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 Mull, C.G., Houseknecht, D.W., Pessel, G.H., and Garrity, C.P., 2004, Geologic map of the Umiat quadrangle, Alaska: U.S. Geological Survey Scientific Investigations Map 2817-A, scale 1:250,000. [http://pubs.usgs.gov/sim/2004/2817a/]
Symbol Kto
Unit name Torok Formation, upper part
Description Dark-gray mudstone and silty mudstone, with lesser amounts of greenish-gray, thin-bedded siltstone. In map area, poorly exposed, present only in axial zone of Big Bend anticline on Chandler River. Unit defined by: (Gryc and others, 1951, Patton, 1956)
Lithology Sedimentary

Correlated geologic units

Label Kto
Description Torok Formation, Aptian to Cenomanian
Geologic age Aptian to Cenomanian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Minor