Highly altered volcanic rocks in southwest Alaska

Unit symbol: KJv
Age range Cretaceous or Jurassic (163.5 to 66 Ma)
Lithology: Igneous - Volcanic
Group name: Volcano-plutonic complexes
Highly altered volcanic and hypabyssal rocks in the west central part of the Bethel and southeastern Russian Mission quadrangle (Box and others, 1993). Unit contains poorly to moderately exposed basalt and andesite lava flows and subordinate altered andesite, dacite, rhyolite, and gabbro. Rhyolitic rocks are orange-weathering, porphyritic and have as much as 10 percent phenocrysts in an altered groundmass of quartz, potassium feldspar, and plagioclase, which locally are in graphic intergrowths. Dacite has phenocrysts of as much as 5 percent each of plagioclase and biotite in a groundmass of plagioclase, quartz, and opaque oxides (Box and others, 1993). Some of these rocks have similar trace element signatures to the Slate Creek pluton (unit TJis) of Box and others (1993). Age is uncertain, though rocks are generally spatially associated with rocks of map units Jab and Jvc of Late Jurassic age; although it is unlikely, some rocks of this unit may be as young as Tertiary. Unit is contact metamorphosed by nearby Late Cretaceous and early Tertiary plutons. Corresponds to the Whitefish Lake volcanic field of Box and others (1993)

Source map information

Correlated geologic units

Label KJba
Description Basalt and andesite flows, Whitefish Lake volcanic field, Nyac terrane
Geologic age Late-Jurassic to Early-Cretaceous
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major