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Source map Karl, S.M., 2013, Unpublished data.
Symbol Dl
Unit name Limestone turbidites
Description Dark gray, thin-bedded limestone and limestone turbidites. Unit includes graded limestone turbidite beds, and subordinate matrix-supported conglomerate, reef talus, and thinly (2-10 cm beds) interbedded black limestone and chert. Unit includes gray, medium-bedded calcarenite and calcareous wacke that have graded bedding and slump structures. Calcareous wacke contains calcite, shelly debris, feldspar, and subordinate chert, quartz, and volcanic rock fragments. The conglomerate includes poorly sorted angular to rounded clasts of limestone, graywacke, volcanic rock, and chert in a calcareous matrix. On eastern Chichagof Island unit includes 170 m of dark gray fine-grained, medium-bedded limestone that grades up to dark gray argillite, calcareous graywacke and argillite, and rare thin andesite flows, in the upper member of the Freshwater Bay Formation (Loney and others, 1963). On Carroll Island near the southern tip of Admiralty Island, the base of the unit gradationally overlies calcareous graywacke turbidites (Stg). The unit consists of gray, rhythmic thin-bedded calcareous turbidites that grade up to dark gray, thin- to medium-bedded limestone that contains corals. Limestone breccia layers locally scour into underlying fetid black limestone. The limestone breccia includes angular fragments of laminated and massive unsorted bioclastic limestone inferred to represent a reef breccia. The top of the unit is not exposed. Unit thickness is at least 100 m. On the north tip of the Cornwallis Peninsula of Kuiu Island, the unit consists of thin-bedded, dark-gray, brownish-gray weathering, fetid limestone that locally contains chert and minor laminae of gray calcareous siltstone, and is depositionally overlain by Carboniferous crinoidal limestone(Muffler, 1967). On Suemez Island, sideritic and dolomitic limestone form the lower member of the Port Refugio Formation, and contain Late Devonian plant fragments, brachiopods, and shark teeth. The limestone was deposited in a tropical shallow marine environment. The limestone is locally tuffaceous. The Port Refugio Formation unconformably overlies deformed volcanic and volcaniclastic rocks of the Silurian-Ordovician Descon Formation, and is conformably overlain by black argillite and chert of the Mississippian Peratrovich Formation
Lithology Sedimentary

Correlated geologic units

Label Dpr
Description Port Refugio Formation
Geologic age Famennian to Devonian
Geologic setting Undivided
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic Minor
Limestone < Carbonate < Sedimentary Bed Incidental