Okpikruak Formation and similar units

Unit symbol: Kof
Age range Lower Cretaceous (145 to 100.5 Ma)
Lithology: Sedimentary
Group name: Okpikruak and Kongakut Formations
Dark-gray to grayish-tan mudstone, siltstone, graywacke sandstone, and minor conglomerate. Locally, unit contains interbeds of distinctive reddish-gray coquinoid limestone, is intensely deformed and, in places, the base is structurally detached. Thickness is unknown. Locally, this unit has been mapped as mélange or olistrostrome between thrust sheets (see, for example, Curtis and others, 1990; Mayfield and others, 1990). The Okpikruak Formation has been dated by fossils to be as young as Valanginian (see, for example, Dover and others, 2004). Locally, the Fortress Mountain Formation is not separated in mapping (Curtis and others, 1990; Mayfield and others, 1990) and is included here; where included it consists primarily of the deep marine facies of the Fortress Mountain and, as discussed above, is also mapped separately when possible (unit Kfm). In the Howard Pass quadrangle, Dover and others (1994) describe a unit of predominantly volcaniclastic rocks and subordinate associated mafic to intermediate volcanic rocks to which they assigned a Jurassic and Early Cretaceous age. They further indicate that it has some lithologic similarities to the Okpikruak Formation in other thrust sequences and has a similar degree of induration. We include it here with some uncertainty. As shown here, this map unit, Kof, includes the Kisimilok Formation of Campbell (1967). The eastern exposures of Okpikruak Formation we show in the Noatak quadrangle (C.G. Mull and H.S. Sonneman, Exxon unpub. report, 1968–1974) may be better assigned to the mélange of the Angayucham assemblage (unit KJm here). As shown here, the Okpikruak Formation also includes small areas of sedimentary rocks in the De Long Mountains, Howard Pass, and Misheguk Mountain quadrangles that most likely belong to the Okpikruak Formation. The Kongakut Formation (unit Kgk) was originally defined by Detterman and others (1975) as a lateral (northern) equivalent of Okpikruak Formation. Also included in this unit is the undivided Arctic Foothills assemblage of Kelley (1990a) in the Chandler Lake quadrangle, which was described as follows: “consists of 7 previously recognized units; Lower Cretaceous coquinoid limestone, undivided Upper Jurassic and Cretaceous strata, Jurassic mafic igneous rocks, Permian and Triassic chert, the Nuka Formation (Carboniferous), marble and mélange.” The unit includes an indeterminate proportion of Cretaceous rocks. As mapped, Kelley’s (1990a) Arctic Foothills assemblage is thrust interleaved with rocks of the undivided Torok and Fortress Mountains Formations (unit Kft) and thrust over rocks of the Otuk Formation (JTro) and Lisburne Group (Clg ). It is interpreted by some (for example, Peapples and others, 2007; Mull and others, 2009; D.W. Houseknecht, written commun., 2014) as part of the Okpikruak mélange

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 Ko3
Unit name Okpikruak Formation, Kelly River Allochthon, Amphitheatre Sequence
Description Interbedded gray to brown, fine- to medium-grained wacke and gray mudstone. Early Cretaceous age inferred from regional stratigraphic correlations. Regionally, is inferred to have an unconformity at base; locally may be conformable on shale of Etivluk Group
Lithology Sedimentary

Correlated geologic units

Label KJo
Description Okpikruak Formation and may include Fortress Mountain Formation, interbedded wacke and mudstone
Geologic age Early-Cretaceous to Albian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone-Mudstone < Mixed-clastic < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major