Mélange facies

Unit symbol: KJm
Age range Cretaceous or Jurassic? (201.3 to 100.5 Ma)
Lithology: Tectonite
Group name: West-central Alaska mélange (Angayucham)
Tectonic assemblage of blocks of carbonate rocks, chert, metagraywacke, and altered mafic extrusive and intrusive rocks in a matrix of phyllite exposed in low hills surrounding the Yukon-Koyukuk Basin. The mélange locally varies in metamorphic grade from pumpellyite to greenschist facies (Patton and others, 2009; Till and others, 2008a). Till and others (2008a) report a block of mafic schist on the Baird Mountains and Ambler River quadrangle boundary that contains abundant blue amphibole, probably crossite. Age of the mélange is not well controlled; Till and others (2008a) report Mississippian radiolarians from metachert and metalimestone bodies yielding Middle and Late Devonian and Ordovician conodonts (Pallister and Carlson, 1988, cited in Till and others, 2008a). Patton and others (2009) suggested that the mélange probably formed during time of tectonic emplacement of Angayucham-Tozitna terrane structurally above the Arctic Alaska and Ruby terranes during the Early Cretaceous. Includes unit KJm of Patton and others (2009), but does not include that part of unit KJm of Karl and others (1989a) in the northwest part of the Baird Mountains quadrangle, which has been assigned here to the Okpikruak Formation (unit Kofm). The outcrop pattern of this part of unit KJm of Karl and others (1989a) projects into the adjoining Noatak quadrangle where, it was mapped as Okpikruak Formation at the top of the Kelly River allochthon by C.G. Mull and H.S. Sonneman (Exxon, unpub. report, 1968–1974)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol KJm
Unit name Melange
Description Mélange consists of blocks of carbonate rocks, chert, metagraywacke, and altered mafic volcanic and intrusive rocks in a matrix of phyllite. Within the mélange unit, large blocks of carbonate rocks from units Pzca and Pzcr and altered basalt, diabase, and gabbro from unit JDv are mapped separately. Mélange probably formed during time of tectonic emplacement of Angayucham-Tozitna terrane structurally above Arctic Alaska and Ruby terranes. Along the faulted contact between Angayucham-Tozitna and Arctic Alaska terranes from Baird Mountains quadrangle eastward to Survey Pass quadrangle and along the faulted contact between Angayucham-Tozitna and Ruby terranes in the Bettles and Tanana quadrangles
Lithology Tectonite

Correlated geologic units

Label MzPzl
Description Melange between Angayucham-Tozitna and Arctic Alaska and Ruby terranes
Geologic age Jurassic to Early-Cretaceous
Geologic setting Melange
Lithology Form Importance
Melange < Tectonite Melange, blocks Major