Red beds

Unit symbol: Trrs
Age range Triassic, Norian and older? (252.2 to 208.5 Ma)
Lithology: Sedimentary
Group name: Chulitna sequence, undivided
Predominantly volcanic and volcaniclastic unit of red-colored lithic tuff, lithic conglomerate, graywacke, and finely laminated tuffaceous siltstone and mudstone. Also contains minor basalt to dacite flows similar to unit JTrrb, but distinct from it because of the occurrence of clasts of gabbro, serpentinite, and fossiliferous Permian(?) limestone (Csejtey and others, 1992; Clautice and others, 2001). Found only in thrust slivers in the southwestern Healy quadrangle. Unconformably overlies volcanogenic and sedimentary rocks of Triassic to Devonian age (unit TrDv). Volcanic members are predominantly calc-alkalic andesite tuff and tuff breccia, but compositions range from calc-alkaline basalt to dacite. Coarser volcaniclastic rocks are frequently calcareous and havea predominantly medium- to very coarse-grained, poorly sorted matrix that contains occasional feldspar, clinopyroxene, and hornblende crystals. Lithic clasts are predominantly mafic volcanic rock fragments altered to chlorite, hematite, and calcite. Other lithic fragments include felsic volcanic, plutonic, and metamorphic rocks. Mineral grains are monocrystalline and polycrystalline quartz. An isolated fault-bounded(?) exposure in Broad Pass (Csejtey and others, 1992) in the Healy quadrangle is included here. Lower part of this section consists of cobble to boulder conglomerate that contains clasts of green volcanic rocks and red radiolarian chert. Chert clasts have yielded Permian Radiolaria. Finer grained volcanogenic conglomerate, higher in the section, locally contains abundant Heterastridium sp., which indicate a Late Triassic, late Norian age (Csejtey and others, 1992). The uppermost part of the section is massive volcanic sandstone. No Permian source area is known for the chert clasts in this unit

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 Trr
Unit name Red beds
Description About 500 m of red sandstone, siltstone, argillite, and conglomerate, similar to red beds of unit JTrrs
Lithology Sedimentary

Correlated geologic units

Label Trr
Description Red beds, Late Triassic
Geologic age Late-Triassic to Norian
Geologic setting Sedimentary, continental
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Minor