Gambier Bay Formation, undivided

Unit symbol: Dgb
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Metamorphic
Group name: Gambier Bay Formation, undivided
Medium-green actinolite schist, semischist, and dark-green garnet amphibolite that was first named by Loney (1964) on Admiralty Island. Unit also contains subordinate gray pelitic schist, calc-schist, chloritic quartzite, and felsic schist and marble. On Kupreanof Island, unit includes chlorite phyllite, schist, and semischist, graphitic schist, siliceous sericite schist, chloritic calc-schist, greenstone, marble, and meta-limestone. Subordinate light-tan to gray quartz-sericite semischist contains quartz porphyroblasts in a groundmass of feldspar, sericite, and pyrite and may have a quartz-porphyritic volcanic protolith. Greenstone semischist locally retains relict primary structures, including vuggy volcaniclastic textures and marble lenses and clasts. Metamorphic mineral assemblages include chlorite-epidote-calcite, quartz-muscovite-chlorite-albite, quartz-talc-calcite, and quartz-albite-muscovite-chlorite, which indicate greenschist-grade metamorphism (S.M. Karl, unpub. data). Unit thickness is unknown. On Kupreanof Island, Karl and others, (1999) report brachiopods from the late Emsian and Eifelian (latest Early to Middle Devonian), conodonts from the late Emsian to Late Devonian, and corals from the Silurian to Middle Devonian; together, the fossils have been interpreted to indicate a unit age of Middle Devonian. Unit yields Permian metamorphic ages (S.M. Karl, unpub. data). Locally, marble separated as map unit Dgbm

Source map information

Source map Karl, S.M., Haeussler, P.J., Himmelberg, G.R., and Brew, D.A., 1998, Unpublished data.
Symbol Pzcs
Unit name Chloritic schists
Description Green and gray mixed chlorite-epidote-albite schists and semischists, with subordinate micaceous marble, sericitic quartzite, dark gray graphitic schist. dominantly sedimentary, volcaniclastic protoliths. Marble is tan to gray, with chlorite and sericite, up to 30 m thick. Also may contain pyrite, titanite, tourmaline, rutile, apatite, zircon.
Lithology Metamorphic

Correlated geologic units

Label Dgp
Description Gambier Bay Formation, dark gray locally pyritic and graphitic semischist and phyllite; some quartz-rich semischist; black siliceous phyllite on Admiralty Island
Geologic age Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Major
Schist < Metamorphic Major