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 Ellersieck, Inyo, Curtis, S.M., Mayfield, C.F., and Tailleur, I.L., 1984, Reconnaissance geologic map of south-central Misheguk Mountain quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1504, 2 sheets, scale 1:63,360.
Symbol IPMcl4
Unit name Black Chert and Limestone
Description Part of the Ipnavik Sequence - Interbedded black chert and fine-grained limestone. Beds range from 1 to 20 cm thick. Rare sponge spicules and crinoids in limestone. Contains persistent mafic sills and dikes. Age based on stratigraphic correlation and intertonguing relation with black chert (unit PMc4). Depositional thickness is estimated to be less than 100 m. Basal contact is sharp on the Kayak Shale.
Lithology Sedimentary

Correlated geologic units

Label Mlku
Description Lisburne Group, Kuna Formation, also includes black carbonaceous metachert, quartzite, and siliceous or calcareous phyllite, commonly distinctive silvery blue (phosphatic) bloom on weathered surfaces
Geologic age Mississippian to Late-Mississippian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Major
Limestone < Carbonate < Sedimentary Bed Incidental