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 Foster, H.L., 1970, Reconnaissance geologic map of the Tanacross quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigation Map I-593, 1 sheet, scale 1:250,000.
Symbol Pza
Unit name Phyllite and schist unit
Description Light-pink, light-green, tan, and gray phyllite, quartz-sericite schist, quartz-sericite-chlorite schist, quartzite and marble. In the Alaska Range in the southwestern part of the quadrangle, the rock is primarily light-pink, light-green, gray, and tan phyllite with some included greenstone; includes several thin ( a few inches to 50 feet) discontinuous marble beads and quartzite. Northward, rocks become more schistose, and quartz-sericite schist, quartz-sericite-graphite schist, quartz-graphite schist, and quartzite are the dominant rocks types. Rocks of this unit are primarily greenschist facies, lower in metamorphic grade to the south and increasing northward.
Lithology Metamorphic

Correlated geologic units

Label Dpmc
Description Phyllite and metaconglomerate
Geologic age Devonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Metasedimentary < Metamorphic Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Major