Mixed assemblage of metasedimentary and metavolcanic rocks in the Brooks Range

Unit symbol: DPxasm
Age range Devonian to Proterozoic (1000 to 358.9 Ma)
Lithology: Metamorphic
Group name: Metasedimentary and metavolcanic rocks of the Central Belt and Northern Thrust assemblage of Till and others (2008a)
“Heterogeneous assemblage of interlayered calcareous, mafic, and siliceous rocks exposed in the * * * Ambler River and Wiseman quadrangles. Includes black quartzite, meta-argillite, and marble; white quartzite; green, buff, and black phyllite and calcareous phyllite; orange-weathering dolostone, orange weathering chloritic marble, chloritic dolomitic marble, gray marble, medium- and dark-green mafic metavolcanic rocks; pale green and orange calcareous schist, and gray-green pelitic schist” (Till and others, 2008a). Unit represents two units of Till and others (2008a), units PzZcm and PzZm, except that they had included in PzZcm a unit called “Metasedimentary and metavolcanic rocks of Tukpahlearik Creek, undivided,” of Karl and others (1989a) in the Baird Mountains quadrangle; that is unit DOtu of this map. Unit D<asm also includes a unit that straddles the Ambler River and Survey Pass quadrangle boundary and consists of “Massive dark greenstone commonly composed of albite, actinolite, epidote, and chlorite, and greenschist composed of albite, chlorite, and minor magnetite” (unit Pzgg of Nelson and Grybeck, 1980). Locally, the greenstone appears to be altered gabbro that intruded Devonian sedimentary rocks, which have been metamorphosed to dark hornfels within 2 meters of altered gabbro. The largest area of greenstone, in the western part of the Survey Pass quadrangle, was previously mapped as biotite schist by Brosgé and Pessel (1977) and by Mayfield and Tailleur (1978) in the adjacent Ambler River quadrangle, but more likely is a metamorphosed volcanic sequence. “Poorly developed pillows in compositionally layered biotite-quartz-chlorite schist and semischist, garnet-epidote-albite amphibolite, and feldspathic biotite-epidote-quartz gneiss with lenticular chloritic patches suggest that these are metamorphosed volcanic rocks of various types” (Nelson and Grybeck, 1980). Also includes unit PzZqs of Till and others (2008a), which they described as a relatively homogeneous assemblage dominated by light greenish-gray fine-grained schist that contains minor layers of metaconglomerate, marble, and calcareous schist. Its eastern exposure has a laminated appearance and contains marble that yielded a conodont of Ordovician to Triassic age (Moore and others, 1997b). These rocks are all part of the Central belt of Till and others (2008a)

Source map information

Source map Till, A.B., Dumoulin, J.A., Harris, A.G., Moore, T.E., Bleick, Heather, and Siwiec, Benjamin, 2008, Bedrock geologic map of the southern Brooks Range, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2008-1149, 88 p., 2 sheets, scale 1:500,000.
Symbol PzZqs
Unit name Quartz-rich metasedimentary rocks
Description Relatively homogeneous assemblage dominated by light greenish-gray fine-grained quartz-rich schist, typically accompanied by minor layers of metaconglomerate, marble, and calcareous schist. Exposed in the southern Central belt in the western Survey Pass quadrangle and straddling the Wiseman/Chandalar quadrangle boundary. This unit is part of the Central belt.
Lithology Metamorphic

Correlated geologic units

Label PzZacs
Description Quartz-rich metasedimentary rocks, Brooks Range Central belt and part of Northern thrust assemblages
Geologic age Neoproterozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Metasedimentary < Metamorphic Indeterminate, major