Andesitic volcanic rocks

Unit symbol: KJiv
Age range Early Cretaceous and Late Jurassic? (163.5 to 100.5 Ma)
Lithology: Igneous - Volcanic
Group name: Andesite and basalt
Widely distributed in western Alaska and on Saint Lawrence Island, unit consists of flows of andesite and basalt, interbedded with tuff, tuff breccia, agglomerate, volcanic conglomerate, and volcanic graywacke. Flows typically have phenocrysts of plagioclase and pyroxene set in a matrix of devitrified glass, altered plagioclase microlites, pyroxene, chlorite, and opaque oxides. Rhyolite and dacite flows are present locally. Tuff is composed chiefly of fine-grained basalt and andesite clasts, plagioclase crystals, and mafic minerals in an altered matrix of devitrified glass. In upper part of the unit, the tuff is highly calcareous and contains abundant shelly debris, including earliest Cretaceous Buchia sublaevis, Buchia subokensis, and Buchia crassicollis (Patton, 1966; Patton and others, 1968) and Simbirskites (Patton, 1967). Tuff commonly occurs in cyclical sequences that grade upward from coarse tuff breccia and lapilli tuff to very fine-grained cherty tuff and blue-green radiolarian chert. Massive agglomerate, breccia, and volcanic conglomerate are present locally. Locally, unit is highly disrupted or intruded by sills and dikes of diabase, diorite, and gabbro. Some of the conglomerate beds east of the Yukon River in the Holy Cross quadrangle contain clasts of altered granitic rocks. Unit is well exposed in the Hughes, Shungnak, Selawik, and Candle quadrangles. Unit also crops out in scattered localities in the Holy Cross quadrangle and northern Russian Mission quadrangle and in several small exposures along the faulted western boundary of the Yukon-Koyukuk Basin in the Kwiguk quadrangle and adjoining Saint Michael quadrangle. Unit is assigned an age of Jurassic to Early Cretaceous based on sparse isotopic and fossil data and on its stratigraphic position below the late Early Cretaceous sedimentary rocks of the Yukon-Koyukuk Basin and above the Triassic rocks of the Angayucham-Tozitna terrane. In the area east of the Yukon River a diorite intrusive body, included in this unit, yielded a K/Ar amphibole cooling age of 128±4.5 Ma (Early Cretaceous) and an andesite porphyry clast from a volcanic conglomerate gave a K/Ar amphibole age of 164.19±4.93 Ma (Middle Jurassic) (Patton and others, 2006)

Source map information

Source map Patton, W.W., Jr., Miller, T.P., and Tailleur, I.L., 1968, Regional geologic map of the Shungnak and southern part of the Ambler River quadrangles, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-554, 1 sheet, scale 1:250,000.
Symbol KJv
Unit name Andesitic volcanic rocks
Description Chiefly dark greenish-gray, andesitic crystal-bearing lithic tuff, volcanic breccia, and volcanic conglomerate with subordinate intercalated trachy andesite and basalt flows. Minor volcanic graywacke and mudstone.
Lithology Igneous

Correlated geologic units

Label Kva
Description Andesitic volcanic rocks (earliest Cretaceous, Neocomian, possibly Jurassic and may include Albian)
Geologic age Late-Jurassic to Early-Cretaceous
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental
Chert < Chemical < Sedimentary Bed Incidental