Akmalik Chert and other black chert of the Lisburne Group

Unit symbol: Mlgac
Age range Mississippian (358.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Akmalik Chert and other black chert of the Lisburne Group
Bedded black chert in beds as much as 10 cm thick, having thin, black siliceous shale partings; locally contains barite deposits and rare interbeds of calcareous radiolarite. In the northwest Howard Pass quadrangle, unit includes abundant interbeds 2–7 cm thick of brownish-black dolostone. These rocks formed in a deep-water, basinal setting. Chert contains abundant radiolarians and lesser sponge spicules. Radiolarians are chiefly Late Mississippian but locally may be as old as late Early Mississippian (Blome and others, 1998; Dover and others, 2004). In the Killik River quadrangle, unit consists of bedded black chert that contains finely disseminated pyrite in beds up to 10 cm thick and has thin siliceous shale partings; contains two laterally persistent thin micritic limestone beds up to 1 m thick near base. Locally includes underlying thin Kayak Shale, which is generally poorly exposed and not mappable at scale of map. Contains Osagean (Middle Mississippian) to Morrowan (lowest Pennsylvanian) or younger conodonts; in Howard Pass quadrangle, contains Osagean conodonts. Thickness about 75 m (Mull and others, 1994). In most areas, units explicitly called the Akmalik Chert are restricted to the Picnic Creek allochthon (see for example, Mull and Werdon, 1994 or Mull and others, 1994). As shown here, unit also includes small exposures of black chert in the Arctic and Table Mountains quadrangles of northeast Alaska

Source map information

Source map Curtis, S.M., Ellersieck, Inyo, Mayfield, C.F., and Tailleur, I.L., 1984, Reconnaissance geologic map of southwestern Misheguk Mountain quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1502, 2 sheets, scale 1:63,360.
Symbol IPMc4
Unit name Black Chert
Description Part of the Ipnavik Sequence - Well-bedded chert with a few shale partings. May contain minor amounts of interbedded limestone and black chert. Contains persistent mafic sills and dikes as much as 100 m thick. Contains Late Mississippian foraminifers of Mamet Zone 11 or younger. Upper part may be Pennsylvanian on the basis of stratigraphic correlation with black chert in structurally lower sequences. Intertongues with black chert and limestone (unit PMcl4). Depositional thickness is estimated to be less than 100 m. Basal contact is sharp on the Kayak Shale.
Lithology Sedimentary

Correlated geologic units

Label IPMlc
Description Lisburne Group, Akmalik Chert
Geologic age Mississippian to Late-Mississippian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Minor
Shale < Mudstone < Clastic < Sedimentary Bed Incidental