Sedimentary member

Unit symbol: Trsy
Age range Upper Triassic (235 to 201.3 Ma)
Lithology: Sedimentary
Group name: Shuyak Formation, undivided
Thin- to medium-bedded lithic sandstone that contains lesser conglomerate, argillite, and siliceous tuff that has been intruded by mafic dikes and sills (Connelly and Moore, 1979). Unit is rich in primary andesitic material and displays flute casts and complete Bouma sequences, which indicate deposition by turbidity currents. Rocks either are broadly folded or dip homoclinally to southeast and have undergone prehnite-pumpellyite-facies metamorphism (Connelly and Moore, 1979). Pelecypod Halobia halorica of Late Triassic (Norian) age, as identified by N. Silberling, was reported by Connelly and Moore (1979). This sedimentary member of the Shuyak Formation is in fault contact with the volcanic member (unit Trsyv) which it is inferred to stratigraphically overlie (Connelly and Moore, 1979)

Source map information

Source map Connelly, William, and Moore, J.C., 1979, Geologic map of the northwest side of the Kodiak and adjacent islands, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-1057, 2 sheets, scale 1:250,000.
Symbol Trss
Unit name Shuyak Formation, sedimentary member
Description Volcaniclastic sequence consisting of thin- to medium-bedded lithic sandstone containing lesser conglomerate, argillite, and siliceous tuff intruded by mafic dikes and sills (unit Trm of Connelly, 1978).
Lithology Sedimentary

Correlated geologic units

Label Trss
Description Shuyak Formation, sedimentary member
Geologic age Late-Triassic
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Felsic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Minor
Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Incidental