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Source map Karl, S.M., 2012, Unpublished data.
Symbol Dsv
Unit name Gambier Bay Formation
Description In Chilkat Mountains, includes medium green actinolite schist to dark green garnet amphibolite. Discontinuous quartz segregations to 15 cm in thickness. Locally intercalated with up to 30% argillaceous marble and minor metachert. Locally contains gray pelitic schist and intervals of felsic schist to 3 m thick. Isoclinally folded. In Chilkat Lake area, includes amphibolite, greenschist, and greenstone. Includes subordinate black phyllite, graphitic metasiltstone, siliceous marble, metafelsite, and metavolcanic breccia
Lithology Metamorphic

Correlated geologic units

Label Dgg
Description Gambier Bay Formation, pyrite-bearing garnet-chlorite-amphibole-quartz-feldspar schist and gneiss
Geologic age Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Gneiss < Metamorphic Major
Quartz-feldspar-schist < Schist < Metamorphic Major