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., 2013, Unpublished data.
Symbol DScg
Unit name Polymictic conglomerate
Description Polymictic conglomerate intercalated with the Kennel Creek and Kuiu Limestones. On Chichagof Island includes the conglomerate member of the Kennel Creek Limestone, consisting of gray to buff polymictic conglomerate containing angular and rounded fragments of syenite, leucocratic granite, mafic volcanic rock, chert, graywacke, and limestone, in a graywacke matrix. Polymictic conglomerate associated with the Kuiu Limestone is interbedded with thin-bedded limestone and massive limestone; conglomerate contains rounded clasts to 15 cm maximum dimension of gray and green chert, gray limestone, red feldspar porphyry, red siltstone, and locally of limestone, coral masses, arkose, pebble conglomerate, and green volcanic rock in a red sandy matrix in the Saginaw Bay area, and a limestone matrix in the Keku Islets (Muffler, 1967). On Kuiu Island, polymictic conglomerate is typically massive or thick-bedded and channeled; southwest of Saginaw Bay includes matrix supported debris flow with dominantly limestone clasts. Clast populations vary as do the graywacke compositions, but generally include, in order of decreasing abundance, graywacke, mudstone, volcanic rock, limestone, and syenitic to dioritic intrusive rock. Well-rounded syenite cobbles are distinctively pink and K-feldspar porphyritic. Conglomerates trend NNW-SSE
Lithology Sedimentary

Correlated geologic units

Label Dcg
Description Conglomerate of the Kennel Creek Limestone
Geologic age Silurian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major