Geologic Map of Alaska: Limestone, southeast Alaska

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 Karl, S.M., 2002, Unpublished data
Symbol	Sab
Unit name	Willougby limestone
Description	In Chilkat Mountains and Glacier Bay includes the Willoughby Limestone, containing massive, bluish gray to light gray limestone and marble; also reefoid limestone with minor limestone breccia, mudstone, and polymictic conglomerate; described and named by Rossman (1963). Heceta Limestone on Prince of Wales Island named by Eberlein and Churkin (1970), is composed of massive limestone, mostly thick-bedded and sublithographic. Light to medium-dark gray, weathers light gray to buff. Bedding generally indistinct; richly fossiliferous, especially with corals, Dasycladacean algae, and brachiopods; contains intraformational limestone breccia. . consists of light gray, massive, sublithographic limestone, with abundant amphiporoid corals, dasycladacean algae, oncoids, and brachiopods. Limestone also contains subordinate stromatoporoids, gastropods, pelecypods, bryozoans, trilobites, graptolites, conodonts, and aphrosalpingid sponges. Aphrosalpingid sponges form cores of stromatolitic mats. Skeletal wackestones, packstones, grainstones, and rudstones contain shelly faunas including corals and stromatoporoids in growth position (Soja, 1991). Algal, coralline, and microbial buildups are interpreted to represent platform margin reefs, with carbonate turbidites, debris flows, and slump deposits that accumulated in deeper water off the shelf margin (Soja, 1991). Bioherms range from 1 to 25 meters thick and 5 to 500 m wide (Soja, 1991). To the north, contains thick lenses of conglomerate and sandstone, with a variety of plutonic, volcanic, and sedimentary rock fragments. Contact with underlying Descon Formation is generally conformable, but limestone detritus resembling the Heceta in polymictic conglomerate that conformably underlies Heceta in several places indicates that carbonate sediments were deposited, lithified, and eroded in the Early Silurian prior to the main period of Heceta limestone deposition. Thickness varies widely due to pre-Karheen erosion but exceeds 3000 m on Heceta Island and thins eastward. In Glacier Bay the Willoughby Limestone contains rugose corals including Tryplasma, gastropods including Coelocyclus?, sp. aff. Ptychocaulus, and Coelocaulus karlae, an Upper Silurian species not known from Laurentian North America (Rohr and others, 2003), and the giant mollusk Pycinodesma of Late Silurian age (Kirk, 1927). The Willoughby Limestone also contains aphrosalpingid sponges (Soja, written commun. September 2001), documented only from the Alexander and Nixon Fork terranes in Alaska and the Ural Mountains of Eurasia (Soja, 1994; Soja and Antoshkina, 1997). On west Heceta Island, basal conglomerate of Heceta Limestone overlies Descon Formation and contains angular slabs of black graptolitic shale containing monograptids of middle Early Silurian age. Contains aphrosalpingid sponges, Middle and Late Silurian brachiopods (Chonchidium, Brooksina, Harpidium), corals (Zelophyllum, Cystophyllum), dasycladacean algae (Microplasma, Tryplasma) (Eberlein and others, 1983; Soja, 1991). Late Silurian (Ludlovian) brachiopods (Atrypella murchisonia), corals, pelecypods (Pycinodesma), gastropods, and conodonts (Polygnathoidas emarginatus and Pterosphathodus amorphognathoides zone); Late Silurian corals (Zelophyllum, Cystiphyllum, and Microplasma), stromatoporoids (Amphipora), brachiopods (Brooksina, Harpidium); Late Early of Early Middle Silurian brachiopods (Atrypella scheii); Silurian corals (Catenipora, Favosites); Middle or Late Silurian dasycladacean algae and corals (Microplasma, sp., Tryplasma), and brachiopods (Conchidium)
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