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 Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2010, Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2009-1254, pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol QTv
Unit name Weathered volcanic rocks, undivided
Description Basalt lava flows, vent deposits, maar volcanoes and associated pyroclastic rocks exposed in northern, central, eastern, and to a minor extent, southern Seward Peninsula (Figure 2). Less weathered rocks are slightly to strongly fragmented by frost riving and locally overlain by windblown silt. These rocks include alkali olivine basalt and olivine tholeiite. Alkalic rocks contain phenocrysts of olivine with plagioclase, augite, and spinel in the groundmass. Tholeiitic rocks contain plagioclase, phenocrysts with augite, hypersthene, olivine, and spinel in the groundmass (Swanson and others, 1981). These less weathered volcanic rocks underlie portions of the Lost Jim Basalt in the central Bendeleben quadrangle, and include the Camille Basalt and Gosling Volcanics of Hopkins (1963), as well as small flows in the valley centers of Lava and Bear Creeks. K-Ar determinations on basalt from the Kugruk River canyon (Turner and Swanson, 1981) and from Minnie Creek (Kaufman and Hopkins, 1985) indicate that the Gosling Volcanics were in part extruded between 0.9 and 0.8 Ma. The Camille Basalt is younger by an unknown amount of time. Equivalent to unit "Qv" of Till and others (1986). In southern Seward Peninsula, a small basaltic vent and associated flows yielded a 40Ar/39Ar whole rock age of approximately 0.8 Ma (Werdon and others, 2006). Other volcanic rocks are thoroughly fragmented by frost action. These are the most widely distributed and voluminous volcanic rocks on the peninsula; they underly most of the Imuruk Lake lava plateau but are mapped only where exposed through an otherwise 1- to 6-m thick mantle of windblown silt. The rocks are mostly alkali olivine basalt with phenocrysts of olivine with plagioclase, augite, and spinel in the groundmass. Lesser olivine tholeiite contains plagioclase phenocrysts with augite, hypersthene, olivine, and spinel in the groundmass (Swanson and others, 1981). This part of the unit includes the Imuruk Volcanics of Hopkins (1963) which are between 2 and 5 Ma (Hopkins and others, 1971; Turner and Swanson, 1981). Flows of the Imuruk Volcanics were confined by modern valley systems that drained north from the Imuruk Lake area and, as a result of subsequent stream incision, are now found as bench remnants high on valley walls. South and east of Imuruk Lake, older volcanic rocks are found capping ridge tops. These older flows were dated at 26-29 Ma (Turner and Swanson, 1981). In the southeastern Bendeleben quadrangle, basalt flows are interlayered with sedimentary rocks that have yielded early Eocene pollen (Dickinson and others, 1987). Equivalent to unit "QTv" of Till and others (1986). The older volcanic units contain xenoliths of dunite, harzburgite, chromite, granite, and schist (Hopkins, 1963). Dunite xenoliths are most common, and reached 7-8 cm across. Granite and schist xenoliths are smaller (less than 2 cm across) and occur only where the volcanics erupted through granitic or metamorphic bedrock. Similar xenoliths in correlative volcanic rocks in western Alaska and on Saint Lawrence Island have had more detailed study (e.g., Wirth and others, 2002). The Late Quaternary Espenberg Maars on the northern Seward Peninsula consist of four large craters ranging from 4 to 8 km in diameter surrounded by coeval tephra layers. These craters, known as Devil Mountain Maar, Whitefish Maar, North Killeak Maar, and South Killeak Maar (Figure 2), are each separate eruption craters excavating 100-300 m into Pleistocene sediments and lavas (Hopkins, 1988). The maars are surrounded by a thick blanket of pyroclastic surge and airfall tephra deposits. Cliffs around Devil Mountain Maar expose sequences of bedded surge deposits, airfall lapilli beds, scoria, massive pyroclastic flows and explosion breccia (Beget and others, 1996). This volcaniclastic sediment provides evidence that the Espenberg Maars were formed by highly explosive hydromagmatic eruptions through permafrost, which ultimately created the unusually large maar craters. Devil Mountain Maar is 8 km long by 6 km wide, as much as 200 m deep, and covers over 30 km2. It is the largest known maar on earth; the three other maars on Seward Peninsula are also larger than any previously described maar. The mafic volcanic rocks of this unit are one of several Cenozoic volcanic fields in western Alaska that extends from western and southwestern Alaska to Saint Lawrence Island and the Pribilof Islands on the Bering Sea shelf (Moll-Stalcup, 1994)
Lithology Igneous

Correlated geologic units

Label QTb
Description Basalt
Geologic age Pliocene to Pleistocene
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Stock or pipe Incidental