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 Karl, S.M., and Brew, D.A., 2002, Unpublished data.
Symbol DSld
Unit name Limestone and dolostone
Description In Glacier Bay includes Pyramid Peak Formation, consisting of light gray, very thin to thick bedded nonfossiliferous limestone with some interbedded argillite near the top; lower part is thin bedded dark gray to black limestone; described and named by Rossman (1963). South of the Tsirku River in the Skagway quadrangle, includes thin-bedded unfossiliferous limestone and marble that overlies Silurian greywacke turbidites and underlies argillaceous limestone turbidites. In the Juneau quadrangle, includes the Kennel Creek Limestone includes gray, thin- to very thick-bedded limestone with few beds of dolostone and limestone breccia, minor shale and siltstone, and rare polymictic conglomerate described and named by Loney and others (1963); the name extended by Loney and others (1975). In Sitka quadrangle, includes gray, thin to thick-bedded limestone, with dolostone, limestone breccia, with minor shale, siltstone, and polymictic conglomerate in the lower part, and thin tuffaceous partings in the upper part of the Kennel Creek Formation. Total unit thickness as much as 1600 meters. Unit includes marble where it is adjacent to younger plutons. On Kuiu Island limestone is massive, poorly bedded, dense, stylolitic, brownish-gray weathering with associated sedimentary breccia and polymictic conglomerate (Muffler, 1967). Associated limestone breccia with dolomitic + pyrite cement or interstitial red siltstone is inferred to represent paleokarstic textures. Approximately 800 m thick. The Pyramid Peak Formation is unfossiliferous, but overlies the Tidal Formation (Rossman, 1963), so occupies a similar stratigraphic position to the Kennel Creek Limestone, which overlies the tidal-correlative Point Augusta Formation. Kennel Creek Limestone contains amphipora, pycinodesma, Silurian corals, and Silurian and Early Devonian conodonts (Loney and others, 1963; 1975; Karl, 1999.) The Kuiu limestone contains Late Silurian megafossils, and Late Silurian to middle Early Devonian conodonts including Panderodus sp., (A.G. Harris in Limestone contains late Middle to Late Silurian ?Parachaetetes sp. algae, ?Striatopora tabulate corals, Microplasma sp., Entelophyllum, and Zelophyllum sp. rugose corals, and brachiopods Conchidium alaskense Kirk and Amsden, Lissatrypa sp., Faardenia, sp., Obturamentella sp., and ?Coelospira sp. (C.W. Merriam in Muffler, 1967). Kuiu Limestone also contains Ludlovian to Praghian (Late Silurian to Early Devonian) Idiostroma? and Amphipora stromatoporoids, Cladopora tabulate corals, and Microplasma? sp., Holmophyllia? sp., Neostringophyllum sp., and Tryplasma T. pachytecum rugose corals (W.A. Oliver, Jr., in Megafossils from the Keku Islands area include stromatoporoids and corals Syringopora sp., Halysites sp., Heliolites sp., Favosites sp., Cystiphyllum sp., Phaulactis sp., Zelophyllum sp., Petrozium cf. P. dewari, and cephalopods Huroniella cf. H. inflect a Parks, of Silurian age, as old as early Wenlock and as young as Ludlovian (C.W. Merriam in Muffler, 1967). Another locality in the Keku Islands yielded ?Striatopora sp., Zelophyllum sp., Entelophyllum sp., Fardenia sp., Obturamentella sp., ?Coelospira sp., and abundant Lissatrypa sp. brachiopods indicating a late Wenlock or early Ludlow age (C.W. Merriam, in Muffler, 1967)
Lithology Sedimentary

Correlated geologic units

Label DSbl
Description Berg Mountain Formation Limestone. Associated with volcanic rocks of Berg Mountain Formation (NSA = 7022). Also Kennel Creek Limestone
Geologic age Silurian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Calcareous Major
Limestone < Carbonate < Sedimentary Calcareous Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Siltstone-mudstone < Mixed-clastic < Clastic < Sedimentary Bed Minor