Crystal tuff, argillite, chert, graywacke, and limestone

Unit symbol: JTrct
Age range Upper Jurassic to Upper Triassic? (235 to 152.1 Ma)
Lithology: Sedimentary
Group name: Chulitna sequence, undivided
Unit dominantly light- to dark-gray tuff and minor argillite, chert, graywacke, and limestone. Csejtey and others (1992) interpreted these as a moderately deep- to deep-marine sequence of tightly folded and internally faulted tuff and other rocks at least several thousand meters thick; tuff constitutes about 80 percent of the sequence. Mapped in southwest Healy and northwest Talkeetna Mountains quadrangles. Csejtey and others (1992) indicate that the argillite, chert, graywacke, and limestone occur only in a narrow belt along the western part of the outcrop area and that the boundary between these rocks and the dominant tuff portion may be tectonic. In the Healy quadrangle, a thin interbed of volcaniclastic sandstone in the tuff yielded ammonites and Weyla sp. fossils of late Sinemurian (Early Jurassic) age (R.W. Imlay, written commun., cited in Csejtey and others, 1992). Csejtey and others (1992) indicate that, because these fossils were from near the top of the tuffaceous section, the section may be as old as Late Triassic. Chert beds in other parts of the unit have yielded Late Jurassic (Callovian to early Tithonian) Radiolaria, and the calcareous rocks yielded early Sinemurian fossils at several localities. An alternative interpretation for these rocks by Clautice and others (2001) results in the assignment of these rocks to five separate units. They assigned the fossil-bearing western part to either a Middle to Late Jurassic cherty argillite and tuff unit or to an Early Jurassic calcareous sandstone, limestone and argillite unit; the remainder of the unit they assigned to three upper Paleozoic tuff, argillite and tuff, and locally chert, argillite and graywacke units. They based age assignment of rocks to these older units on Silurian to late Paleozoic radiolarians in chert; Csejtey and others (1992) assigned the same rocks to their Cretaceous mélange unit (unit Kmar here). All of these upper Paleozoic units are found east of the Broad Pass Fault of Csejtey and others (1992), which defines the western boundary of the mélange 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 JTrta
Unit name Crystal tuff, argillite, chert, graywacke, and limestone
Description Four-fifths of the sequence is composed of massive cliff-forming crystal tuff, while remaining rocks form a narrow belt along western margin of unit. Contact between two parts of sequence may be tectonic. Crystal tuff is light- to dark-gray, locally having a greenish tint, and weathers to various shades of brown. Tuff composed of abundant small feldspar crystals (albite?) in a very fine-grained matrix of devitrified volcanic glass. Argillite and chert are dark-gray to black; the graywacke is medium- to dark-gray, very fine- to medium-grained, locally having graded bedding. Limestone is medium-gray, generally phosphatic, in part sandy, and locally associated with limy siltstone and conglomerate. Some cherty beds yielded radiolarians
Lithology Igneous

Correlated geologic units

Label JTrct
Description Crystal tuff, argillite, chert, graywacke, and limestone, deep marine, Late Jurassic to Late Triassic(?)
Geologic age Late-Triassic to Late-Jurassic
Geologic setting Extrusive
Lithology Form Importance
Volcanic < Igneous Pyroclastic, tuff Major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Chert < Chemical < Sedimentary Bed Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow, pillows Incidental