Upper Saginaw Bay Formation and similar rocks of southeast Alaska

Unit symbol: IPsb
Age range Pennsylvanian (323.2 to 298.9 Ma)
Lithology: Sedimentary
Group name: Upper Saginaw Bay Formation and similar rocks of southeast Alaska
Unit includes the upper members of the Saginaw Bay Formation, the Ladrones Limestone, and the Klawak Formation. Upper two members of the Saginaw Bay Formation are silty limestone and chert and limestone. Silty limestone is thin- to medium-bedded, medium-gray, brown-weathering limestone that contains variable amounts of terrigenous cherty debris. Light-gray-weathering biotherms are present locally throughout the member, and a conglomerate near the base has chert and limestone cobbles. Worm borings are conspicuous on bedding surfaces (Muffler, 1967). Chert and limestone member is thin- to medium-bedded, light-brown-weathering calcareous chert and subordinate thin-bedded, locally dolomitic, brown-weathering, medium-gray cherty limestone. Ladrones Limestone is massive limestone and minor dolostone that contains oolites and light-gray chert nodules and is more than 300 meters thick (Eberlein and others, 1983). Klawak Formation is 150–300 m of mainly calcareous, orange-weathering sandstone and siltstone that has chert pebbles and nodules and minor limestone and chert pebble conglomerate (Eberlein and others, 1983). Fossils indicate that the chert and limestone member of the Saginaw Bay Formation, at least, was deposited during Middle and Early Pennsylvanian time. Black chert and volcanic rocks at base of Saginaw Bay Formation are included here in unit MDls

Source map information

Source map Brew, D.A., Ovenshine, A.T., Karl, S.M., and Hunt, S.J., 1984, Preliminary reconnaissance geologic map of the Petersburg and parts of the Port Alexander and Sumdum 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open File Report 84-405, 43 p., scale 1:250,000.
Symbol Cl
Unit name Crinoidal Limestone
Description Medium-bedded to massive crinoidal limestone, white or medium gray fresh, weathers white, contains crinoids, brachiopods, corals, and (locally) fusulinids.
Lithology Sedimentary

Correlated geologic units

Label IPl
Description Crinoidal limestone
Geologic age Pennsylvanian to Middle-Pennsylvanian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Coquina < Limestone < Carbonate < Sedimentary Calcareous Major