Limestone and basalt sequence

Unit symbol: Trlb
Age range Upper Triassic, Norian (228 to 208.5 Ma)
Lithology: Sedimentary
Group name: Chulitna sequence, undivided
Interlayered limestone and amygdaloidal basalt flows. Limestone is medium-gray, massive to thick-bedded, and locally metamorphosed to fine- to medium-grained marble. Basalt is dark- to greenish-gray, aphanitic, and contains numerous amygdules. Csejtey and others (1992) suggested that the unit was deposited in shallow water and that the chemistry of the basalts suggests an ocean island shield volcano. Unit is mapped as thrust slivers in Chulitna sequence in the Healy quadrangle (Csejtey and others, 1992) and extends a short distance into the Talkeetna and Mount McKinley quadrangles (Wilson and others, 1998). Included here is a slightly metamorphosed and unevenly sheared sequence of shallow marine, interbedded dark-greenish-gray amygdaloidal metabasalt flows and dark-gray to black argillite and slate. Monotis subcircularis and Heterarstridium sp., of late Norian age, have been collected from argillite (Csejtey and others, 1992)

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 Trlb
Unit name Limestome and basalt sequence
Description Interlayed sequence, at least several hundreds of meters thick, of limestone, partly recrystallized to marble, and flows of altered amygdaloidal basalt.
Lithology Sedimentary

Correlated geologic units

Label Trlb
Description Limestone and basalt sequence, Late Triassic, Norian
Geologic age Norian
Geologic setting Undivided
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Limestone < Carbonate < Sedimentary Bed Major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Indeterminate, major
Chert < Chemical < Sedimentary Bed Minor
Sandstone < Clastic < Sedimentary Calcareous Minor