Calcareous sedimentary rocks

Unit symbol: Trcs
Age range Upper Triassic, middle? Norian and upper Carnian (235 to 208.5 Ma)
Lithology: Sedimentary
Group name: Calcareous sedimentary rocks
Thin-bedded, dark- to medium-gray, commonly cross-bedded, carbonaceous, intercalated, calcareous shale, argillite, sandstone, siltstone, and sandy-to-silty and argillaceous limestone; generally intensely deformed. Basal beds are predominantly fine-grained gray sandstone and siltstone and subordinate cherty limestone that has black chert clasts (Bundtzen and others, 1997a). Age established by conodonts Negondolella polynathiformis and Epigondolella primitia and the occurrence of the pelecypod Monotis cf. M. subcircularis (Csejtey and others, 1992; Bundtzen and others, 1997a). Extensively exposed both north and south of the McKinley strand of the Denali Fault System. Distinguished from other Late Triassic carbonate units, such as the Chitistone and Nizina Limestones, by turbiditic depositional character, lack of evaporite or sabkha facies rocks, inclusion of clastic debris, and intense deformation. Rocks in eastern and southern exposures are metamorphosed to greenschist and amphibolite facies in the MacLaren metamorphic belt of Smith and Turner (1974), and we tentatively include unit sq of the MacLaren terrane of Nokleberg and others (1992a) in this unit. Metamorphosed sedimentary rocks in the Mount Hayes quadrangle of the Aurora Peak terrane of Nokleberg and others (1992a, unit as) are correlated with these rocks and are shown as such here. Associated with this unit are large dikes, sills, and plugs of altered diabase and gabbro

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 as
Unit name Metamorphosed sedimentary rocks
Description Chiefly of fine- to medium-grained, mylonitic, calc-schist, marble, quartz-mica schist, and quartzite. Protoliths thought to be marl, limestone, pelite, and sandstone. Unit intensely ductily deformed and schistosity dips steeply to vertically and strikes west-northwest. Exhibits evidence of multiple metamorphism, earlier at upper amphibolite facies and later at middle greenschist facies
Lithology Metamorphic

Correlated geologic units

Label Trcs
Description Fine- to medium-grained mylonitic metasedimentary rocks, calc-schist, marble, quartz-mica schist
Geologic age Silurian to Triassic
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Major
Calc-silicate-schist < Schist < Metamorphic Greenschist Major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Minor
Mica-schist < Schist < Metamorphic Greenschist Minor