Welded tuff and other felsic volcanic rocks

Unit symbol: TKwt
Age range early Tertiary? and early Late Cretaceous (100.5 to 47.8 Ma)
Lithology: Igneous - Volcanic
Group name: Flows and pyroclastic rocks
Unit consists of at least two ages of tuff, welded tuff, volcanic breccia, rhyolitic volcanic rocks, and shallow intrusive rocks in the Big Delta, Eagle, and Tanacross quadrangles of east-central Alaska. Existing mapping is not adequate to reliably distinguish the Paleocene tuffaceous units that properly belong in unit Tpt from the older Cretaceous rocks. The older part of the unit is represented by the West Fork and Sixtymile Butte calderas (Bacon and others, 1990), which yield K/Ar ages of approximately 93 Ma. U/Pb zircon dating (LA-ICPMS) by Mortensen (2008), however, yielded ages of 107.8±0.5 and 108.6±1.5 Ma for the Sixtymile Butte and the Dennison Fork calderas, respectively (note that Mortensen (2008) suggested that the West Fork and Dennison Fork exposures may represent a single caldera complex). Unit also includes the South Fork Volcanics of Yukon, which yield K/Ar and U/Pb ages between about 108 and 90 Ma, most ages were between 95 and 90 Ma (Gordey and Makepeace, 2003). Bacon and Lanphere (1996) reported an 40Ar/39Ar biotite age from tuff on the Middle Fork complex of 69.10 Ma and obtained zircon ages from rhyolitic tuff and a granite porphyry intrusion of 71.1±0.5 to 69.7±1.2 Ma and a revised age of 69.94±0.52 Ma for the previously dated biotite (Bacon and others, 2014). Most rocks of unit are highly altered (Bacon and others, 1990). Age of both parts of this unit, as well as unit Tpt, correspond to times of extensive plutonic intrusions in the Yukon-Tanana Upland and mostly likely represent the extrusive equivalent of these plutons

Source map information

Source map Foster, H.L., 1976, Geologic map of the Eagle quadrangle, Alaska: U.S. Geological Survey, Miscellaneous Investigations I-922, 1 sheet, scale 1:250,000.
Symbol Tw
Unit name Welded tuff
Description Siliceous light-gray welded tuff that weathers tan or orange brown
Lithology Igneous

Correlated geologic units

Label Kwt
Description Intermediate and silicic pyroclastic deposits
Geologic age Late-Cretaceous to Ypresian
Geologic setting Extrusive
Lithology Form Importance
Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Major