Geologic Map of Alaska: Andesitic volcanic rocks

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., Wilson, F.H., and Taylor, T.A., 2011, Geologic map of Saint Lawrence Island, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-3146, pamphlet, 13 p., 1 sheet, scale 1:250,000, and database.
Symbol	Kv
Unit name	Andesitic volcanic rocks
Description	Unit includes a wide variety of flows, hypabyssal intrusive rocks and volcaniclastic rocks composed chiefly of andesite and trachyandesite but range from basalt to rhyolite. Volcaniclastic rocks vary from lithic and crystal tuffs and breccias to volcanic conglomerate. All rocks have been altered, in varying degrees, to an aggregate of chlorite, epidote, sericite, clay minerals, calcite, and pyrite; and, where intruded by the Sevuokuk pluton, have been metamorphosed to a hornblende hornfels or albite-epidote hornfels. The unit is assigned to the Cretaceous but may have a wide age range within the Cretaceous. It is locally intruded by and therefore, at least in part, older than the Sevuokuk pluton. However, near Powooliak Point, on the south coast, this unit yielded a K/Ar age of 91 Ma and may be, in part, younger than the Sevuokuk pluton. Location-Unit exposed in the western part of the island
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