Mafic to intermediate volcano-plutonic complexes

Unit symbol: Kmvi
Age range Late Cretaceous (100.5 to 66 Ma)
Lithology: Igneous - Volcanic
Group name: Volcano-plutonic complexes
Complexes composed primarily of altered basalt, andesite, and trachyandesite porphyritic lava flows, hypabyssal intrusive bodies, altered mafic and intermediate crystal and lithic tuffs, and subordinate olivine basalt and dacite flows that occur in a belt in southwest Alaska from the Medfra quadrangle to the Hagemeister Island quadrangle. In the Iditarod, Sleetmute, and Medfra quadrangles, unit consists chiefly of altered basaltic andesite and trachyandesite porphyry flows and hypabyssal intrusive bodies. Elsewhere includes volcanic rocks that include rhyolite to olivine basalt flows, dacitic to andesitic tuff and tuffaceous sandstone, and rhyolitic domes (Hoare and Coonrad, 1978; Patton and others, 1975; Box 1985; Box and others, 1993). Also includes moderately to steeply dipping, fresh to weakly altered porphyritic basaltic andesite flows of calc-alkaline affinity and interbedded sandy to boulder-rich volcaniclastic rocks mapped by Globerman (1985) and Box (1985) in the Hagemeister Island quadrangle, which were originally mapped as unit Tv by Hoare and Coonrad (1978). In the Hagemeister Island and southern Goodnews Bay quadrangles, unit overlies Upper Cretaceous Summit Island Formation on Summit Island and adjacent mainland. On Saint Matthew Island, unit consists of flat-lying to gently dipping mafic flows of andesite and basalt, and volcaniclastic rocks that consist of andesitic and basaltic tuff and conglomerate. K/Ar and 40Ar/39Ar ages range from 79.2±2.4 Ma to 64.5±2.0 Ma. A basalt sill from Saint Matthew Island yielded a whole-rock K/Ar age of 52.2±1.6 Ma (Wittbrodt and others, 1989), however, as this age is significantly younger than other dated rocks from Saint Matthew Island, Wittbrodt and others (1989) suggested that the basalt sill was derived from a significantly younger event, which intruded this map unit. Overall, the most reliable age determinations range between approximately 79 and 64 Ma

Source map information

Source map Patton, W.W., Jr., Miller, T.P., Berg, H.C., Gryc, George, Hoare, J.M., and Ovenshine, A.T., 1975, Reconnaissance geologic map of St. Matthew Island, Bering Sea, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-642, 1 sheet, scale 1:125,000.
Symbol Ki
Unit name Intermediate and silicic pyroclastic deposits
Description Intermediate and silicic pyroclastic deposits including: dacite, andesite, and rhyolite nonwelded tuff breccia and crystal tuff, dacite and rhyolite welded tuff, massive dacite breccia (lahar?), fine ash-fall tuff, volcanic conglomerate, and andesite and dacite porphyritic and vitrophyritic plugs, dikes, sills, and flows.
Lithology Igneous

Correlated geologic units

Label Kpd
Description Intermediate and silicic pyroclastic rocks
Geologic age Campanian to Late-Cretaceous
Geologic setting Extrusive
Lithology Form Importance
Latite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Rhyolite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, air fall Indeterminate, major