McCarthy Formation

Unit symbol: JTrmc
Age range Lower Jurassic and Upper Triassic (235 to 182.7 Ma)
Lithology: Sedimentary
Group name: Spiculitic rocks
Exposed in eastern Alaska, unit is divided into two informal members: an upper member of dominantly thin-bedded, very fine-grained spiculite, impure chert, impure limestone, and shale; and a lower member that is characteristically thin-bedded impure limestone, calcareous carbonaceous shale, and impure, locally spiculitic chert (MacKevett, 1978). Fossils in upper member indicate an Early Jurassic age, from Hettangian to Pliensbachian, and Monotis subcircularis, M. alaskana, and M. jakutica (Silberling and others, 1997) from the lower part of the lower member indicate a Late Triassic, mainly late Norian, age. Unit as a whole is deep marine and the depositional character indicates a restricted environment, possibly a starved basin. Unit also includes some rocks in west-central Alaska that are similar to the lower part of the McCarthy Formation (unit Trsl)

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 JTrm
Unit name McCarthy Formation
Description Chiefly rhythmically, thin- to medium-bedded, calcareous argillite and impure limestone, mainly spiculitic, skeletal, or intraclastic lime packstone. Estimated maximum thickness of a few hundred meters.
Lithology Sedimentary

Correlated geologic units

Label JTrmc
Description McCarthy Formation, argillite and impure limestone (Early Jurassic-Late Triassic)
Geologic age Carnian to Pliensbachian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Argillite < Metaclastic < Metasedimentary < Metamorphic Major
Shale < Mudstone < Clastic < Sedimentary Calcareous Major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Limestone < Carbonate < Sedimentary Calcareous Minor
Chert < Chemical < Sedimentary Bed Incidental