Metavolcanic and metasedimentary rocks

Unit symbol: Trms
Age range Late Triassic (235 to 208.5 Ma)
Lithology: Metamorphic
Group name: Skolai and Mankomen Groups, undivided
Consists, in part, of the informal Amphitheatre group of Smith (1981), which ranges in western exposures from thinly stratified greenish volcaniclastic metasedimentary rocks, chert, and greenschist interbedded with basalt and dark carbonaceous argillite to thicker bodies of undifferentiated pelitic and volcaniclastic metasedimentary rocks. The thinly stratified volcaniclastic sedimentary rocks have common graded laminae, cross bedding, and other depositional textures. The thicker undifferentiated pelitic and volcaniclastic metasedimentary rocks typically show lateral change from argillite to thin tuffaceous interbeds (Smith, 1981; Smith and others, 1988). Unit also includes clayey and silty lenses and interbeds of microsparite that are locally carbonaceous, laminated, or fossiliferous; presence of Halobia cf. H. superba and Tropites sp. indicate a Late Triassic age. Marble contains structures suggestive of scleractinian colonial corals such as Thamnastraea(?). Smith (1981) also reported thickly bedded greenish flows that show numerous exposures of columnar jointing and pillows, and he suggested that these and their underlying rocks may be as old as late Paleozoic. Eastern exposures, such as in the Mount Hayes and Nabesna quadrangle, are chiefly fine-grained quartz–white-mica schist, chlorite-calcite-quartz schist, marble, greenstone, and sparse metadacite that exhibits a weak schistosity that strikes northeast to east and dips moderately to steeply northward (Nokleberg and others, 1992a). This map unit is contiguous with rocks of the Permian to Pennsylvanian Mankomen and Skolai Groups (unit PIPms and subunits), but available data does not allow distinction at map scale)

Source map information

Source map Smith, T.E., 1981, Geology of the Clearwater Mountains, south-central Alaska: Alaska Division of Geological and Geophysical Surveys Geologic Report 60, 73 p., 1 sheet, scale 1:63,360.
Symbol Tras
Unit name Amphitheatre group
Description Thicker bodies of undifferentiated pelitic and volcaniclastic sedimentary rocks in lower part of volcanic section. These subunits typically show lateral change from argillite outward into thin tuffaceous interbeds
Lithology Sedimentary

Correlated geologic units

Label Trnm
Description Metavolcanic, metavolcaniclastic, and subordinate metasedimentary rocks, Late Triassic, late Norian
Geologic age Norian
Geologic setting Extrusive
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Minor
Slate < Metaclastic < Metasedimentary < Metamorphic Minor