Pyroclastic rocks

Unit symbol: Tpt
Age range Tertiary, early Eocene or Paleocene (66 to 47.8 Ma)
Lithology: Igneous - Volcanic
Group name: Flows and pyroclastic rocks
Tuffaceous rocks distributed in several areas of Alaska. The largest area of exposure is a belt in the Bettles, Tanana, and Melozitna quadrangles where light-gray to pink rhyolite tuff, welded(?) tuff, flows, and breccia and subordinate pumice, dark vitrophyre, and obsidian constitute the unit (Patton and others, 2009). Obsidian chips and artifacts found in archeological sites in northwest Alaska may have originated from this unit. In southwest Alaska in the northeast Dillingham quadrangle, unit includes crystal tuff that contains variable amounts of biotite and feldspar crystals and varies in general appearance from crystal tuff to porphyritic plutonic rock (Wilson and others, 2003), but in all cases the groundmass is tuffaceous. The proportion of tuff appears to increase from east to west. Wallace and others (1989) report K/Ar ages of 58.6±1.8 Ma (biotite) and 57.9±1.7 Ma (hornblende) for this unit, whereas multiple 40Ar/39Ar age determinations on biotite yield a tight age range between 59.69±0.05 and 59.25±0.05 Ma (Iriondo and others, 2003). On the northern part of Saint Matthew Island in the Bering Sea, unit is chiefly rhyolite and dacite welded tuff, tuff breccia, and dark rhyolite vitrophyre (Patton and others, 1975) and also includes minor intercalated andesite and basalt flows and dikes. On southwestern part of island, unit is chiefly light-colored rhyolite and dacite hypabyssal rocks (Patton and others, 1975). These felsic rocks appear to overlie mafic flows and volcaniclastic rocks and may be extrusive and hypabyssal cogenetic equivalent of granodiorite on the island (Patton and others, 1975). Age thought to be Eocene or Paleocene (see Wittbrodt and others, 1989). Tuffaceous rocks of similar age are known from the interior of Alaska in the Tanana (Reifenstuhl and others, 1997) and Big Delta quadrangles (Weber and others, 1978; Day and others, 2007), described by Reifenstuhl and others (1997) as “white and pink, purple and white, light-orange and pink, glassy-aphanitic to very fine grained, flow-banded rhyolite, rhyolite tuff breccia, ignimbrite, and potassium feldspar-porphyritic rhyolite. The rock types present suggest that the rhyolite was emplaced as flows, domes, tuffs, breccia, and rare obsidian, and suggest extrusion over a significant period of time.” Bacon and others (1990) described areas of tuffaceous rocks in east-central Alaska that they interpreted as caldera complexes. In the easternmost Big Delta quadrangle, a complex that they called “Slate Creek,” another complex they called “EC” in the easternmost Tanacross quadrangle, and additional exposures along Taylor Highway in the Eagle quadrangle have yielded a range of K/Ar and U/Pb ages that date these rocks to the Paleocene and Eocene, between 61.6±2.0 and 54.6±1.6 Ma (Foster and others, 1979; Bacon and others, 1990). In the northern Tyonek quadrangle, more mafic andesite to dacite welded tuff occurs, where it consists of massive welded tuff in beds thicker than several meters (P.J. Haeussler, USGS, written commun., 2007). Unit also includes the Porcupine Butte andesite of Solie and Layer (1993), which consists of columnar jointed andesite forming the neck of a Paleocene volcanic center (Solie and others, 1991a). Pyroclastic volcanic rocks are also found in southeast Alaska in the Juneau and Taku River quadrangles and range from tuff to coarse block-and-ash-fall breccia, which are recognized as part of the Sloko Group of Canada. They include minor sedimentary rocks, andesite, trachyte, dacite, rhyolite, and minor andesite and basalt flows. Age in inferred from the intimate association of the volcanic rocks with granodiorite of the Coast plutonic complex of Brew and Morrell (1979b)

Source map information

Source map Weber, F.R., Foster, H.L., Keith, T.E.C., and Dusel-Bacon, Cynthia, 1978, Preliminary geologic map of the Big Delta quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-529A, scale 1:250,000.
Symbol Tf
Unit name Felsic igneous rocks
Description Lava, shallow, intrusives and dikes and sills. Lava in eastern part of quad, gray with smokey quartz, sanidine, plagioclase, biotite, and hornblende phenocrysts. Phenocrysts may be as long as 2 - 5 mm. Some are 10 mm long. Rock generally fractured and crumbly. Locally glass matrix; glass has perlitic dehydration cracks.
Lithology Igneous

Correlated geologic units

Label Tfv
Description Intermediate to felsic biotite tuff
Geologic age Paleocene
Geologic setting Extrusive
Lithology Form Importance
Dacite < Felsic-volcanic < Volcanic < Igneous Flow Major
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major
Rhyolite < Felsic-volcanic < Volcanic < Igneous Flow Minor
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Minor