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 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 Trsi
Unit name Ivishak member ot the Sadlerochit Formation
Description The Sadlerochit Formation is well exposed and is one of the most persistent units within the mapped area. It crops out in a belt 1 to 3 miles wide extending northeasterly along the north front of the Brooks Range. Typically the rocks are the flanking beds of the east- and west-plunging en echelon anticlines which form the mountain front, and generally are exposed best in cutbanks on smaller streams. The Ivishak member ranges from 1000 to 2000 feet thick. Near Eagle Creek the unit contains more sandstone than elsewhere. The rocks of the member are nearly the same at one locality as another. The member typically consists of two parts; a lower nonresistant shale and minor siltstone unit and an upper resistant siltstone, shale, and sandstone unit
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