Ivishak Formation

Unit symbol: Trif
Age range Lower Triassic (252.2 to 247.2 Ma)
Lithology: Sedimentary
Group name: Sadlerochit Group, undivided
Consists of three lithologic units. The uppermost unit, the Fire Creek Siltstone Member, is medium-dark-gray, thin-bedded to massive, commonly laminated siliceous siltstone, minor silty shale, and argillaceous sandstone. The middle unit, the Ledge Sandstone Member, is clean light-gray massive sandstone that weathers red to reddish-brown and is locally conglomeratic. The lowermost unit, the Kavik Member, is dark-colored, laminated to thin-bedded silty shale and siltstone that has minor argillaceous sandstone interbeds; some workers (Jones and Speers, 1976; Wilson and others, 2001) consider this a distinct formation in the subsurface as mentioned above. Total thickness of the three members is about 85 m at the type section (Detterman and others, 1975). Crowder (1990) describes the basal Kavik Member as an upward-fining and then coarsening depositional assemblage that records a retrograde then prograde migration of prodelta environments. The overlying Ledge Sandstone Member records the evolution of delta-fringe, distributary-channel, and crevasse-splay environments of the lower delta plain. The uppermost Fire Creek Siltstone Member is an aggradational and transgressive assemblage deposited in shallow-marine environments that reworked sediment originally deposited on the lower delta plain. In the subsurface, the Ivishak Formation is considered of fluvial origin—possibly braided stream deposits on a coastal plain—and becomes finer grained and marine southward (Jones and Speers, 1976; Melvin and Knight, 1984; Tye and others, 1999; Wilson and others, 2001). The Kavik Member contains an ammonite and pelecypod fauna of Early Triassic age; the upper members are more sparsely fossiliferous (Detterman and others, 1975; Reiser and others, 1980)

Source map information

Source map Reiser, H.N., Brosge, W.P., Dutro, J.T., Jr., and Detterman, R.L., 1980, Geologic map of the Demarcation Point quadrangle: U.S. Geological Survey Miscellaneous Investigations Series Map I-1133, 1 sheet, scale 1:250,000.
Symbol Tri
Unit name Sadlerochit Group, Ivishak Formation, undivided
Description Comprised of 3 lithologic units. Upper unit, Fire Creek Siltstone Member is medium-dark-gray thin-bedded to massive commonly laminated siliceous siltstone, minor silty shale, and argillaceous sandstone. Middle unit, Ledge Sandstone Member is clean light-gray massive sandstone that weathers red to reddish-brown. Locally conglomeratic. Lower unit, Kavik Member is dark-colored laminated to thin-bedded silty shale and siltstone having minor argillaceous sandstone interbeds
Lithology Sedimentary

Correlated geologic units

Label Tri
Description Sadlerochit Group, Ivishak Formation
Geologic age Early-Triassic
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Siltstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Minor
Shale < Mudstone < Clastic < Sedimentary Bed Minor