Fortress Mountain Formation

Unit symbol: Kfm
Age range Cretaceous, Albian (125 to 93.9 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of the North Slope
Cyclic marine and nonmarine units of polymictic conglomerate, dusky yellow-green to dark-gray lithic wacke, dark-gray siltstone and shale; rocks in upper cycles generally finer grained and thinner bedded than those in lower cycles (Brosgé and others, 1979). Fortress Mountain includes nonmarine, shallow marine, marine slope, and deep basin facies. It contains lenticular and crossbedded beds, shows locally prominent pebble imbrication, rip-up clasts, scarce mud cracks in thin, discontinuous mudstone intervals, and has plant debris ranging from small carbonized wood to coalified logs. Conglomerate clasts include varieties of chert, varieties of mafic igneous rocks, limestone, argillite, organic shale, and granitic rocks. Conglomerate is also interbedded with bioturbated marine sandstone and with sandstone that shows local ripple and wave crossbedding, gravel lenses, abundant wood debris and, locally, marine mollusks (Kelley, 1990a). Common turbidite features in the marine and deep marine facies are graded bedding, sole marks, and flute, groove, and striation casts. Very small-scale crossbedding is common in most wacke beds. Rocks are texturally and compositionally immature and have clasts of chert, quartz, claystone, carbonaceous and kerogenous rocks, igneous rocks, and carbonate rocks in a matrix of chlorite, calcite, quartz, and clay minerals. Thickness is as much as 1,300 m (Brosgé and others, 1979, Sable and Mangus, 1984). Includes Mount Kelly Graywacke of Mull (1985), which is divided into upper and lower parts. The upper part consists of fine- to medium-grained, dark brownish-gray to greenish-gray sandstone that contains interbedded, poorly exposed, slightly micaceous silty shale; unit generally forms poorly exposed, low, rubble-covered ridges. The lower part of the Mount Kelly Graywacke consists predominantly of fine- to medium-grained sandstone and some coarse-grained, medium gray-green to brown sandstone, interbedded with poorly exposed dark-gray silty shale and local conglomeratic channels and contains abundant carbonaceous plant material on the top of some beds (Mull and others, 2000). Mull and others (2000) posit that the Mount Kelly Graywacke’s abundant detrital muscovite and carbonate grains suggest a provenance in the southern Brooks Range schist belt (units D<aqm and D<acs); T.H. Nilsen, (consulting geologist, written commun., 1996, cited in Mull and others, 2000) suggests a storm-dominated shelf setting, mainly on the basis of wave-generated sedimentation features. Houseknecht and Wartes (2013) suggest that the Fortress Mountain Formation spans the boundary between an orogenic wedge (Okpikuark Formation) and foredeep, with proximal strata onlapping the tectonic wedge-front and distal strata downlapping the floor of the foreland basin. In the Noatak, De Long Mountains, and Misheguk Mountain quadrangles Inoceramus and Aucellina (Mayfield and others, 1987; Curtis and others, 1990) and rare ammonites of Albian age (Sable and others, 1984a) provide the age control for the Fortress Mountain Formation, as do Albian pelecypods collected in the Philip Smith Mountains quadrangle (Brosgé and others, 1979)

Source map information

Source map Sable, E.G., Mangus, M.D., Morris, R.H., and Dutro, J.T., Jr., 1984, Geologic map of the east-central Kukpowruk-Nuka Rivers region, northwestern Alaska: U.S. Geological Survey Miscellaneous Field Investigations Map MF-1670, 2 sheets, scale 1:63,360.
Symbol Kfw
Unit name Fortress Mountain Formation, wacke and conglomerate member
Description Roughly 50 percent greenish-gray wacke and granule to pebble wacke conglomerate interbedded with siltstone and mudstone. Thickness estimated to range from 50 to 1000 m. Common turbidite features are graded bedding, sole marks and flute, groove, and striation casts. Very small-scale crossbedding common in most wacke beds. Rocks are texturally and compositionally immature, with clasts of chert, quartz, claystone, carbonaceous and kerogenous rocks, igneous rocks, and carbonate rocks in a matrix of chlorite, calcite, quartz and clay minerals
Lithology Sedimentary

Correlated geologic units

Label Kfm
Description Fortress Mountain Formation, interbedded wacke and mudstone
Geologic age Albian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Minor