Sadlerochit Group, undivided

Unit symbol: TrPsg
Age range Lower Triassic to Permian (298.9 to 247.2 Ma)
Lithology: Sedimentary
Group name: Sadlerochit Group, undivided
A “heterogeneous assemblage of rocks that includes orthoquartzite, chert, limestone, sandstone, siltstone, and shale” (Detterman and others, 1975) that has been divided into two formations: an upper Ivishak Formation and a lower Echooka Formation, each of which is subdivided into formal members. Largely restricted to northeast Alaska and in the subsurface of the North Slope, it begins as a regressive sequence sourced from the north (Wilson and others, 2001) and transitions upward from shelf to fluvial deposits. A third formation, stratigraphically in the middle of the group, the Kavik Shale is recognized in the subsurface and is a medium to dark gray silty shale with minor siltstone and sandstone (Jones and Speers, 1976; Crowder, 1990; Wilson and others, 2001). The Kavik Shale of the subsurface is considered equivalent to the Kavik Member of the Ivishak Formation on the surface. The Sadlerochit Group includes the main reservoir rocks for the Prudhoe Bay oil field, which is estimated to hold as much as 25 billion barrels of oil and 46 trillion cubic feet (tcf) of natural gas (British Petroleum, 2013). Unit locally subdivided into the following two units, Trif and Pe

Source map information

Source map Wilson, F.H., 2011, Unpublished data.
Symbol TrPs
Unit name Sadlerochit Group
Description Composed of Ivishak Formation (Lower Triassic) and Echooka Formation (Permian), which are not shown separately on this map. Ivishak Formation -- Laminated gray siltstone and shale in upper third; massive, fine-grained quartz arenite in middle third; gray laminated shale and siltstone with abundant limestone nodules and concretions, and locally with barite nodules in lower third. Marine. Early Triassic ammonites and pelecypods. Thickness about 550 to 720m in the north, about 170m in the south. Echooka Formation -- Calcareous siltstone and shale; local thick units of thin to medium bedded limestone; siliceous siltstone and massive quartz arenite beds in upper part. Marine. Early Permian brachiopods; early late Permian brachiopods locally northeast of Lupine River. Thickness about 110m to 260m. Unit assigned to Autochthonous rocks of Colville Basin
Lithology Sedimentary

Correlated geologic units

Label TrPsg
Description Sadlerochit Group, undivided
Geologic age Permian to Early-Triassic
Geologic setting Undivided
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Calcareous Minor