Freshwater Bay and Port Refugio Formations

Unit symbol: Dfr
Age range Late Devonian (382.7 to 358.9 Ma)
Lithology: Igneous - Volcanic
Group name: Basalt, andesite, and sedimentary rocks
Freshwater Bay Formation on Chichagof Island is composed of green and red andesite and basalt flows, breccia, and tuff, pyroclastic rhyolite deposits, minor amounts of interbedded conglomeratic volcanic graywacke, grayish-black argillite, and dark-gray limestone (Loney and others, 1963). The correlative but more sedimentary-rock-rich Port Refugio Formation on Prince of Wales Island consists of km-thick sections of siltstone, shale, volcanogenic graywacke, conglomerate, and minor limestone that alternate with km-thick sections of pillow basalt intercalated with minor chert, shale, limestone and aquagene tuff (Eberlein and others, 1983). Unit also includes the Coronados Volcanics and the Saint Joseph Island Volcanics found on western Prince of Wales Island and adjacent islands (Eberlein and others, 1983). The Port Refugio Formation may be a distal facies of the Freshwater Bay Formation. Eberlein and Churkin (1970, p. 43) stated that “many of the graywackes are largely reworked basaltic lavas that contain euhedral crystals of plagioclase and pyroxene that resemble the phenocrysts in the basaltic flows of the formation,” and that many of the conglomerate clasts are andesitic or basaltic rocks. Volcanic flows are found throughout the unit and are up to a hundred meters thick (Eberlein and Churkin, 1970). Age control from the Freshwater Bay is derived from included brachiopods, including Cyrtospirifer, mollusks, and corals of Frasnian (Late Devonian) age (Loney and others, 1975) and conodonts of Famennian (Late Devonian) age (Karl, 1999). Eberlein and Churkin (1970) reported Late Devonian “beautifully preserved” brachiopods that Savage and others (1978) assigned a middle to late Famennian age and that are associated with vascular plant fossils

Source map information

Source map Karl, S.M., 1999, Preliminary geologic map of northwest Chichagof Island, Alaska: U.S. Geological Survey Open-File Report 96-53, 21 p., 1 sheet, scale 1:63,360.
Symbol Df
Unit name Freshwater Bay Formation
Description Green and red andesite and basalt flows, breccia, and tuff, pyroclastic rhyolite, subordinate conglomerate, graywacke, black argillite and dark gray limestone. 500 m felsic volcanic deposits, including welded tuff and devitrified silicic agglomerate, are between 1000 m mafic flows. Unconformably overlies Cedar Cove and Kennel Creek Formations.
Lithology Sedimentary

Correlated geologic units

Label Dfb
Description Freshwater Bay Formation and Kasaan Island Volcanics, Basalt and rhyolite with subordinate sedimentary rocks, Middle to Late Devonian
Geologic age Late-Devonian
Geologic setting Extrusive
Lithology Form Importance
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Rhyolite < Felsic-volcanic < Volcanic < Igneous Pyroclastic Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Minor
Limestone < Carbonate < Sedimentary Bed Minor
Graywacke < Sandstone < Clastic < Sedimentary Bed Minor