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Source map Wilson, F.H., 2012, Unpublished data.
Symbol KMu
Unit name Andesitic and basaltic volcaniclastic rocks, lava flows, chert, and mudstone (KJv) and altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone (TrMv), undivided
Description Andesitic and basaltic volcaniclastic rocks, lava flows, chert, and mudstone (KJv) and altered basalt, chert, chert breccia, argillite, tuff, volcaniclastic graywacke, and limestone (TrMv), undivided. Undivided Koyukuk and Angayucham-Tozitna terranes
Lithology Metamorphic

Correlated geologic units

Label KMu
Description Melange-like assemblage of mafic volcanic rocks, chert, mudstone, argillite, graywacke and limestone
Geologic age Mississippian to Hauterivian
Geologic setting Extrusive
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Greenstone < Metaigneous < Metamorphic Indeterminate, major
Sandstone-mudstone < Mixed-clastic < Clastic < Sedimentary Tuffaceous Indeterminate, major
Chert < Chemical < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Bed Incidental