Nasorak and Utukok Formations

Unit symbol: Mlgnu
Age range Mississippian (358.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Nasorak and Utukok Formations (Lisburne Group)
These formations represent the lowest part of the Lisburne Group in northwestern Alaska. The Nasorak is the more western of the two units, which are generally equivalent in age and stratigraphic position and is divided into three members. Upper member of the Nasorak Formation is about 550 m thick and is characterized by rhythmically interbedded thin- to medium-bedded dark-gray limestone and by thin-laminated to very thin-bedded silty calcareous shale. Shale interbeds decrease both in abundance and thickness progressively upward through the member. The Cape Thompson Member, about 70 m thick, is the middle member of the Nasorak Formation and consists of massive light-gray limestone, chiefly a crinoid biosparite that is almost entirely coarse sand- to fine-pebble-sized crinoid stem fragments and columnals. Unit locally contains very minor very fine-grained quartz silt (Campbell, 1967). Lower member consists of 50 m of interbedded dark-gray to grayish-black, locally calcareous, silty clay shale and medium-gray to dark-gray cherty limestone. Dark-gray limestone is predominantly medium-to coarse-grained biomicrite (Campbell, 1967). The Utukok Formation is a buff-weathering limestone and fine-grained, locally calcareous sandstone, locally as thick as 1,400 m in the western De Long Mountains quadrangle and possibly structurally thickened (Sable and others, 1984a, b, c). Elsewhere the Utukok is typically less than 100 m thick and may represent a thin, discontinuous tongue below the Kogruk Formation (unit Clgk), or may have not been deposited locally within this sequence. Base is probably gradational into Devonian limestone (Dover and others, 2004). Upper part contains light-gray, medium-bedded ferruginous sandy limestone, which weathers to a distinct dark-yellowish-brown and yellowish-orange rust color and is commonly blocky-weathering on talus slopes. Lower part contains sandy limestone, calcareous siltstone, shale, and fine-grained sandstone, less resistant to erosion than the Kogruk Formation (unit Clgk) or Baird Group (unit DCbg), thus commonly forms saddles or recessive zones. Contains Late Mississippian foraminifers, conodonts (Dumoulin and others, 2004, 2006); common megafossils are crinoids and brachiopods and locally contains abundant spiriferoid brachiopods, gastropods, pelecypods, cephalopods, trilobites, and crinoidal debris of Early Mississippian age (Sable and others, 1984a, b, c; Mayfield and others, 1987; Dutro, 1987)

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 Mu2
Unit name Utukok Formation
Description Approximately 60-70% buff-weathering limestone of sandy limestone and 30-40% interbedded shale. Part of the Picnic Creek allochthon, Amaruk sequence.
Lithology Sedimentary

Correlated geologic units

Label Mlut
Description Lisburne Group, Utukok Formation - Limestone, Mississippian to Devonian
Geologic age Early-Mississippian to Late-Mississippian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Calcareous Minor
Siltstone < Clastic < Sedimentary Bed Minor
Shale < Mudstone < Clastic < Sedimentary Bed Minor