Graywacke of the Yenlo Hills

Unit symbol: KJyh
Age range Cretaceous? and uppermost Jurassic (145 to 66 Ma)
Lithology: Sedimentary
Group name: Graywacke of the Yenlo Hills
Consists of graywacke containing “extremely angular, and oscillatory zoned plagioclase, volcanic rock fragments, hornblende, epidote, and calcite grains” (Reed and Nelson, 1980). According to Reed and Nelson (1980), this unit differs significantly from the other sedimentary rocks in the vicinity, which are mapped as part of the Kahiltna assemblage (unit Kfy) flysch. The plagioclase, volcanic rock fragments, hornblende, epidote, and calcite are more abundant in the Yenlo Hills and Reed and Nelson (1980) suggest “that the sandstone may have been derived from the Jurassic magmatic arc (Reed and Lanphere, 1973) now covered by younger rocks in Cook Inlet basin. Contemporaneous volcanism is suggested by rare interbedded light tuffaceous sediments in the Yenlo Hills” (Reed and Nelson, 1980). New detrital zircon analyses indicate a significant peak at 150 Ma, which suggests unit has a maximum age of uppermost Jurassic or is younger in age (Hults and others, 2013)

Source map information

Source map Wilson, F.H., Dover, J.H., Bradley, D.C., Weber, F.R., Bundtzen, T.K., and Haeussler, P.J., 1998, Geologic map of central (interior) Alaska: U.S. Geological Survey Open-File Report 98-133, pamphlet, 63 p., 3 sheets, scale 1:500,000, and appendix.
Symbol Kyh
Unit name Graywacke
Description Part Reed and Nelson (1980 [TL002]) unit KJs in the Yenlo Hills. Was coded as NSA 3140 on Central Alaska compilation, recoded here. Consists of: graywacke in the Yenlo Hills of the Talkeetna quadrangle differs significantly from other sedimentary rocks in the vicinity '*** in that extremely angular, and oscillatory zoned plagioclase, volcanic rock fragments, hornblende, epidote, and calcite grains are more abundant, suggesting that the sandstone may have been derived from the Jurassic magmatic arc (Reed and Lanphere, 1973) now covered by younger rocks in Cook Inlet basin. Contemporaneous volcanism is suggested by rare interbedded light tuffaceous sediments in the Yenlo Hills (Reed and Nelson, 1980)'
Lithology Sedimentary

Correlated geologic units

Label Kyh
Description Graywacke of the Yenlo Hills
Geologic age Cretaceous
Geologic setting Sedimentary, marine, shallow to deep association, undivided
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major