Volcanic rocks of southern Alaska

Unit symbol: Tvu
Age range Tertiary, Pliocene? and older (66 to 1.806 Ma)
Lithology: Igneous - Volcanic
Group name: Volcanic rocks, undivided
Widely distributed throughout southern Alaska, unit is composed of andesite, basalt, and dacite lava flows, tuff, lahar deposits, volcanic breccia, and hypabyssal intrusions, typically poorly age constrained, and all locally hydrothermally altered or hornfelsed. Rocks of this unit tend to fall within two groups on the basis of age, where it has been determined. In south central Alaska, unit includes subaerial flows and associated pyroclastic rocks that range in composition from rhyolite to basalt. These rocks tend to be dominantly moderately altered rhyolite and basalt, but andesite, dacite, and latite also occur. Similar volcanic rocks are mapped in the Anchorage and Talkeetna Mountains quadrangles (unit Tv, Winkler, 1992; Csejtey and others, 1978, respectively), where they also include small lenses of fluvial conglomerate. Flows possibly correlative with this unit occur within the Arkose Ridge Formation (included in unit Ttk here) and yield K/Ar ages that range from 56.2±1.7 to 39.2±2.4 Ma (Csejtey and others, 1992; Winkler, 1992). A crude stratification is described (Csejtey and others 1978; Winkler, 1992), where felsic rocks and pyroclastic rocks occur stratigraphically lower and basaltic and andesitic flows occur in the upper part of the section. Unit includes rhyolitic breccia, ash-flow tuff, flows, and intrusive rocks and subordinate mafic and intermediate flows in the Lake Clark quadrangle and is thought to encompass entire Tertiary and may include Jurassic rocks of the Talkeetna Formation. Much of this unit was included in map unit Tv of Nelson and others (1983), but those rocks that we believe correspond in age with the Oligocene to Eocene Meshik Arc (Wilson, 1985) were placed within map unit Tmv here. In the Sleetmute quadrangle, brownish black olivine basalt that locally displays columnar jointing and possibly pillow structures (Miller and others, 1989) is included here, as well as olivine basalt in the Livengood, Fairbanks, and Lime Hills quadrangles. On Kavalga, Ogliuga, Ulak, and Skagul Islands (Fraser and Barnett, 1959), basaltic or andesitic lava and pyroclastic deposits were undivided in mapping; however, K/Ar age determinations suggest a bimodal age distribution of these rocks. Dates on altered rock samples from Ulak Island were 33.9±1.1 Ma and 43.4±4.6 Ma (DeLong and others, 1978; Marvin and Cole, 1978), whereas samples from Skagul and Kavalga Islands yielded 5.2±0.9 Ma and 6.2±0.4 Ma on andesite (Marvin and Cole, 1978). Marine deposits of volcanic breccia, tuff, and a few pillow lava flows, all of basaltic composition, with a few interbeds of conglomerate and sandstone of similar material are found on Kiska Island (Coats and others, 1961). Samples from flows, a dike, and a basalt boulder within the Vega Bay Formation yielded K/Ar ages that range from 55.3 to 14.7 Ma (DeLong and others, 1978; Marvin and Cole, 1978); the oldest age is inconsistent with the presumed stratigraphic position of the Vega Bay Formation and is therefore viewed as incorrect. The next oldest age of 29.2±4.4 Ma is on an olivine-basalt boulder within the sedimentary rocks of the unit and clearly indicates a protolith age. An age of 17.8±1.1 Ma was determined on a dike cutting the unit, presumably suggesting a minimum age for the unit; however a flow within the unit yielded an age of 14.7±1.2 Ma, resulting in some doubt as to the age of the unit. Scattered outcrops of felsic to mafic volcanic rocks and related shallow intrusive bodies in the northwest part of the Talkeetna Mountains quadrangle (unit Tv of Csejtey and others, 1978) are also included here; other parts of Csejtey and others’ (1978) unit Tv have been assigned to unit Tepv. Finally, includes “basaltic pyroclastic and flow rocks, including minor pillowed lava flows; locally interbedded with marine sedimentary rocks, including tuffaceous or glauconitic strata, probably related genetically to mafic dikes, sills, and plugs on Middleton Island” (Winkler and Plafker, 1993). Locally subdivided into the following seven units Twv, Tpv, Tvm, Tob, Tca, Tmv, and Tev:

Source map information

Source map Day, W.C., Aleinikoff, J.N., Roberts, Paul, Smith, Moira, Gamble, B.M., Henning, M.W., Gough, L.P., and Morath, L.C., 2003, Geologic map of the Big Delta B-2 quadrangle, east-central Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Series I-2788, scale 1:63,360.
Symbol Tb
Unit name Basalt
Description Dark-gray to black, nonfoliated basalt dike containing small, randomly oriented plagioclase phenocrysts set in devitrified aphanitic groundmass. Age uncertain, but may be correlative with a 50 Ma basaltic dike swarm occurring throughout Yukon-Tanana Upland
Lithology Igneous

Correlated geologic units

Label Tb
Description Basalt
Geologic age Tertiary
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Gabbro < Gabbroic < Plutonic < Igneous Dike or sill Incidental
Mafic-volcanic < Volcanic < Igneous Pyroclastic Incidental
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Incidental