Volcanic and sedimentary rocks of southwest Alaska

Unit symbol: KTrvs
Age range Cretaceous to Triassic? (235 to 100.5 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks of southwest Alaska
Thick unit that consists of low-grade metamorphic or contact metamorphosed marine volcanic and sedimentary rocks; restricted to mainland part of southwest Alaska. According to Hoare and Coonrad (1978), “* * * the volcanic rocks range in composition and type from mafic pillow basalts to more abundant andesitic and trachytic flows, tuffs, and breccias. Interbedded with the volcanic rocks are thick sections of tuffaceous siltstone, tuffaceous cherts, and massive or thin-bedded argillite. Tuffs and tuffaceous sedimentary rocks associated with the intermediate composition volcanic rocks are commonly laumontized.” Radiolarians of Late Jurassic to Early Cretaceous age and fragmentary ammonites of Jurassic age have been collected from this unit in the Goodnews Bay quadrangle (Hoare and Coonrad, 1978). In the Bethel quadrangle, Box and others (1993) subdivided this unit into their units KJc, KJb, Ja, and Jvs, whereas, in the Hagemeister Island and southern Goodnews Bay quadrangles, Box (1985) subdivided this unit into Jlt, Jvt, and JTrcv. Units KJc and KJb of Box and others (1993) are included herein; unit KJc consists of thin-bedded green to brown argillite with occasional 1- to 4-cm thick tuffaceous chert, siltstone, and fine-grained tuff interbeds. A Late Jurassic and (or) Early Cretaceous age is suggested from poorly preserved radiolarians (Box and others, 1993). Unit KJb, south of Crooked Mountain in the Bethel quadrangle, is a highly altered pillow basalt sequence exposed along a narrow trend about 1 km wide and 10 km long (Box and others, 1993) and interbedded with the argillite and chert. Unit also includes olivine basalt flows and fragmental mafic volcanic rocks in the Dillingham quadrangle. Locally subdivided here into units Jvs and JTrvs

Source map information

Source map Wilson, F.H., Hudson, T.L., Grybeck, Donald, Stoeser, D.B., Preller, C.C., Bickerstaff, Damon, Labay, Keith, and Miller, M.L., 2003, Preliminary geologic map of the northeast Dillingham quadrangle (D-1, D-2, C-1, and C-2), Alaska: U.S. Geological Survey Open-file Report 03-105, 13 p., scale 1:100,000.
Symbol KJvs
Unit name Volcanic and sedimentary rocks
Description Low-grade metamorphic or contact metamorphosed volcanic and lesser sedimentary rocks. Altered volcanic breccia is relatively abundant; however in the southwestern part of T.4 S., R.40 W., unit consists largely of vertically standing beds of contact-metamorphosed shale, siltstone, and lesser sandstone. In its southwestern outcrop area, locally includes unaltered fragmental volcanic rocks. Contact metamorphism evidenced in outcrop by recrystallization where weathered surfaces show textures and structures but fresh surfaces appear aphanitic. In thin-section, mafic minerals are largely replaced by chlorite and feldspars are altered in part to sericite. Inferred to correlate with a thick marine unit (KJvs) consisting of volcanic and sedimentary rocks mapped by Hoare and Coonrad (1978) in the western Dillingham and adjacent Goodnews Bay quadrangles. According to Hoare and Coonrad (1978), "the volcanic rocks range in composition and type from mafic pillow basalts to more abundant andesitic and trachytic flows, tuff, and breccia. Interbedded with the volcanic rocks are tuffaceous siltstone, tuffaceous chert, and massive or thin-bedded argillite."
Lithology Sedimentary

Correlated geologic units

Label KJvs
Description Volcanic and sedimentary rocks
Geologic age Late-Triassic to Early-Cretaceous
Geologic setting Undivided
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Pyroclastic Indeterminate, major
Mixed-clastic < Clastic < Sedimentary Bed Indeterminate, major