Volcanic and sedimentary rocks

Unit symbol: TrPzvs
Age range Lower Triassic to upper Paleozoic (252.2 to 247.2 Ma)
Lithology: Sedimentary
Group name: Chulitna sequence, undivided
Intercalated greenish-gray to black tuffaceous chert, volcanic conglomerate and lesser maroon volcanic mudstone, basaltic breccia, laminated flysch-like graywacke and shale, large lenses of light-gray thick-bedded limestone, and poorly exposed beds of ammonite-bearing limestone (Csejtey and others, 1992) mapped in the Healy and Talkeetna quadrangles. Clautice and others (2001) included and described an ammonite-bearing limestone as thin-bedded, light-gray, light-brown-weathering mudstone and packstone recognized only within small, high-angle fault slivers at two localities. Both exposures are less than 4 m in strike length and 1–2 m thick. In the easternmost exposure, Nichols and Silberling (1979) recognized at least 13 species of ammonite fossils in a single 10-cm-thick bed. The collection is significant because the closest rocks of similar Lower Triassic (Olenekian) age and apparent equatorial paleolatitude (10 degrees) are found in northern Washington. The second locality, about 2 km to the west, is orange-brown weathered, silicified packstone to wackestone with poorly-preserved fossil material in rubbly outcrop over a few square meters (loc. 23 in Blodgett and Clautice, 2000). Fossils include Devonian and Carboniferous fossils in the chert, Permian fossils including brachiopods in the thick-bedded limestone, and Early Triassic ammonites and conodonts from the limestone. Considered the stratigraphically lowest part of the Chulitna sequence

Source map information

Source map Reed, B.L., and Nelson, S.W., 1980, Geologic map of the Talkeetna quadrangle, Alaska: U.S. Geological Survey, Miscellaneous Investigation Series Map I-1174, 15 pages, 1 plate, scale 1:250,000.
Symbol Trvs
Unit name Chulitna sequence -- Volcanic and sedimentary rocks
Description Distinctive light-green to maroon volcaniclastic rocks that range from tuffaceous shale to a quartz-pebble conglomerate having a tuffaceous matrix. Commonly contain interbeds of massive micritic and bioclastic limestone, minor chert, argillite, rust-weathering siltstone and sandstone, and basaltic tuff.
Lithology Igneous

Correlated geologic units

Label TrDv
Description Chulitna Sequence -- volcanic and sedimentary rocks, Early Triassic to late Paleozoic
Geologic age Late-Devonian to Early-Triassic
Geologic setting Undivided
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Minor
Limestone < Carbonate < Sedimentary Lens Incidental
Shale < Mudstone < Clastic < Sedimentary Bed Incidental
Graywacke < Sandstone < Clastic < Sedimentary Bed Incidental