Torok Formation

Unit symbol: Kto
Age range Cretaceous, Cenomanian to Aptian (125 to 93.9 Ma)
Lithology: Sedimentary
Group name: Sedimentary rocks of the North Slope
Thick sequence of dominantly nonresistant, dark-gray to black silty shale, mudstone, and clay shale that has interbedded thin-bedded siltstone and lesser amounts of greenish-gray, thin-bedded siltstone and fine-grained sandstone as thick as 5,700 m (Mull and others, 2003). Unit is generally exposed only in discontinuous stream cutbanks, where most exposures are tightly folded; lower part of unit interfingers with Fortress Mountain Formation (Mull and others, 1994). Upper part of the Torok Formation is age equivalent with the Nanushuk Formation (unit Knf here). Locally, in the western Brooks Range, the Torok and Fortress Mountain Formations are mapped undivided, where they consist of at least 500 m of dark-gray shale, subgraywacke and wacke sandstone lithologically similar to the Torok Formation and to the sandstone member of the Fortress Mountain Formation, but contain intermediate proportions of those rocks. This may represent a relatively proximal facies of the Torok Formation (Sable and Mangus, 1984, Sable and others, 1984a, b, c). The Torok Formation is a thick slope-to-prodelta deposit derived from a western source that is gradational and interfingers with both overlying and underlying units (Mull and others, 2000; 2009). The Torok is relatively incompetent and is generally poorly exposed; it acts as a detachment surface for decollement folding of the overlying competent Nanushuk Formation (Mull and others, 2009)

Source map information

Source map Dover, J.H., Tailleur, I.L., and Dumoulin, J.A., 2004, Geologic and fossil locality maps of the west-central part of the Howard Pass and part of the adjacent Misheguk Mountain quadrangles, western Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2413, pamphlet, 75 p., 3 map sheets, scale 1:100,000.
Symbol Ktfm
Unit name Torok Shale and Fortress Mountain Formation, undivided
Description Undifferentiated and regionally interfingering units of the Torok Shale, characterized by folded, thinly interbedded, black clay shale and silty shale; and the Fortress Mountain Formation, consisting predominantly of greenish-gray graywacke, interbedded with dark-gray mudstone and shale, and massive beds of pebble to cobble-conglomerate containing clasts of variegated chert and altered mafic igneous rocks.
Lithology Sedimentary

Correlated geologic units

Label Ktfm
Description Torok and Fortress Mountain Formations, undivided, Aptian to Cenomanian
Geologic age Aptian to Albian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major