Seventymile assemblage

Unit symbol: TrMsm
Age range Triassic to Mississippian (358.9 to 201.3 Ma)
Lithology: Metamorphic
Group name: Seventymile assemblage (Yukon-Tanana crystalline complex)
Massive greenstone, amygdaloidal pillow basalt and metatuff and associated peridotite, metachert, quartzite, phyllite, silica-carbonate lenses, and other low metamorphic grade sedimentary rocks. White and orange-brown silica-carbonate lenses crop out in the northern Big Delta quadrangle (Weber and others, 1978). Foster (1992) interpreted the massive greenstone as “originally mostly basaltic pillow lava and mafic lava flows; generally associated with peridotite bodies. * * * In places, the greenstone contains amygdules (generally filled with calcite) and pillows. Locally tuff and silicified tuff, commonly red, gray, or green, are interlayered.” Foster (1992) reported that basalt metamorphosed to blueschist facies is included and also reported a glaucophane-bearing sample associated with greenstone yielded a late Permian 40Ar/39Ar age, based on a G.W. Cushing written communication in 1991 (Foster, 1992, p. 9); however, this age can not be confirmed. Cushing (written commun., 1992), the apparent source of the Permian date, does not report any age determinations on glaucophane-bearing rocks. Elsewhere the greenstone is intercalated with chert that yields Radiolaria and rare shelly fossils of Mississippian through Triassic age (Foster, 1976, 1992; Weber and others, 1978; Dusel-Bacon and Harris, 2003) of similar age and composition as rocks of the Angayucham and Tozitna assemblages (unit ^Dtz). Beranek and Mortensen (2011) analyzed detrital zircon samples of Triassic rocks from this assemblage as defined here; these zircon populations contain a significant number of Carboniferous and older Paleozoic zircons, as well as a smaller number of Proterozoic and rare Archean zircons. Includes units Pgc and Pq of Weber and others (1978) in the Big Delta quadrangle, unit Pzg of Foster and others (1983) in the Circle quadrangle, units Pq and MzPzb of Foster (1976) in the Eagle quadrangle, and units gs and gsi of Brabb and Churkin (1969) in the Charley River quadrangle

Source map information

Source map Foster, H.L., 1992, Geologic map of the eastern Yukon-Tanana region, Alaska: U.S. Geological Survey Open-File Report 92-313, 26 p., 1 plate, scale 1:500,000.
Symbol MzPzs
Unit name Sedimentary and slightly metamorphosed sedimentary rocks
Description Includes chert, limestone, graywacke, argillite, sandstone, conglomerate, phyllite, quartzite, and tuff. Locally interlayered with greenstone. Chert is green, light and dark gray, red, mottled green and gray; massive, closely fractured with abundant silica veins and veinlets, and commonly has a sugary texture recrystallization
Lithology Sedimentary

Correlated geologic units

Label MzPzs
Description Low-grade metasedimentary rocks of the Seventymile/Slide Mountain terrane
Geologic age Mississippian to Triassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Metasedimentary < Metamorphic Major