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 Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., 1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1961, 63 p., 2 sheets, scale 1:250,000.
Symbol Dy
Unit name Yanert Fork sequence
Description Intensely deformed, thick marine sequence of dominantly metasedimentary and metavolcanic rocks. Bulk of sequence is dark-gray to black carbonaceous shale with intercalations of thin-bedded, commonly bioturbated, medium- to dark-gray, normally graded, both cross- and parallel-laminated, fine- to medium-grained lithic sandstone and siltstone (turbidites). Most regionally metamorphosed into siliceous mudstone, argillite, slate, phyllite, semischist, and impure quartzite. Also contains thin interbeds of dark- to medium-gray, locally greenish-gray, banded chert or metachert and other units, largely metavolcanic.
Lithology Metamorphic

Correlated geologic units

Label Dy
Description Yanert Fork sequence, thick sequence of marine metasedimentary and metavolcanic rocks, Late Devonian
Geologic age Late-Devonian
Geologic setting Undivided
Lithology Form Importance
Black-shale < Shale < Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Minor
Siltstone < Clastic < Sedimentary Bed Minor