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 Gillis, R.J., Decker, P.L., Wartes, M.A., Loveland, A.M., and Hubbard, T.D., 2014, Geologic map of the south-central Sagavanirktok Quadrangle, North Slope, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2014-4, 24 p., 2 sheets, scale 1:63,360. doi:10.14509/29138
Symbol Kt
Description The Torok Formation is organized into discrete packages dominated by either sandstone or silty mudstone. Sandstone is indurated, lithic, light to medium olive-gray- or light brown- to tan-weathering, dominantly fine to medium grained, and moderate to poorly sorted. Mica and phyllitic(?) rock fragments are locally conspicuous. Sandstone beds are typically decimeters thick and amalgamated along obscure bed boundaries; most are internally massive, although subtle normal grading occurs, particularly in coarser-grained examples with granules at the base; sole marks on the base of sandstone beds include flutes, grooves, and various load features. Mud rip-ups and carbonaceous debris are locally abundant. Convolute lamination is present locally; low-angle and possible small-scale trough cross-stratification are locally developed, but subordinate to diffuse, plane-parallel lamination is also present. Ripple cross-stratification ranges from rare to ubiquitous and is most common at the top of beds. This assemblage of sedimentary structures is consistent with deposition from medium- to high-concentration sediment gravity flows, probably in deep water. Mudstone facies are not commonly exposed, but where observed are moderately hard siltstone and silty shale ranging from monotonous medium gray to brownish maroon with a conspicuous iridescent-purple-weathering sheen. Silty shales are locally punctuated by densely rippled, very-thin-bedded, very-fine-grained sandstone interpreted as the product of dilute turbidity currents. Bentonite and tuff are rare although local “popcorn” weathering suggests a possible volcanic component in recessive intervals. The stacking of the resistant sandstone and recessive silty facies is well expressed near the hinge of the Lupine anticline on the Sagavanirktok River just downstream of the confluence with the Lupine River. The sandstone packages appear to be sharp-based and generally fine upward over a thickness of 5 to 15+ m (16 to 50+ ft). The juxtaposition of these facies can also be seen in the steeply dipping strata at the Ice Cut (fig. 2), an easily accessible location along the Dalton Highway (Mull and Harris, 1989) just south of the mapped position of the Ice Cut thrust. The discrete occurrence of sandstone-dominated facies can also be inferred from the intermittent pattern of rubble-crop traces in the southernmost part of the map area, east of the Sagavanirktok River. Thick sandstone-dominated zones also cap resistant, mesa-like knobs, especially in the southeasternmost part of the map area. This grain-size partitioning is consistent with the log character in the Lupine Unit 1 well and also mirrors that of the correlative Gilead sandstone in the Ivishak River and Gilead syncline areas just east of the map area (Decker and others, 2008). Based on the overall sedimentology, stacking pattern, and context in the basin, the unit is interpreted to represent a base-of-slope or basin-floor setting where migrating sandstone lobes or channels are interspersed with off-axis, finer-grained facies. Limited paleocurrent data from sole marks and ripples indicate northward and north–northeastward paleoflow. The Torok Formation was defined along the upper Chandler River, considerably southwest of the map area (Gryc and others, 1951). The inferred depositional settings for the type Torok Formation in the inner Brooks Range foothills are similar to that interpreted above, although the thickness of the sandstone packages and overall sand:shale ratio are generally higher in the map area. Given the east–northeast progradation of this interval observed in regional seismic data (for example, Houseknecht and others, in press), much of the Torok Formation in the map area is likely to be younger than exposures of the formation to the west and south. Outcrops near the top of the formation along the Sagavanirktok River near its confluence with the Lupine River probably include Torok of Cenomanian age. This younger age assignment is not constrained by existing biostratigraphic interpretations, but is supported by the presence of bentonitic shales and a subtly lighter gray-weathering character inferred to be the result of increased tuffaceous material characteristic of the Upper Cretaceous (LePain and others, 2002). In addition, similar facies along the Ivishak River and high in the section at Gilead syncline have yielded the Cenomanian bivalve Inoceramus Dunveganensis (Reifenstuhl, 1991; W. Elder, written comms., 2008, 2009). South of the Ice Cut thrust, the Torok Formation is interpreted to be largely middle to late Albian, representing the lower and/or middle parts of the formation. This assignment is supported by the occurrence of the late middle Albian ammonite Gastroplites sp. aff. G. kingi McLearn (W. Elder, written communication, 2008), and a middle to late Albian pelecypod fauna including Inoceramus anglicus Woods and Inoceramus cf. I. kedroviensis Pergament (W. Elder, written communication, 2009). The total thickness of the formation is not known from outcrops in the map area. We tentatively interpret the slightly deviated Lupine Unit 1 well to have penetrated a true vertical Torok thickness of approximately 1,200 m (3,940 ft), represented by the measured depth interval 2,109–3,337 m (6,920–10,950 ft). This thickness includes an upper 840-m-thick (2,756-ft-thick) interval with a high proportion of sandstone relative to mudstone, and a basal Brookian mudstone interval approximately 360 m (1,180 ft) thick. East of the map area, Torok-equivalent strata of the Gilead succession are at least 850 m (2,780 ft) thick (Reifenstuhl, 1991), with striking similarities in thickness and stratigraphic architecture to the sandier upper section in the Lupine Unit 1 well. Both the Torok Formation and the partially age-equivalent Gilead succession thin dramatically to the north, probably by a combination of distal taper of the clinoform slope wedge (Houseknecht and others, in press) and onlap of basinal strata onto the north flank of the Colville basin (Decker and others, 2008); equivalent strata are essentially absent due to nondeposition at the Echooka Unit 1 well in the northeastern part of the map area, and have not been recognized in Neocomian through Upper Cretaceous age exposures on the Echooka River northeast of Gilead syncline.
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