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 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 Trcs
Unit name Calcareous sedimentary rocks
Description A generally thin-bedded, commonly cross-bedded, carbonaceous, dark- to medium-gray, marine, intensely deformed thick sequence of intercalated calcareous shale, argillite, sandstone, siltstone, and sandy to silty and argillaceous limestone. Sequence also contains numerous large dikes, sills as much as several tens of meters thick, and small plugs of altered diabase and gabbro. Unit locally metamorphosed (east and south). Turbidites and intercalated hemipelagic sedimentary rocks
Lithology Sedimentary

Correlated geologic units

Label Trcs
Description Calcareous sedimentary rocks, Late Triassic, middle(?) Norian and Late Karnian
Geologic age Carnian to Norian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Argillite < Metaclastic < Metasedimentary < Metamorphic Major
Limestone < Carbonate < Sedimentary Calcareous Major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Major
Sandstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Siltstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Minor