Volcanic rocks in southern Alaska

Unit symbol: TKv
Age range early Tertiary to Late Cretaceous (100.5 to 47.8 Ma)
Lithology: Igneous - Volcanic
Group name: Flows and pyroclastic rocks
Primarily consists of basalt and andesite, but ranges from basalt to rhyolite, largely exposed in southwest Alaska. A common association is with rocks of the Kuskokwim Group; (unit Kk) however, this volcanic rocks unit is somewhat more widespread in southwest Alaska. Unit varies compositionally across exposure area, and incorporates the full lithologic range of flows, tuff, and breccia and minor interbedded sandstone and shale in the Ruby, Iditarod, and Ophir quadrangles (Cass, 1959; Chapman and Patton, 1979; Chapman and others, 1985; Miller and Bundtzen, 1994). In the Iditarod quadrangle, includes that part of the Iditarod Volcanics that overlies the Kuskokwim Group. Andesite and basalt flows and volcaniclastic rocks are widely exposed in the central part of the Holy Cross quadrangle and in a small area in the north-central part of the Russian Mission quadrangle between the Yukon and Kuskokwim Rivers. Flows are generally porphyritic and are composed of phenocrysts of plagioclase and pyroxene in a groundmass of plagioclase microlites. Some of the flows are columnar jointed and locally vesicular. Volcaniclastic rocks in this unit include breccia, tuff, and agglomerate. The andesitic and basaltic rocks commonly are interlayered with or intruded by small bodies of dacite and rhyolite (Patton and others, 2006). In the southwest part of Talkeetna quadrangle, Reed and Nelson (1980) mapped interbedded medium- to coarse-grained greenish-gray crystal-lithic lapilli tuff and mafic volcanic rubble flows in units as much as 150 m thick, as well as associated sandstone, shale, and minor calcareous mudstone. In the McGrath, Melozitna, Unalakleet, Tanana, and Medfra quadrangles (Patton and others, 1978; Patton and others, 1980; Bundtzen and others, 1997a), unit consists of dacite, rhyolite, and trachyandesite lava flows, domes, sills, dikes, and interlayered breccia and tuff. In the Bethel quadrangle, unit includes felsic rocks of the Swift Creek, Tulip, and Eek volcanic fields of Box and others (1993) as well as rhyolitic rocks in the Ruby (Cass, 1959), Mount McKinley (Bela Csejtey, Jr., written commun., 1993), Tanana (Chapman and others, 1982), and Kantishna River quadrangles (Chapman and others, 1975); unit also includes felsic tuff in the Tyonek quadrangle (Solie and others, 1991a). Locally, in Unalakleet and Medfra quadrangles, tuff at the base of TKv unit contains interbeds of quartz-chert-pebble conglomerate, sandstone, siltstone, and thin coaly layers that contain abundant plant fossils. Palynflora collected from coaly layers at the base of the unit in the Medfra quadrangle are latest Cretaceous in age. Unit overlaps compositionally and spatially with units Tpt and TKwt, described below. Where known, age determinations generally range between approximately 70 and 50 Ma

Source map information

Source map Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol Tg
Unit name MICROGABBRO/DIABASE
Description Fine- to medium-grained (1–5 mm) tholeiitic microgabbro or diabase. Typical mineralogy is 50–70 percent felted plagioclase laths, 30–50 percent altered clinopyroxene + olivine, 3–5 percent magnetite + ilmenite, and 0–3 percent biotite. Although previously mapped as fine- to coarse-grained basalt (Foster, 1976), the absence of columnar jointing, layering, vesicles, volcanic breccias, or other evidence of extrusive character suggests that this unit is predominantly of shallow (?) intrusive origin or the interior portion of a thick flow. Contacts with adjacent units, where exposed, are high-angle and sheared, suggesting faulted intrusive contacts. Ar/ Ar biotite plateau age of 57.5 Ma (sample 1; table 1). Probably correlates with Tertiary basalts of similar major- and trace-element compositions (within-plate) throughout Interior Alaska. Early Tertiary resets in Ar spectra for minerals of the Chicken area and some early Tertiary Ar/ Ar isochrons (Layer and others, 2001) indicate this magmatism may have been extensive in the study region. Relatively unaltered mafic dikes (too small to be shown at map scale) are sporadically present elsewhere in the map area and probably represent the same early Tertiary magmatism. Magnetic susceptibility of the unit is moderate to high, usually 1–11 x 10 SI. Large bodies spatially correspond to aeromagnetic lows, indicating they are reversely magnetized, similar to Tertiary basalt in the Fairbanks area (Roe and Stone, 1993).
Lithology Igneous

Correlated geologic units

Label TKmv
Description Flat-lying to gently dipping mafic flows of andesite and basalt, and volcaniclastic deposits of andesitic and basaltic tuff and conglomerate
Geologic age Campanian to Paleocene
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Andesite < Mafic-volcanic < Volcanic < Igneous Volcaniclastic Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Volcaniclastic Indeterminate, major