Kogruk Formation

Unit symbol: Clgk
Age range Upper Mississippian (330.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Tupik and Kogruk Formations (Lisburne Group)
Light-gray-weathering limestone and lesser light-brown weathering dolostone that contain less than 25 percent black chert nodules and lenses (Curtis and others, 1984; Ellersieck and others, 1984; Mayfield and others, 1990). Depositional thickness ranges from about 30 m in the southeastern part of Misheguk Mountain quadrangle to more than 300 m in southwestern part of Misheguk Mountain quadrangle. Base is gradational into the Utukok Formation (unit Mlgnu) (Curtis and others, 1984; Ellersieck and others, 1984). Common and abundant fossils are Late Mississippian corals, crinoids, brachiopods, foraminifers, and conodont (Sable and Dutro, 1961; Patton and Dutro, 1969, Armstrong and Mamet, 1977, Lane and Ressmeyer, 1985) reflecting shallow-water deposition. Includes deformed and metamorphosed limestone and marble on the Seward Peninsula (Till and others, 2011) and Saint Lawrence Island (Patton and others, 2011) that are thought to correlate with this unit. On the eastern part of Saint Lawrence Island, unit is composed of an upper and lower member. Upper member consists chiefly of light- to medium-gray, coarsely bioclastic limestone that contains interbedded limey mudstone in its upper part. Lower member is composed of dark-gray thin-bedded limestone that contains abundant dark chert nodules. Total thickness of the unit is estimated to be between 400 and 500 m. Unit is metamorphosed to a coarse-grained marble near the contacts with granitic plutons (Patton and others, 2011) on Saint Lawrence Island

Source map information

Source map Mayfield, C.F., Curtis, S.M., Ellersieck, Inyo, and Tailleur, I.L., 1990, Reconnaissance geologic map of the De Long Mountains A-3 and B-3 quadrangles and parts of A-4 and B-4 quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1929, 2 sheets, scale 1:63,360.
Symbol Mkoc3
Unit name Cherty limestone member, Kogruk Formation
Description Consists of medium-grained limestone with 25-50% black chert lenses and beds. Part of the Kelly River allochthon, Kelly sequence.
Lithology Sedimentary

Correlated geologic units

Label Mlkg
Description Lisburne Group, Kogruk Formation
Geologic age Mississippian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Chert < Chemical < Sedimentary Bed Incidental