Kugururok Formation and Eli Limestone

Unit symbol: Dke
Age range Devonian (393.3 to 358.9 Ma)
Lithology: Sedimentary
Group name: Baird Group and similar rocks
Kugururok Formation consists of three informal lithologic members: (1) a lower, dominantly clastic unit of shale that has with interbedded sandstone, granule conglomerate, siltstone and limestone; (2) a middle calcarenite unit containing some conglomeratic limestone, sparse chert lenses and dolomitic(?) sandstone; and (3) an upper, light-colored, laminated to cross-bedded dolomite unit. Thickness at the type section in the Misheguk Mountain quadrangle is 1,370 ft (417 m) and elsewhere over 2,000 ft (610 m). Age is Late Devonian (Frasnian and Famennian) based on paleontological evidence (Sable and Dutro, 1961). “Eli Limestone is yellow-brown to orange weathering, medium- to dark-gray, fine- to coarse-grained, commonly argillaceous limestone and lesser dolostone; beds are planar to irregular and bioturbated and rock types range from lime mudstone to bioclastic packstone. Conodonts denote an age of late Middle or early Late to late Late Devonian (late Givetian or Frasnian to Famennian); it also contains brachiopods of Late Devonian (possibly mid-Famennian) age” (Till and others, 2008a). These units were “deposited in a range of shallow-water, inner to middle shelf environments; the shallowest and most restricted depositional regimes prevailed during the late Middle Devonian. * * * Middle Devonian carbonate rocks correlate at least in part with youngest part of Baird Group * * *” (Till and others, 2008a). As originally defined, these units were part of the Baird Group: however, as they are younger than any part of the Baird Group type section and, unlike the Baird Group, are not metamorphosed, they were excluded from the Baird Group by Till and others (2008a) and are part of the Eli River sequence of Till and others (2008a). Unit also includes Devonian limestone mapped in the Arctic (Brosgé and others, 2000), Table Mountain (C.G. Mull and H.S. Sonneman, Exxon, unpub. report, 1968–1974), and Coleen (Brosgé and Reiser, 1969) quadrangles, and dolostone and dolomitic limestone on Saint Lawrence Island (Patton and others, 2011)

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 Dbs3
Unit name Limestone and shale, Baird Group, upper part
Description Gray shale interbedded with light-gray- to buff-weathering limestone. Most limestone is fine grained and weathers to platy and flaggy fragments. Part of the Kelly River allochthon, Kelly sequence.
Lithology Sedimentary

Correlated geologic units

Label Dbe
Description Baird Group, Eli Limestone
Geologic age Middle-Devonian to Late-Devonian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Dolostone < Carbonate < Sedimentary Bed Minor
Shale < Mudstone < Clastic < Sedimentary Bed Minor