Yanert Fork sequence of Csejtey and others (1992) and correlative rocks

Unit symbol: DOmvs
Age range Devonian to Ordovician? (541 to 358.9 Ma)
Lithology: Metamorphic
Group name: Schist and phyllite of the Alaska Range
Consists of a metamorphosed marine sequence of phyllite, metavolcanic rocks, schist, and marble exposed along the north side of the Denali Fault System and found between the Hines Creek strand and main Denali Fault, in the Healy and Mount Hayes quadrangles. Mainly phyllite, but unit also includes argillite, slate, phyllonite, semischist, impure quartzite, schistose stretched-pebble conglomerate, banded metachert, felsic metatuff, metabasalt, and marble. Metamorphic grade is dominantly greenschist facies but ranges to amphibolite facies locally. Conodonts from a marble interbed in the Healy quadrangle indicate a Late Devonian age (Csejtey and others, 1992), whereas, in the Nabesna quadrangle, “rugose and tabulate corals from widely scattered localities indicate a Middle Devonian age” (Richter, 1976). U/Pb zircon analysis of metavolcanic rocks in this unit yielded an apparent age of 372±8 Ma, which is a composite age based on multiple samples in a concordia plot (Aleinikoff and Nokleberg, 1985). Nokleberg and others (1992a) mapped fine-grained metavolcanic and metasedimentary phyllonite and blastomylonite and fine-grained schistose metasedimentary rocks in the Mount Hayes quadrangle that are on strike with the Yanert Fork sequence of Csejtey and others (1992). In the Tanacross quadrangle Foster (1970) mapped an undivided phyllite and schist unit of which these rocks are a part. In the Nabesna quadrangle, this unit includes the Devonian phyllite and limestone mapped by Richter (1976). Csejtey and others (1992) mapped a unit of flysch along the Healy-Mount Hayes quadrangle boundary that they believed was of Cretaceous or Jurassic age; however, we have included it within this map unit because of its direct association with rocks thought to be Devonian in the Mount Hayes quadrangle

Source map information

Source map Nokleberg, W.J., Aleinikoff, J.N., Lange, I.M., Silva, S.R., Miyaoka, R.T., Schwab, C.E., and Zehner, R.E., with contribution for selected areas from Bond, G.C., Richter, D.H., Smith, T.E., and Stout, J.H., 1992, Preliminary geologic map of the Mount Hayes quadrangle, eastern Alaska Range, Alaska: U.S. Geological Survey Open File Report 92-594, scale 1:250,000 [Received as CAD coverage produced in 1996].
Symbol hgs
Unit name Fine-grained schistose sedimentary rocks and volcanic rocks
Description Chiefly fine-grained, pale-yellow, pale-green, and graphitic black metasedimentary phyllonite and blastomylonite with minor metavolcanic blastomylonite. Predominant lithologies are quartz-mica phyllonite, quartz-calc phyllonite, quartzite, marble, and metavolcanic blastomylonite. Protolith thought to be siltstone, shale, marl, limestone, quartz-rich volcanic rocks, and volcanic graywacke. Intense recrystallization and ductile deformation, exhibits strong schistosity. Metamorphosed to lower to middle greenschist facies.
Lithology Metamorphic

Correlated geologic units

Label Dys
Description Fine-grained, schistose metasedimentary and metavolcanic rocks, Devonian or older, [Evidence suggests that this unit is most likely Triassic, at least in its western exposures and correlates with similar units all along the north side of the Denali, unit colored same as 4033, is likely correlative] [Overprint pattern 14]
Geologic age Paleozoic to Devonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Metasedimentary < Metamorphic Major
Metavolcanic < Metaigneous < Metamorphic Minor