Nanushuk Formation

Unit symbol: Knf
Age range Cretaceous, Cenomanian to Albian (113 to 93.9 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of the North Slope
Consists of a regressive depositional sequence as thick as 2,750 m that includes marine, transitional, and nonmarine intervals (Alhbrandt and others, 1979) and has the most extensive exposure area of any unit in northern Alaska. Upper part consists of numerous thick horizons of typically nonmarine fine- to medium-grained and, locally, coarse-grained, gray to light-gray lithic arenite and quartz- and chert-pebble conglomerate interbedded with poorly exposed dark-gray carbonaceous shale and coal. Lower part is dominantly marine, greenish-gray, very fine- to fine-grained and locally fossiliferous sandstone and minor conglomerate. Alhbrandt (1979) reported that paleo-transport data from outcrops as well as from seismic and dipmeter data in wells on the North Slope indicate a generally northeast progradation of the Nanushuk, away from the Brooks Range. He hypothesized at least two river-dominated deltas for the onshore Nanushuk: “a western Corwin delta and, in the central North Slope, the Umiat delta.” Huffman (1985) described the slightly older (early Albian or late Aptian? origin) Corwin delta as prograding from west to east onto a shelf consisting of fine-grained sediment of the Torok Formation. The Umiat delta developed in mid-Albian time and has an elongate to lobate form that prograded northward from the vicinity of the Endicott Mountains (Huffman, 1985). Having different source areas, the deposits of the two deltas are compositionally different: the Corwin delta sourced sedimentary rocks, shale, limestone and chert, whereas the Umiat delta sourced metamorphic rocks of the central Brooks Range (Huffman, 1985) that includes detrital blueschist facies minerals (Till, 1992). Huffman (1985) reported that petrographic analyses indicate sediment of the Torok Formation is closely related to the sources of the Corwin delta and quite different than the Umiat delta. LePain and others (2009) report a detailed study in the central part of the North Slope that led to recognition of 20 facies in the unit that are combined “to form ten facies associations, including (1) offshore–prodelta, (2) storm-influenced shoreface, (3) distributary channel and mouth-bar successions, (4) tidal inlet, (5) bayfill–estuarine, (6) crevasse channel, (7) crevasse splay, (8) sandy fluvial channel fill, (9) conglomeratic fluvial sheet, and (10) alluvial flood basin successions. Facies associations 1, 2, and 3 record deposition in open marine settings; facies associations 4 and 5 record deposition in open marine and marginal-marine settings; facies associations 6 and 7 are interbedded in both marginal-marine and nonmarine deposits of the bayfill–estuarine association and alluvial flood basin associations, respectively; facies associations 8, 9, and 10 record deposition in nonmarine settings. The abundance of storm-wave-generated structures, such as hummocky and swaley cross-stratification in marine deposits, demonstrates deposition in high-energy, storm-wave-modified deltas and associated inter-deltaic shoreface settings.” Unit “is time transgressive, becoming younger to the north and northeast away from the Brooks Range” (Alhbrandt and others, 1979). The Nanushuk in northwestern Alaska consists of eastward-prograding deltaic rocks that overlie and interfinger with the Torok Formation and, together, the Torok and the Nanushuk prograde across the Colville Basin (Mull and others, 2000). Abundant and varied megafauna of pelecypods and lesser gastropods and ammonites are reported. Plant fossils common in nonmarine beds and microfauna recovered from shale. Although unit ranges in age, it is primarily Albian (Mull and others, 2003). Includes former Kukpowruk, Tuktu, Grandstand, Corwin, Chandler, and Ninuluk Formations of northern Alaska, all now abandoned

Source map information

Source map Brosge', W.P., Reiser, H.N., Dutro, J.T., Jr., and Detterman, R.L., 1979, Bedrock geologic map of the Phillip Smith Mountains quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-879-B, 2 sheets, scale 1:250,000.
Symbol Knt
Unit name Tuktu Formation
Description Gray; fine to very fine grained; thin to medium-bedded quartz wacke; ripple marked and crossbedded; interbedded micaceous siltstone and shale more abundant in lower part. Flute casts, load casts and drag marks common. Marine. Early Cretaceous pelecypods, and cephalopods. Thickness 150m
Lithology Sedimentary

Correlated geologic units

Label Knl
Description Nanushuk Formation, lower part
Geologic age Albian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Minor