Cherty tuff and breccia

Unit symbol: JMct
Age range Jurassic? to Mississippian (358.9 to 174.1 Ma)
Lithology: Sedimentary
Group name: Cherty tuff and breccia (Angayucham)
Dark greenish, very fine-grained cherty tuff that grades into greenish-gray radiolarian chert in west-central Alaska; commonly considered part of the Innoko assemblage. Unit also contains fine- to coarse-grained crystal and lithic tuff, as well as volcanic breccia and conglomerate composed of poorly sorted clasts of mafic volcanic rocks and cherty tuff in a crystal and lithic tuff matrix. Unit includes several small bodies of slightly recrystallized limestone and a few small intrusive bodies of gabbro and diabase. Age assignment is from widely distributed Radiolaria and conodonts in the cherty beds and range in age from Mississippian to possibly as young as Jurassic (Patton and others, 1980). Unit is unique in this group of assemblages (Angayucham, Tozitna, and Innoko) because of its tuffaceous character. Unit crops out in a distinctive band east of the more typical Innoko terrane rocks. Unit is similar in age and lithology to unit JTrct of the Chulitna sequence, but there is little reason to assert that these are the same unit

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol JMc
Unit name Chert, cherty tuff, argillite, crystal-lithic tuff, volcanic breccia, and conglomerate
Description Chiefly banded radiolarian chert interbedded with argillite, cherty tuff, fine- to coarse-grained crystal and lithic tuff, and volcanic breccia and conglomerate. Breccia and conglomerate are composed of poorly sorted clasts of mafic volcanic rocks and cherty tuff in a matrix of crystal and lithic tuff. Unit also includes several small bodies of slightly recrystallized limestone and a few small intrusive bodies of gabbro and diabase. Exposed along a belt extending from southern Ruby quadrangle through northwestern Medfra quadrangle and into eastern Ophir quadrangle
Lithology Sedimentary

Correlated geologic units

Label JTrta
Description Crystal tuff, argillite, chert, graywacke, and limestone, deep marine, Late Jurassic to Late Triassic(?), thought largely Triassic
Geologic age Triassic to Early-Jurassic
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Pyroclastic, tuff Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major