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 Mull, C.G., Moore, T.E., Harris, E.E., and Tailleur, I.L., 1994, Geologic map of the Killik River quadrangle, Brooks Range, Alaska: U.S. Geological Survey Open-File Report 94-679, 1 sheet, scale 1:125,000.
Symbol TrIPi
Unit name Imnaitchiak Chert, of Etivluk Group; Picnic Creek Allochthon, Akmalik Creek Sequence
Description Yellowish-orange weathering gray to greenish gray thin-bedded chert or silicified mudstone, with interbedded siliceous shale, overlain by greenish-gray to maroon silty mudstone, base contains a distinctive dark gray evenly laminated glauconitic, phosphatic siltstone bed up to 1 m thick; locality at Akmalik Creek marked by an oncolitic conglomerate replaced by barite. Unit commonly structurally contorted, thickness of < 75 m thick.
Lithology Sedimentary

Correlated geologic units

Label TrIPic
Description Imnaitchiak Chert, Etivluk Group, chert of Siksikpuk Formation
Geologic age Pennsylvanian to Triassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Major