Limestone, southeast Alaska

Unit symbol: Sl
Age range Silurian (443.4 to 419.2 Ma)
Lithology: Sedimentary
Group name: Shallow-marine, carbonate-dominated rocks
Primarily very thick sequences (up to 3,000 m) of cliff-forming, dominantly reefoid limestone, which is locally fossiliferous, commonly massive, but is also thin- to thick-bedded. It locally contains minor interbeds of sandstone and argillite, conglomerate, and limestone breccia (Rossman, 1963; Eberlein and Churkin, 1970; Brew and Ford, 1985; Karl, 1999). “Conglomerates are typically polymictic, with a variety of plutonic, supracrustal volcanic, and sedimentary lithologies represented among the clasts. Locally, however, the clasts are almost entirely of a single rock type (oligomictic), as exemplified by beds of chert pebble or limestone pebble conglomerate and by beds of limestone breccia * * *” (Eberlein and Churkin, 1970, p. 16). Locally present are limestone turbidites that are probably interchannel and overbank deposits (Brew and Ford, 1985). Where clastic rocks are mapped separately from the calcareous units, they are included in unit St. Unit also includes thin-bedded calcareous argillite (Rossman, 1963), which locally has ripple marks and mud cracks that indicate shallow-water deposition (Eberlein and others, 1983). Rocks of unit are thought to have formed fringing reefs or shallow carbonate banks surrounding islands and are coeval with the deeper water turbidites (unit St) (Brew and Ford, 1985; Karl and others, 1999). Includes the Willoughby, Kennel Creek, and Heceta Limestones, carbonate rocks of the Rendu Formation, and those associated with the Point Augusta and Tidal Formations in the Glacier Bay and Chichagof Island areas (Rossman, 1963; Eberlein and Churkin, 1970; Brew and Ford, 1985), and limestone on Kuiu Island (Brew and others, 1984). The Willoughby, Kennel Creek, and Heceta Limestones contain abundant tabulate and rugose coral fossils and the distinctive bivalve fossil Pycinodesma (Seitz, 1959; Rossman, 1963; Eberlein and Churkin, 1970; Loney and others, 1975; Rohr and Blodgett, 2013), which is considered endemic to late Silurian strata of southeast Alaska (Blodgett and others, 2010). These units and the Kuiu Limestone also contain less abundant cephalopod, brachiopod, conodont, and stromatoporoid fossils. Neither the Rendu Formation nor the underlying Pyramid Peak Formation contain fossils, but, based on their stratigraphic position underlying the Black Cap Limestone (unit Dl) and possibly unconformably overlying the Tidal Formation (included in unit St), they are thought to be Silurian or possibly Early Devonian (Rossman, 1963). The Kennel Creek Limestone grades upward into the Cedar Cove Formation (included in unit DSl), but the lower contact is faulted (Loney and others, 1963). Karst topography and features are commonly developed in the rocks of this unit

Source map information

Source map Brew, D.A., and Ford, A.B., 1985, Preliminary reconnaissance geologic map of the Juneau, Taku River, Atlin, and part of the Skagway 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 85-395, pamphlet 23 p., 2 sheets, scale 1:250,000.
Symbol DScw
Unit name Willoughby limestone
Description ALEXANDER BELT: GLACIER BAY SEQUENCE: CARBONATE ROCKS-- Interpreted to have formed as fringing reefs or carbonate banks in an island arc environment dominated by volcanic turbidites. Probably range in age and are not a single stratigraphic horizon. Massive, bluish gray to light gray limestone and marble; described and named by Rossman (1963); exposed north of Endicott River in Chilkat Mountains.
Lithology Sedimentary

Correlated geologic units

Label Sab
Description Heceta Limestone, reefoid limestone
Geologic age Silurian to Ludlow
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Reef Major