Oldest volcanic rocks

Unit symbol: OCdv
Age range Ordovician and Cambrian? (541 to 443.4 Ma)
Lithology: Sedimentary
Group name: Older rock units of the Doonerak Window
Andesitic to basaltic volcaniclastic rocks and local tuffaceous phyllite, gabbro, and diabase, and black phyllite, metamorphosed to lower greenschist and prehnite-pumpellyite facies (Dillon and others, 1986) and exposed as part of the basement assemblage of the Doonerak Window (Till and others, 2008a) in the Wiseman quadrangle. These rocks, called the Apoon assemblage by Julian and Oldow (1998), include flows and pyroclastic rocks that have island-arc chemical affinities. 40Ar/39Ar ages on hornblende from mafic dikes were 392±12, 474±14, 487±20, and 529±17 Ma (Dutro and others, 1976; Till and others, 2008a)

Source map information

Source map Dillon, J.T., Brosge, W.P., and Dutro, J.T., Jr., 1986, Generalized geologic map of the Wiseman quadrangle, Alaska: U.S. Geological Survey Open-File Report 86-219, 1 sheet, scale 1:250,000.
Symbol OCv
Unit name Volcanic rocks
Description Andesitic to basaltic volcaniclastic rocks and local tuffaceous phyllite, gabbro, and diabase, and black phyllite. Basement rocks of the Doonerak Window. Lower greenschist and prehnite-pumpellyite. Unconformably overlain by Mississippian Kekiktuk conglomerate.
Lithology Metamorphic

Correlated geologic units

Label OCv
Description Andesitic to basaltic volcaniclastic rocks and local tuffaceous phyllite, gabbro, and diabase, and black phyllite
Geologic age Cambrian to Ordovician
Geologic setting Extrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Volcaniclastic, volcanic breccia Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Conglomerate < Clastic < Sedimentary Tuffaceous Indeterminate, major
Sandstone < Clastic < Sedimentary Tuffaceous Indeterminate, major
Dolostone < Carbonate < Sedimentary Tuffaceous Incidental
Limestone < Carbonate < Sedimentary Tuffaceous Incidental