Geologic Map of Alaska: Siksikpuk Formation and Immaitchiak Chert

Siksikpuk Formation and Immaitchiak Chert

Unit symbol:	TrIPeg
Age range	Triassic to Pennsylvanian (323.2 to 201.3 Ma)
Lithology:	Sedimentary
Group name:	Etivluk Group, undivided

Predominantly greenish-gray argillite; contains variable but generally very minor amounts of quartz-rich siltstone and very fine sandstone in beds 2–15 cm thick (Curtis and others, 1990; Ellersieck and others, 1990; Mayfield and others, 1990; Mull and others, 1994; Dover and others, 2004). Base contains a distinctive dark-gray, evenly laminated, glauconitic, phosphatic siltstone bed up to 1 m thick (Mull and others, 1994). Greenish-gray chert is common as interbeds in the argillite at several horizons, the most striking of which is the topmost zone of the formation where argillite is greatly subordinate to chert. Thickness is less than 100 m (Mull and others, 1994). Contains mainly Pennsylvanian to Late Triassic radiolarians; but locally has yielded Early Jurassic radiolarians, which may properly belong to the Blankenship Member of the Otuk Formation (unit JTro). Ages of radiolarian collections from this unit include Permian (possibly middle and late Permian), Middle and Late Triassic (probably Ladinian, Carnian, and Norian), and Early Jurassic (Hettangian or Sinemurian) (Dover and others, 2004). As mapped, locally may include rocks of the Sadlerochit Group

Source map information

Source map	Ellersieck, Inyo, Curtis, S.M., Mayfield, C.F., and Tailleur, I.L., 1990, Reconnaissance geologic map of the De Long Mountains A-2 and B-2 quadrangles and part of the C-2 quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1931, 2 sheets, scale 1:63,360.
Symbol	TrIPs1
Unit name	Siksikpuk Formation, Etivluk Group
Description	Gray chert and gray, olive-gray, and maroon siliceous shale. Middle part of unit is mostly shale that grades both up and down into well-bedded chert with thin siliceous shale partings. Mammillary bedding structures locally common especially near base. Part of Brooks Range allochthon, Key Creek sequence.
Lithology	Sedimentary

Correlated geologic units

Label	TrIPsk
Description	Siksikpuk Formation, Etivluk Group, chert and shale
Geologic age	Permian
Geologic setting	Sedimentary, slope-and-deep-water

Lithology	Form	Importance
Chert < Chemical < Sedimentary	Bed	Major
Shale < Mudstone < Clastic < Sedimentary	Bed	Major
Slate < Metaclastic < Metasedimentary < Metamorphic		Indeterminate, major
Limestone < Carbonate < Sedimentary	Bed	Indeterminate, major