Kuna Formation

Unit symbol: Mlgk
Age range Mississippian (358.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Kuna Formation (Lisburne Group)
Predominantly black siliceous mudstone and sooty, carbonaceous shale, including minor light-gray bioclastic limestone interbeds and concretions. Siliceous beds are rich in sponge spicules and radiolarians. Thin carbonate layers are chiefly dolomitic mudstone and calcified radiolarite. Sedimentological and faunal evidence suggests that the Kuna was deposited in a deep-water setting in which low oxygen conditions prevailed. Maximum thickness about 100 m (Mull and others, 1982). Conodonts from carbonate layers near base of type section in Howard Pass quadrangle are early middle Osagean, or approximately Middle Mississippian (Dover and others, 2004); conodont-bearing layers also contain rare cephalopods of Osagean and Meramecian (approximately Middle Mississippian) age. Siliceous beds in the uppermost Kuna yield radiolarians of Late Mississippian to Early Pennsylvanian age (Mull and Werdon, 1994; Dover and others, 2004). Unit is considered Mississippian, although an early Pennsylvanian age is locally possible for the uppermost beds. Unit is primarily exposed in the western Brooks Range, but as mapped here includes small exposures of similar rocks in northeast Alaska

Source map information

Source map Brosgé, W.P., and Reiser, H.N., 1969, Preliminary geologic map of the Coleen quadrangle, Alaska: U.S. Geological Survey Open-File Report 69-25, 1 sheet, scale 1:250,000.
Symbol Ms
Unit name Brooks Range sequence -- Siltstone
Description Black laminated silicified siltstone; chert; black shaly phosphatic limestone with horn corals. Silicified crinoidal limestone with Carboniferous or Permian bryozoans in upper part of unit. About 1,200 ft. thick.
Lithology Sedimentary

Correlated geologic units

Label Mlk
Description Lisburne Group and Kayak Shale, undivided
Geologic age Mississippian to Pennsylvanian
Geologic setting Undivided
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Minor