Kongakut Formation

Unit symbol: Kgk
Age range Lower Cretaceous, Hauterivian and younger (132.9 to 100.5 Ma)
Lithology: Sedimentary
Group name: Okpikruak and Kongakut Formations
Upper part mostly dark-gray to black, locally manganiferous pebbly shale (pebble shale member) and dark-olive-gray brownish-gray-weathering siltstone (siltstone member) that contains minor interbedded sandstone, as shown by Reiser and others (1980) and Brosgé and others (1979). Lower part is mostly dark-gray fissile shale (“clay shale member” of Reiser and others, 1980) that has nodules and lenticular beds of red-weathering clay ironstone and is overlain by thin interval of quartz arenite, the Kemik Sandstone Member (shown here, where possible, as unit Kke). Thickness is more than 300 m. Unit is restricted to northeast Alaska and is thought to be lateral (northern) equivalent of Okpikruak Formation (Brosgé and others, 1979). Unit contains Early Cretaceous (Neocomian) pelecypods. Pebble shale unit, which is sometimes mapped separately (see, for example, Bader and Bird, 1986), is thin-bedded, tan to dark-brown, gray-weathering pebbly silty shale, siltstone, sandstone, and quartzite. Worm borings are locally abundant (Reiser and others, 1971). It is distinctive, as a non-bentonitic shale about 100 m thick that contains matrix-supported, well-rounded, frosted quartz sand grains and common to rare chert and quartzite pebbles (Bader and Bird, 1986). In the Ignek Valley region, the lower part of the Kongakut Formation was subsequently reassigned (Molenaar, 1983) to the Kingak Shale (unit KJks here), extending the age range of the Kingak Shale into the Early Cretaceous (Valanginian)

Source map information

Source map Reiser, H.N., Brosge, W.P., Dutro, J.T., Jr., and Detterman, R.L., 1971, Preliminary geologic map, Mt. Michelson quadrangle, Alaska: U.S. Geological Survey Open-File Report 71-237, 2 sheets, scale 1:200,000.
Symbol KJ
Unit name Pebbly Shale
Description Pebbly silty shale, siltstone, sandstone and quartzite; thin bedded; weathers gray, tan, dark brown. Minor ironstone concretions and orange-weathering ironstone beds. Worm borings locally abundant.
Lithology Sedimentary

Correlated geologic units

Label Kps
Description Pebble Shale unit
Geologic age Hauterivian to Albian
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major