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 Curtis, S.M., Ellersieck, Inyo, Mayfield, C.F., and Tailleur, I.L., 1984, Reconnaissance geologic map of southwestern Misheguk Mountain quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1502, 2 sheets, scale 1:63,360.
Symbol TrIPs1
Unit name Siksikpuk Formation
Description Part of the Key Creek Sequence - Gray chert and gray, olive, and maroon siliceous shale. Middle part of unit is mostly shale which grades both up and down into well-bedded gray chert with thin siliceous shale partings. Mammillary bedding structures locally command. Chert contains Pennsylvanian to Triassic radiolarians. Depositional thickness is approximately 40-60m. Base contains bedded chert which appears to be gradational into the Kuna Formation, and one or more prominent orange-and yellow-weathering clay-rich horizons (benzonite?) that are of regional extent.
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