they are mapped as units JMoc and JMct. Age range based on probable Permian microfossils and bryozoan and echinoderm fragments collected in the Ruby quadrangle (Chapman and Patton, 1978); Permian foraminifera, conodonts, and brachiopods (Weber and others, 1992); and Mississippian Radiolaria, conodonts, and foraminifera collected from unit in the Ruby and Medfra quadrangles (Chapman and Patton, 1979). Latest Devonian Radiolaria were collected in two localities in the southwestern Ruby quadrangle (Chapman and Patton, 1979) and are the only Devonian fossils known from rocks that are unequivocally Innoko assemblage; Patton and others (1994a) also report Devonian palynflora from the Wiseman and Christian quadrangles in the similar Angayucham assemblage. Around the Yukon-Koyukuk Basin, unit also includes “Interbedded white to light-gray banded quartzite, dark phyllite, and gray laminated limestone. * * * Thin layers of white mica folia give the quartzite a faint foliation. The white and light-gray banding, the purity of the quartzite, and the even texture of the quartz grains suggest that the quartzite is a recrystallized chert (metachert). The quartzite is locally interlayered with dark-gray, finely laminated, slightly foliated siliceous argillite, dark phyllite, and talc-chlorite schist. In the Nulato quadrangle, the limestone is partly recrystallized and silicified and locally contains unidentifiable coral and crinoid fragments. In the Melozitna quadrangle, thin marble beds contain conodonts and crinoids of Devonian age (Anita Harris, written commun., 1983). Some of the metachert may be as young as Mesozoic and correlative with the Mesozoic chert in unit JDv [JMoc here]” (Patton and others, 2009). A K/Ar age of 302±9 Ma on amphibole from tuff suggests a Pennsylvanian age in, at least, part of the unit (Miller and Bundtzen, 1994). Parts of unit were originally described by Chapman and Patton (1979). Corresponds to the Rampart Group of Brosgé and others (1969) in the Tanana quadrangle, the TrMra, TrMrb, TrMrs, and TrMrl units of Reifenstuhl and others (1997) in the Tanana B-1 quadrangle, the TrMrs unit of Weber and others (1992) in the Livengood quadrangle, and unit IPMc of Foster and others in the Circle quadrangle. Corresponds to the TrMc and TrMs units of Chapman and others (1985) in the Ophir quadrangle and unit TrMc of Miller and Bundtzen (1994) in the Iditarod quadrangle. Includes the IPMc unit of Patton and others (1980) in the Medfra quadrangle. In Livengood quadrangle, includes interlayered black shale or slate; light, olive-greenish-gray thinly bedded to massive and thickly bedded radiolarian-bearing chert; and light greenish-gray tuff (Weber and others, 1992, 1994). In the Coleen quadrangle, unit includes shale and chert as thick as 600 m, previously mapped as part of the Strangle Woman and Christian River sequences of Brosgé and Reiser (1969), who presumed a Triassic or Permian age. Unit also includes 3 to 8 m of cherty limestone, which is possibly equivalent to the Lisburne Group (C.G. Mull, written commun., 2012)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Labay, K.A., in press, Reconnaissance geologic map of the lower Yukon River Region, Alaska: U.S. Geological Survey Scientific Investigations Series Map SIM-3015, scale 1:500,000.
Symbol ls
Unit name Limestone
Description Scattered limestone bodies within unit TrMv
Lithology Sedimentary

Correlated geologic units

Label TrMica
Description Chert, volcanic rocks, metasandstone, and limestone
Geologic age Mississippian to Triassic
Geologic setting Undivided
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Chert < Chemical < Sedimentary Bed Major
Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental