Chitistone and Nizina Limestones and Kamishak Formation

Unit symbol: Trcnk
Age range Upper Triassic, Norian to Carnian (235 to 208.5 Ma)
Lithology: Sedimentary
Group name: Marble and limestone of Wrangellia
The Chitistone and Nizina Limestones, correlative unnamed units in the Mount Hayes and Healy quadrangles, and the Kamishak Formation to the southwest are widespread in southern Alaska. The Chitistone Limestone is stratigraphically lower than the Nizina Limestone and is as thick as 600 m. Its lower part consists of abundant dolostone, algal-mat chips, stromatolites, and relicts of evaporites, whereas the upper part consists of varieties of limestone including lime mudstone, wackestone, packstone, and grainstone; both parts are exposed in southern east-central Alaska. The overlying Nizina Limestone is as thick as 500 m and consists of varieties of limestone that generally contain subordinate chert as nodules, lenses, and coalescing masses; its lithology is gradational into the overlying McCarthy Formation (unit J^mc). The upper Chitistone and cherty Nizina were deposited in a shallow to moderately deep marine neritic environment, transitioning from an intertidal to supratidal setting with local sabkha facies full of algal-mat chips, stromatolites, and evaporate relics typical of the dolostone-rich lower Chitistone rocks (Richter and others, 2006). Sparse fossils characterized by the ammonite genus Tropites indicate a late Carnian age for the Chitistone (MacKevett, 1971). Fossils are also sparse in the Nizina; those that are found consist mainly of pelecypods of the genus Halobia. Paleontologic studies by N.J. Silberling (written commun., 1962, to E.M. McKevett) indicate that the Nizina Limestone is Late Triassic and ranges in age from late Karnian or early Norian to early middle Norian. Along with the underlying Nikolai Greenstone, these limestone units are defining parts of the Wrangellia terrane (Jones and others, 1977). Kamishak Formation consists of limestone, chert, porcellanite, and minor tuff and volcanic breccia that divided into two formal members and an informal middle member; they are, in descending order: the Ursus, middle, and Bruin Limestone Members (Detterman and Reed, 1980). Unit primarily found along west side of Cook Inlet, east of the mountain range crest where a measured reference section is about 800 m thick. Depositional environment of Kamishak Formation was shallow water and high energy; intervals of unit include both reefs and biohermal buildups. Fossils found within the middle and Bruin Limestone Members of the Kamishak Formation yield a Norian age (Detterman and Reed, 1980; C.D. Blome, oral commun., 1981). As mapped here, unit also includes a small area of Triassic limestone and chert associated with the Chilikadrotna Greenstone in the central Lake Clark quadrangle. Kamishak Formation is cut by abundant dikes and sills that are related either to the Cottonwood Bay Greenstone or the Talkeetna Formation (Detterman and Reed, 1980)

Source map information

Source map Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., 1 sheet, scale 1:250,000.
Symbol Trkb
Unit name Kamishak Formation -- Briun Limestone Member
Description Massive to thin-bedded, light- to dark-gray limestone; coral and echinoid bioherms, banded green and white chert. Environment was apparently high energy, shallow water. Unit cut by abundant dikes and sills either related to the Cottonwood Bay Greenstone and the Talkeetna Formation. Possibly age equivalent to Nizina Limestone.
Lithology Sedimentary

Correlated geologic units

Label Trkb
Description Kamishak Formation, Bruin Limestone Member
Geologic age Norian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Dike or sill Minor
Chert < Chemical < Sedimentary Bed Minor