Carbonates and associated rocks

Unit symbol: Trls
Age range Triassic (235 to 208.5 Ma)
Lithology: Sedimentary
Group name: Marble and limestone of Wrangellia
This unit is exposed in southwest Alaska in isolated small outcrops associated with a variety of poorly described rock assemblages. In the northeast part of the Taylor Mountains quadrangle, a cream colored to dark-gray, limestone, silty limestone, and chert outcrops along the boundary between early Paleozoic calcareous rocks and the younger flysch of the Kuskokwim Group (unit Kk). Potentially divisible into two subunits, but grouped here; consists of a lower subunit of massive- to thick-bedded, light-gray to cream colored limestone that contains common scleractinian corals, spongiomorphs, and few brachiopods (Blodgett and others, 2000a); this lower unit has a minimum thickness 50 m and grades upward to a subunit of thinner bedded platy limestone, silty limestone, and minor chert that contain a locally well-developed silicified megafauna (brachiopods, bivalves, gastropods) of late Norian age (see McRoberts and Blodgett, 2002; Blodgett and others, 2000a); minimum thickness estimated at 100 m. Upper subunit represents slightly deeper, more offshore environment. Many of the gastropods present in upper subunit (for example, Chulitnacula alaskana Smith and Andangularia wilsoni Blodgett) are also known from coeval rocks in the Hyd Group of southeastern Alaska. A marine unit that consists of chert, tuffaceous cherty rocks, argillite, siltstone, volcanic wacke, conglomerate, limestone, and mafic flows and breccia occurs in what are possibly fault-bounded occurrences along the south shore of Nuyakuk Lake in the northwest Dillingham quadrangle and in the northwestern Taylor Mountains quadrangle (Sainsbury and MacKevett, 1965; J.M. Hoare and W.H. Condon, unpub. data, 1970). The limestone is generally white to cream colored and recrystallized; however, locally it is dark-gray and only finely crystalline. Triassic (Norian) fossils were reported by Mertie (1938) and others in written communications cited in Hoare and Coonrad (1978). Includes map unit Trvs of Hoare and Coonrad (1978) and as mentioned by them, was only shown “* * * in the vicinity of fossil localities because the rocks resemble other rocks of Paleozoic and Mesozoic ages with which they are tectonically associated” (Hoare and Coonrad, 1978)

Source map information

Source map Hoare, J.M., and Coonrad, W.L, 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-File Report 78-9-B, 2 sheets, scale 1:250,000.
Symbol Trvs
Unit name Volcanic and sedimentary rocks
Description Marine unit consisting of chert, tuffaceous cherty rocks, argillite, siltstone, volcanic wacke, conglomerate, limestone, and mafic flows and breccias.
Lithology Sedimentary

Correlated geologic units

Label Trvs
Description Limestone, chert and some greenstone
Geologic age Norian
Geologic setting Undivided
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Lithic-wacke < Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Chert < Chemical < Sedimentary Tuffaceous Minor