Young volcanic rocks, undifferentiated

Unit symbol: QTv
Age range Quaternary or Tertiary (5.333 to 0.0117 Ma)
Lithology: Igneous - Volcanic
Group name: Young volcanic and shallow intrusive rocks
In western Alaska, from Nunivak Island and northward to the Seward Peninsula, these rocks are dominantly alkaline and tholeiitic basalt and locally contain ultramafic inclusions (Hoare, 1975; Cox and others, 1976). Analysis of rocks of this unit from the Pribilof Islands and Nunivak Island were used to establish the radiometric time scale for geomagnetic reversals (Cox and others, 1968). Unit includes numerous alkali basalt, basanite, and hawaiite cones, short flows, and maar craters. Cones and flows have little or no vegetative cover and still preserve some primary flow structures (Patton and others, 2009). Includes tholeiitic basalt of Binakslit Bluff on Nunivak Island (Hoare and others, 1968); massive, columnar-jointed flows; normally polarized flows of Gauss polarity epoch as well as normally and reversely polarized flows older than Gauss polarity epoch. Multiple samples yielded K/Ar ages between 5.01±.15 and 3.24±.10 Ma. Also includes alkalic basalt of Ahzwiryuk Bluff on Nunivak Island; nubbly mottled flows and pyroclastic ejecta; that also includes both normally and reversely polarized rocks older than Gauss polarity epoch (Hoare and others, 1968). Two samples from this unit yielded K/Ar ages of 6.28±.18 and 5.19±.15 Ma. Additionally, unit includes vesicular and dense basalt and olivine basanite flows and sills in the Pribilof Islands (Barth, 1956). Along the Alaska Peninsula and in the Aleutian Islands, unit includes a wide range of volcanic products, similar to unit Qv; the main distinction is that this unit includes rocks where the age is not unequivocally Quaternary. As such, this unit includes the Pochnoi Volcanics of Semisopochnoi Island (Coats, 1959b), as well as volcanic rocks of ancestral Mount Kanaton volcano on Kanaga Island (Coats, 1956b; Miller and others, 2003), the Massacre Bay Formation of Attu Island (Gates and others, 1971), the Williwaw Cove Formation of Little Sitkin Island (Snyder, 1959), the flows and tuff-breccia of olivine-, hypersthene-, and hornblende-bearing andesite associated with Andrew Bay volcano on Adak Island (Coats, 1956a) and agglomerate on Kanaga Island (Tva of Coats, 1956b). On Great Sitkin Island, unit includes flows and agglomerate of the Sand Bay Volcanics (Tsl and Tsa of Simons and Mathewson, 1955). Locally, also includes sandstone from reworked pyroclastic deposits, as well as the pyroclastic rocks and lava flows (unit QTpl of Coats, 1959b) and crystalline vent plugs (unit QTp of Coats, 1959b) on Semisopochnoi Island. Includes interbedded flows, pyroclastic deposits, sedimentary rocks, and fine-grained dikes and sills on Tanaga, Kanaga, and Unalga Islands (unit QT of Fraser and Barnett, 1959), andesitic and basaltic tuff and tuff-breccia on Shemya Island (unit QTt of Gates and others, 1971), and Quaternary or Tertiary basaltic rocks of Bobrof Island as reported by Coats (1956c). On Little Sitkin Island, this unit locally contains areas of kaolinized, silicified, and pyritized rock (Snyder, 1959). Undated columnar-jointed flows of fine-grained tholeiitic and alkaline-olivine basalt in the western Holy Cross quadrangle (Csejtey and Keith, 1992) are included here because of similarity to volcanic rocks in the adjacent quadrangles to the north, west, and southwest. Unit includes Pliocene rocks of the Wrangell volcanic field in the Gulkana, Nabesna, Valdez, and McCarthy quadrangles (Nichols and Yehle, 1969; Richter, 1976; Richter and others, 2006; MacKevett, 1978; Winkler and others, 1981; W. Nokleberg, written commun., 1997)

Source map information

Source map Karl, S.M., 2001, Unpublished data.
Symbol QTv
Unit name Volcanic flows, undivided
Description Dominantly basalt, subordinate andesite, dacite, flowbanded rhyolite, obsidian, interbedded breccia, tuff, and agglomerate. Volcanic rocks include cinder cones, columnar jointed basalt, pahoehoe flows, and rubbly aa flows, pumice, scoria, ash and lapilli pyroclastic deposits up to tens of meters in thickness. The volcanic rocks are dominantly mafic and alkalic in composition. The volcanic rocks have alkaline chemical compositions. Tertiary volcanic rocks are undeformed and are not different in appearance or composition from the Quaternary volcanic rocks. If the volcanic rocks overlie glacial deposits, they are inferred to be Quaternary. In southeast Alaska, the Admiralty Volcanics are characterized by voluminous flows and large volcanic edifices that are generally aligned along a northwest-southeast trend, and they range from 30 Ma at the northwest end to 18 Ma at the southeast end, with a clear age progression. The Quaternary volcanic rocks are broadly distributed throughout southeast Alaska (and British Columbia) with no regional pattern except that their distribution appears to be controlled by association with major fault structures. The volcanic rocks assigned to this unit are the alkalic volcanic rocks that 1) are not located along the trend of the Admiralty Volcanics, and 2) are younger than Early Miocene. There are a few places where the broadly distributed On Kupreanof Island, the unit includes thick undated flows of basalt, andesite, and flowbanded rhyolite in an area where the dated volcanic rocks have ages ranging from Oligocene to Holocene. On Suemez Island, an undated flow that is isolated from the the Quaternary volcanic center is interlayered with alluvial deposits that contain coal seams (Buddington and Chapin, 1929)
Lithology Igneous

Correlated geologic units

Label QTv
Description Quaternary and/or Tertiary volcanic rocks, undifferentiated
Geologic age Miocene to Pleistocene
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Felsic-volcanic < Volcanic < Igneous Flow Minor