Geologic Map of Alaska: Red and brown sedimentary rocks and basalt

Red and brown sedimentary rocks and basalt

Unit symbol:	JTrrb
Age range	Lower Jurassic and Upper Triassic (235 to 174.1 Ma)
Lithology:	Sedimentary
Group name:	Chulitna sequence, undivided

Red-bed sequence of sandstone, siltstone, argillite, and conglomerate that grades upward into highly fossiliferous brown sandstone and brownish-gray siltstone. Thickness is more than 2,000 m. Mapped only in the Healy and Mount McKinley quadrangles, unit is part of the Chulitna sequence of Csejtey and others (1992) (see also unit TrPzvs). Red-bed sequence contains a few thin interbeds of brown fossiliferous sandstone, pink to light-gray dense limestone, and intercalated basalt flows. Clasts in the red beds are dominantly basalt cobbles and pebbles derived from underlying basalt and from flows within the sequence. Quartzite pebbles and flakes of white mica and red radiolarian chert pebbles and grains are present in subordinate amounts. Fossils indicate a Late Triassic and Early Jurassic age. Also includes thin- to medium-bedded, brown-weathering calcareous sandstone, sandy limestone, and argillite. Unit is locally phosphatic and fossiliferous and contains ammonites of Sinemurian (Early Jurassic) age (Blodgett and Clautice, 2000)

Source map information

Source map	Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., 1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1961, 63 p., 2 sheets, scale 1:250,000.
Symbol	JTrrs
Unit name	Red and brown sedimentary rocks and basalt
Description	Maximum thickness of more than 2,000m. Basal part consists of a red-bed sequence of sandstone, siltstone, argillite, and conglomerate, with a few thin interbeds of brown fossiliferous sandstone, pink to light-gray dense limestone, and intercalated basalt flows. This sequence grades upward into brownish-gray siltstone with yellowish-brown limy concretions.
Lithology	Sedimentary

Correlated geologic units

Label	JTrsu
Description	Red and brown sedimentary rocks and basalt (Early Jurassic-Late Triassic)
Geologic age	Late-Triassic to Early-Jurassic
Geologic setting	Sedimentary, continental

Lithology	Form	Importance
Siltstone < Clastic < Sedimentary	Bed	Major
Argillite < Metaclastic < Metasedimentary < Metamorphic	Bed	Indeterminate, major
Conglomerate < Clastic < Sedimentary	Bed	Indeterminate, major
Sandstone < Clastic < Sedimentary	Bed	Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous	Flow	Minor
Limestone < Carbonate < Sedimentary	Bed	Incidental