Younger schist

Unit symbol: Pzncs
Age range Devonian and Silurian? (443.4 to 358.9 Ma)
Lithology: Metamorphic
Group name: Layered sequence (Nome Complex)
Unit is a composite of units best described by Till and others (2011, units Dcs, and Ds) and consists of pelitic, calcareous, and graphitic schist interlayered on a scale of centimeters to meters in the Nome and Solomon quadrangles. Unit is predominantly pale-brown and gray, weakly to well foliated schist that consists primarily of plagioclase, calcite, quartz, white mica, and graphite. Pelitic schist weathers light- to dark-gray and contains chloritoid, glaucophane, or pseudomorphs of chlorite and albite after glaucophane, and rarely, garnet. Calcareous schist is light- to dark-brownish-gray and commonly contains chloritoid and rare garnet, as well as dolomite. Locally gray pelitic schist is interlayered with orange- or brown-weathering impure marble, calcareous schist, or mica- and graphite-rich schist. Dark-gray- to black-weathering, graphitic, locally micaceous schist “contains mm-scale laminae enriched in graphite, white mica, and iron oxide, and occurs in layers meters to tens of meters thick” (Till and others, 2011). Unit Dcs of Till and others (2011) consists of pelitic, calcareous, and graphitic schist and typically does not outcrop; rather, it usually occurs as loose rubble on hills, or in stream cuts; in low areas unit is covered by tundra. Unit Ds, Till’s pelitic schist unit, forms tors of resistant, well-foliated quartz-rich schist; pelitic schist is the dominant lithology in this unit and calcareous schist is minor. This schist also contains chloritoid, and locally graphite, glaucophane, and garnet. Many detrital zircon samples collected from the Dcs unit contain small populations of Middle and Late Devonian zircons and large populations of early-middle Silurian zircons, as well as older populations (Till and others, 2006b, 2008b). Only a single sample from Ds yielded sufficient detrital zircons for analysis; the youngest zircon population contains grains that are Early to Middle Devonian (Till and others, 2006b). Till and others (2011) described a distinctive white to light gray, generally impure marble that occurs in small lenses within or adjacent to this unit in the Nome quadrangle. The clast-bearing marble typically contains sand- to pebble-size clasts of dark-gray to black, organic-rich marble. Most clast-bearing intervals appear to be clast-supported, some may be graded, and the clasts are rounded to angular, commonly laminated, and generally about 2 cm in diameter, though some are as big as 12 cm long. Two collections of conodonts having CAI values of 5 are known. “One collection yielded a single element of Silurian(?) through Triassic age. The other collection, also a single conodont, is an Sb element of late Permian through Triassic (likely Triassic) age” Till and others (2011). Till and others (2011) suggest that the Triassic marble was juxtaposed with the rocks of the Nome Complex prior to metamorphism and, therefore, chose to not extend the protolith age of the Nome Complex to Triassic; the Jurassic metamorphism of the unit (Armstrong and others, 1986) provides an upper limit for the age of the protolith of the unit

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2011, Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Scientific Investigations Map 3131, 2 sheets, scale 1:500,000, 1 pamphlet, 75 p., and database, available at
Symbol Ds
Unit name Pelitic schist, Nome Group
Description Tors of resistant, well-foliated quartz-rich schist. Pelitic rocks are the dominant lithology in the unit; calcareous schist is a minor component. Outcrop appearance of the pelitic schist is diagnostic of the unit, and shows 1- to 2-cm-thick bands of granular interlocking quartz grains interlayered with micaceous minerals. In thin section, major minerals include quartz, muscovite, chlorite, chloritoid, and locally graphite, glaucophane and garnet. Quartz-segregation layers parallel foliation and trace intrafolial isoclinal and chevron-style folds axial planar to the foliation. Minor lithologic variation may occur on outcrop scale, typically with relatively thin calc-schist layers in the quartz-rich schist. Four detrital zircon samples were collected from this unit, and only one yielded sufficient zircons for analysis. The youngest zircon population contains grains that are Early to Middle Devonian (Till and others, 2006). At least part of the protolith of the unit is Devonian or younger. The thickness of the unit is unknown. Typical outcrops of the unit occur in the northeast Solomon D-5 quadrangle, along the ridgeline south of Bermudez Bluff to the headwaters of Camp Creek. First described by Smith (1910). Equivalent to "pCst" of Sainsbury (1974) and "CpCs" of Till and others (1986), and "Ds" of Werdon and others (2005a,b) and Newberry and others (2005)
Lithology Metamorphic

Correlated geologic units

Label Dnps
Description Nome Complex, pelitic schist
Geologic age Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Quartz-feldspar-schist < Schist < Metamorphic Major
Calc-silicate-schist < Schist < Metamorphic Minor