Karheen and Cedar Cove Formations

Unit symbol: Dcc
Age range Devonian (419.2 to 372.2 Ma)
Lithology: Sedimentary
Group name: Karheen and Cedar Cove Formations
Karheen Formation has two facies: an upper shallow-water sandstone, shale and conglomerate facies, and a lower deep-water facies. According to R.B. Blodgett (written commun., 2014), the Karheen Formation, as used here, is much more complex than generally described. Much of it is Silurian in age and there is another unnamed unit likely included here. The shallow-water facies, about 1,800 m thick, contains minor well-bedded and penecontemporaneously deformed platy limestone in addition to its clastic components and is characterized by red beds, calcareous cement, festoon crossbedding, ripple marks, and mud cracks. Clasts are mainly volcanic rocks and chert, but pebbles to boulders of sedimentary rocks and felsic to mafic plutonic rocks are also present (Eberlein and others, 1983). Locally the limestone contains abundant brachiopods. Unconformably overlies Staney Creek unit of Eberlein and others (1983) and the Descon Formation. Eberlein and others (1983) reported that abundant detrital K-feldspar and bronze-colored biotite in the sandstone distinguishes the Karheen from older sandstone, but S.M. Karl (unpub. data, 2013) noted that, upon thin-section examination, detrital K-feldspar is not abundant in this facies. The deep water facies consists of matrix-supported debris-flow deposits, which were described by Gehrels (1992) as a sedimentary breccia composed of unsorted clasts of plutonic rocks and highly deformed volcanic, sedimentary and intrusive rocks that are as large as 50 cm in diameter. The clasts are moderately flattened, tectonically brecciated, and locally semischistose (Gehrels, 1992). Eberlein and Churkin (1970) described the deep-water facies as green-gray, gray, and reddish-brown lithic wacke and graywacke and minor siltstone; red, red-brown, and green shale; thin-bedded sandy limestone; contorted platy limestone; pebble-to-cobble polymictic conglomerate; and biostromal limestone and reef breccia. Sandstone and shale is commonly graded and contains festoon cross bedding, ripple marks, and mud cracks (Eberlein and Churkin, 1970). Latest Lower Devonian or earliest Middle Devonian (Pragian or younger) graptolites Monograptus pacificus are reported (Churkin and others, 1970). Eberlein and others (1983) also describe “graptolite and plant-bearing shale interbedded with graywacke, sandstone, and conglomerate.” The vascular plants preserved in this unit are the oldest plant fossils known in North America (Churkin and others, 1969). Eberlein and others (1983) also report subordinate andesitic volcanic rocks and that “the youngest beds of the sequence appear to be a 200 m thick section of interbedded andesitic flows, broken pillow breccia, and tuff.” These volcanic rocks are included in unit Dmv here. The Cedar Cove Formation, on Chichagof Island, consists of up to 900 m of thin-bedded argillite and minor limestone, turbiditic graywacke, and conglomerate. Conglomerate clasts include “volcanic rock, granite, alaskite, syenite, graywacke, quartz, chert, and limestone” (Loney and others, 1975, p. 10). Graywacke contains large pink K-feldspar, plagioclase, quartz, pyrite grains, and volcanic rock fragments (Loney and others, 1975; Karl and others, 1999). Locally, includes the limestone member of the Cedar Cove Formation (unit Dlse). Loney and others (1975) reported Middle Devonian to Frasnian (lower Upper Devonian) corals, stromatoporoids, brachiopods, and a trilobite. Karl (1999) reported Emsian (upper Lower Devonian) conodonts from the lower member of the Cedar Cove Formation

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 Dch
Unit name Cedar Cove Formation, hornfelsed
Description Biotite hornfels of Cedar Cove Formation adjacent to Devonian syenite plutons
Lithology Metamorphic

Correlated geologic units

Label Drl
Description Cedar Cove Formation
Geologic age Early-Devonian to Frasnian
Geologic setting Undivided
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Limestone < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major