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 Trvs
Unit name Metavolcanic, metavolcaniclastic, and subordinate metasedimentary rocks
Description Consists of intercalated sequences several hundreds of meters thick, occurring as thrust slices. Most common rock type is dark-greenish-gray aphanitic metabasalt containing numerous amygdules and occurring in thick, commonly pillowed massive flows. Metavolcaniclastic rocks comprise dark-gray or dark-greenish-gray, generally fine-grained, tuffaceous metasandstones and metasiltstones. Metasedimentary rocks comprise dark-gray, locally greenish-gray slate and argillite and subordinate dark-gray finely crystalline, thinly bedded marble.
Lithology Metamorphic

Correlated geologic units

Label Trnm
Description Metabasalt and slate, Late Triassic
Geologic age Norian
Geologic setting Extrusive
Lithology Form Importance
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Major
Slate < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major