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., Ellersieck, Inyo, and Tailleur, I.L., 1987, Reconnaissance geologic map of the Noatak C5, D5, D6 and D7 quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations I-1814, 1 sheet, scale 1:63,360.
Symbol MDl3
Unit name Limestone and dolomite, Kelly River Allochthon, Eli Sequence
Description Light-gray weathering carbonate rocks that are correlative with either part of the Mississippian Kogruk Formation or the Devonian part of the Baird Group.
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