Iyoukeen and Peratrovich Formations

Unit symbol: MDip
Age range Mississippian and Devonian (393.3 to 323.2 Ma)
Lithology: Sedimentary
Group name: Iyoukeen and Peratrovich Formations
Iyoukeen Formation (Loney and others, 1963) consists of three informal members: lower member is 130 m of thin-bedded, dark-gray limestone that contains thin dark-gray chert innerbeds; middle member is about 200 m of dark-gray, sparsely fossiliferous, partly calcareous shale; upper member is as much as 1,000 m of medium-bedded, dark-gray, fossiliferous limestone that contains dark-gray chert nodules. Fossils indicate shallow, warm, marine environment. Lower and middle members have Early Mississippian conodonts, corals, brachiopods and gastropods, whereas upper member contains conodonts of Late Mississippian age (Karl, 1999). Peratrovich Formation (Eberlein and Churkin, 1970) also has three informal members: lowest member consists of about 60 m of thin-bedded grayish-black chert that contains rare lenses of medium-dark-gray, aphanitic limestone, dolostone, and crinoidal limestone; middle member about 120 m thick, is mainly medium- to thick-bedded, massive, medium-gray limestone and grayish-black chert nodules and lenses which may form up to 25 percent of the middle member; uppermost member is about 75 to 90 m of thick-bedded massive limestone and minor dolomitic limestone that is as much as 25 percent black chert nodules and lenses. Limestone is composed largely of echinoderm, bryozoan, and foraminiferal fragments and oolites (Churkin and Eberlein, 1975). Peratrovich Formation contains abundant shelly fossils, especially rugose corals (Eberlein and others, 1983), which indicate an age range of Middle to Late Mississippian for the Peratrovich Formation. Unit also includes the black chert and limestone member of the Saginaw Bay Formation, which consists of thin-bedded black chert and minor thin-bedded dense dark-gray limestone (Muffler, 1967). Muffler (1967) was uncertain about the nature of the contact between this unit and the other members of the Saginaw Bay Formation because outcrops are poor, and the rocks locally exhibit complex folds. The upper members of the Saginaw Bay Formation are of Pennsylvanian age (unit IPsb), whereas the lower volcanic rocks are of Lower and Middle Devonian age. The age of the black chert is not known directly; volcanic rocks yielded earliest Late to latest Early Devonian conodonts (Dutro and others, 1981; Brew and others, 1984)

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 k
Unit name Iyoukeen Formation with karst
Description Karst and epikarst features, such as solution pits and fissures, mapped within gray weathering, dark gray, fossiliferous, petroliferous limestone and carbonaceous shale. Unconformably overlies Freshwater Bay Formation. Lower member is 130 m thin-bedded dark gray limestone with thin dark gray chert beds. Middle member is about 200 m of dark gray sparsely fossiliferous, partly calcareous shale. Upper member is up to 1000 m of dk gray medium bedded fossiliferous limestone with dark gray chert nodules. Fossils indicate shallow warm marine environment.
Lithology Sedimentary

Correlated geologic units

Label Mls
Description Iyoukeen and Peratrovich Formations, limestone, black chert, and black shale
Geologic age Mississippian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Black-shale < Shale < Mudstone < Clastic < Sedimentary Bed Major