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 Mayfield, C.F., and Tailleur, I.L., 1978, Bedrock geology map of the Ambler River quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-120-A, 1 sheet, scale 1:250,000.
Symbol Pzq
Unit name Quartzite
Description Clean white or gray, fine-grained quartzite weathering orange and yellow; mapped as a unique unit independent of Pzmq when possible
Lithology Metamorphic

Correlated geologic units

Label PzZcm
Description Metasedimentary and metavolcanic rocks, undivided
Geologic age Paleozoic to Devonian
Geologic setting Sedimentary, undivided
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Major
Metasedimentary < Metamorphic Major