Shublik Formation

Unit symbol: Trgs
Age range Triassic (298.9 to 201.3 Ma)
Lithology: Sedimentary
Group name: Shublik Formation and lower Glenn Shale
Black, marine, carbonaceous, partly calcareous shale and thin-bedded limestone. Upper part is dark gray to black, calcareous, phosphatic siltstone and shale, and contains thin gray limestone interbeds. Locally, varicolored chert beds are present, as are locally abundant limestone concretions within the siltstone and shale. Lower part is black clay shale that contains limestone concretions and laminated silty limestone beds. Shale locally weathers rust-colored. Unit contains abundant fossils including Triassic pelecypods, such as Halobia and Monotis, and ammonites. Thickness is about 30 to 150 m. Unit locally may include rocks of the Karen Creek Sandstone and the Siksikpuk Formation (Brosgé and others, 1979). Generally exposed in the autochthonous part of the eastern Brooks Range and known from the subsurface of the North Slope, unit is exposed in the Surprise Creek anomaly of Mull and others (2000) in the De Long Mountains quadrangle, where it is contrasted with the Otuk Formation (unit J^o), which is more commonly exposed in the western Brooks Range. “Unit is interpreted to record deposition in a low-energy, restricted marine environment characterized by high organic productivity. * * * Organic-rich shale and limestone contain up to ~4 percent total organic carbon characterized by Type I and II kerogen, and constitute excellent potential hydrocarbon source beds” (Mull and others, 2000). The Shublik is considered to be the main source rock for oil in Arctic Alaska (D.W. Houseknecht, USGS, written commun., 2014)

Source map information

Source map Keller, A.S., Morris, R.H., and Detterman, R.L., 1961, Geology of the Shaviovik and Sagavanirktok Rivers region, Alaska, in Exploration of Naval Petroleum Reserve No. 4 and adjacent areas, northern Alaska, 1944-1953, Part 3, Areal Geology: U.S. Geological Survey Professional Paper 303-D, p. 169-222, 6 plates, scale 1:63,360.
Symbol Trs
Unit name Shublik Formation
Description Rocks of the Shublik formation crop out in a belt, generally less than 2000 feet wide, which can be traced along the Brooks Range front from Canning River to Elusive Lake. The Triassic rocks are present also about 2 miles west of the Sagavanirktok River in a breached and faulted anticlinorium. The formation is easily identified by its characteristic black shale, its abundant Late Triassic fossil suit, and its buff-weathering limestone. It is especially important as a marker unit, inasmuch as it represents the only predominantly calcareous unit between the Lisburne Group and the marine formation of the Nanushuk group. The unit overlies the Sadlerochit Formation without angular discordance; it is overlain disconformably by the Kingak Shale and, depending on the amount of post-Tiglukpuk and post-Okpikruak erosion, may be overlain locally by the Tiglukpuk, Okpikruak, or Fortress Mountain formations. The lower part of the formation consists of phosphatic siltstone, shale and very fine grained sandstone. Shale is characteristically black, owing to the high carbon content, limy, and clayey or silty; it contains abundant dark-gray nodules which are coated with a white salt
Lithology Sedimentary

Correlated geologic units

Label Trsf
Description Shublik Formation, black shale, sandstone, and silty limestone. Possibly equivalent to lower Glenn Shale [4034]
Geologic age Middle-Triassic to Late-Triassic
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Calcareous Major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Major
Sandstone < Clastic < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Quartzose-sandstone < Sandstone < Clastic < Sedimentary Bed Indeterminate, major