Wrangell Lava

Unit symbol: Twv
Age range Tertiary, Pliocene to Oligocene (33.9 to 1.806 Ma)
Lithology: Igneous - Volcanic
Group name: Volcanic rocks, undivided
Consists of basaltic andesite, andesite, and dacite lava flows; andesitic lahar deposits and andesite to dacite porphyry, diorite, and quartz diorite. This is the older part of the Wrangell Lava in the McCarthy and Nabesna quadrangle, exposed on either side of the Totshunda Fault and in the adjacent Yukon. Includes the Sonya Creek and other Tertiary volcanic centers. Includes rhyolitic rocks, domes, hypabyssal, and plutonic rocks (Richter and others, 2006). K/Ar ages of volcanic rocks range between 26.3±0.8 and 8.8±0.9 Ma, whereas intruding plutons have K/Ar ages that range from 9.4±0.7 to 3.41±0.46 Ma

Source map information

Source map Richter, D.H., Moll-Stalcup, E.J., Duffield, W.A., and Shew, Nora, 1997, Geologic map of the Nabesna A-6 quadrangle, south-central Alaska: U.S. Geological Survey Open-File Report 97-475, 1 pamphlet, 24 p., 1 sheet, scale 1:63,360.
Symbol Tau
Unit name Older andesites and basaltic andesites, undivided
Description Chiefly flat-lying to gently dipping flows, 3-8 m thick, exhibiting scoriaceous tops and bottoms, and lesser intercalated lahars and flow breccias. Flows range from medium gray to olive gray to dark gray, are locally amygdaloidal, and generally porphyritic. Most prevalent are pophyritic two-pyroxene andesites containing fine- to medium-grained phenocrysts (up to 4 mm) of oscillatory-zoned plagioclase (2-25%), clinopyroxene (trace to 3%), orthopyroxene (trace to 5%), olivine (0-1%), opaque minerals (trace to 1%) , and occasional traces of quartz and altered amphibole in a pilotaxitic groundmass of plagioclase laths, minute pyroxene grains, opaque minerals, and cryptocrystalline material. Glomerotrystic clots of plagioclase ± opaque minerals ±. clinopyroxene ± orthopyroxene are locally common. Olivine, where present, is generally rimmed by orthopyroxene. An olivine-bearing flow contains 54.5% SiO2 (sample 91ASh-7, Table 2) and an olivine-free flow contains 57.1 % SiO2 (sample 91AM1-11A, Table 2). Correlative flows in the adjacent Nabesna B-6 quadrangle give K-Ar ages of 1.99 and 2.21 Ma (Richter and others, 1993). A light-colored small intrusive body (labelled i) exposed on the steep valley wall above Copper Glacier was not examined on the ground
Lithology Igneous

Correlated geologic units

Label Twl
Description Wrangell Lava, Pliocene to Oligocene lower part, undivided
Geologic age Late-Oligocene to Pliocene
Geologic setting Extrusive
Lithology Form Importance
Hypabyssal-andesite < Mafic-hypabyssal < Hypabyssal < Igneous Dome Major
Basalt < Mafic-volcanic < Volcanic < Igneous Dome Major
Volcanic < Igneous Flow Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Volcanic < Igneous Pyroclastic Minor
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Minor
Dacite < Felsic-volcanic < Volcanic < Igneous Flow Incidental