West-central Alaska mélange-like rocks

Unit symbol: KMmu
Age range Cretaceous or older (358.9 to 129.4 Ma)
Lithology: Sedimentary
Group name: West-central Alaska mélange (Angayucham)
“Unit composed of a wide variety of mafic and intermediate volcanic and intrusive rocks including basalt, andesite, diabase, gabbro, and volcaniclastic rocks that are weakly metamorphosed to prehnite-pumpellyite and locally to greenschist facies assemblages. The volcanic and intrusive rocks are interbedded with varicolored radiolarian chert, chert breccia, chert pebble conglomerate, argillite, graywacke, and small limestone bodies. The chert, argillite, graywacke, and limestone are locally metamorphosed to metachert, slate, pelitic schist and marble” (Patton and others, 2006). Unit contains locally abundant Mississippian to Triassic fossils, chiefly from scattered limestone bodies in southwestern Holy Cross and northwestern Russian Mission quadrangles. Unit may also include rocks that probably represent unit KJiv, elsewhere mapped separately and known to contain mollusks of Early Cretaceous (Hauterivian and Valanginian) age. “A small body of amphibolite from near the northwestern border of the Russian Mission quadrangle yielded a K/Ar isotopic cooling age of 349.27±10.48 Ma” (Patton and others, 2006). This unit is located in one of the least-mapped parts of the state and has only been mapped at a reconnaissance level. It primarily occurs in fault blocks surrounded by younger rock units

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Labay, K.A., in press, Reconnaissance geologic map of the lower Yukon River Region, Alaska: U.S. Geological Survey Scientific Investigations Series Map SIM-3015, scale 1:500,000.
Symbol TrMv
Unit name Complexly deformed assemblage of altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone, Angayucham-Tozitna terrane
Description Composed of a wide variety of mafic and intermediate volcanic and intrusive rocks including basalt, and andesite, diabase, gabbro, and volcaniclastic rocks that are weakly metamorphosed to prehnite-pumpellyite and locally to greenschist facies assemblages. Angayucham-Tozitna terrane
Lithology Metamorphic

Correlated geologic units

Label MzPzo
Description Ophiolitic metamorphosed mafic rocks; Melange-like assemblage of mafic volcanic rocks, chert, mudstone, argillite, graywacke and limestone
Geologic age Mississippian to Toarcian
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Greenstone < Metaigneous < Metamorphic Major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Volcanic < Igneous Pyroclastic, tuff Incidental
Marble < Metacarbonate < Metasedimentary < Metamorphic Incidental