Limestone, dolostone, and shale

Unit symbol: SOyld
Age range Silurian and Ordovician (470 to 419.2 Ma)
Lithology: Sedimentary
Group name: Sedimentary and metasedimentary rocks of York terrane
Unit combines three units described by Till and others (2011) on the Seward Peninsula: Ols, SOul, and Ol—all largely carbonate rocks. Till and others’ (2011) unit Ols is thin- to medium-bedded, pale-orange- to pale-yellow-brown-weathering, medium-gray to dark-gray limestone, dolomitic limestone, and dolostone that contains local shale and chert nodules. Ols occurs forms subdued slopes in the vicinity of the York Mountains, and is generally in fault-bounded, although locally, it depositionally overlies Till and others’ (2011) unit Ol. Till and others’ (2011) unit Ols is at least 300 m thick and consists of a relatively thin unit of fissile black shale and lesser interbedded black limestone and dolomitic limestone that contains calcified and pyritized radiolarians and sponge spicules. These limestones grade upward into flaggy, thin-bedded black limestone containing shaly partings and soft-sediment deformational features, which then grades into thin-bedded, graded, cross-bedded, and bioturbated sparsely bioclastic limestone and dolostone. Unit SOul of Till and others (2011) consists either of dolostone that contains corals of Late Ordovician (Sainsbury, 1969; Sainsbury and others, 1971) or late Middle to early Late Ordovician (Oliver and others, 1975) age or, alternatively, very fine-grained, locally cherty, gray dolostone that contains long-ranging conodonts, domal to laminar stromatoporoids, and diverse tabulate and rugose corals of Silurian age. Till and others’ (2011) unit Ol is massive to thick-bedded, light-brownish-gray to medium-gray, fine-grained limestone with local chert nodules and lesser interbeds of argillaceous limestone and shale, widely exposed in and adjacent to the York Mountains. Parts of this unit correlate well in age and lithology with Early and Middle Ordovician rocks of the Baird Group in the western Brooks Range (unit DOb of Till and others, 2008a)

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2011, Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Scientific Investigations Map 3131, 2 sheets, scale 1:500,000, 1 pamphlet, 75 p., and database, available at http://pubs.usgs.gov/sim/3131/.
Symbol Ols
Unit name Limestone and shale, York terrane
Description Thin- to medium-bedded, pale-orange- to pale-yellow-brown-weathering, medium-gray to dark-gray limestone, dolomitic limestone, and dolostone with local shale and chert nodules. Ols forms subdued slopes within, north of, and east of the York Mountains; it is generally fault bounded, but locally depositionally overlies unit Ol. The lower 7 to 30 m of Ols is fissile black shale and lesser interbedded black limestone and dolomitic limestone that contains calcified and pyritized radiolarians and sponge spicules (T. Carr and T. Hudson, written commun, 1982; Dumoulin and Harris, 1994). These strata grade upward into flaggy, thin-bedded black limestone with shaly partings and soft-sediment deformation features, and then into thin-bedded, sparsely bioclastic limestone and dolostone that is graded, cross-bedded, and bioturbated. Ols is at least 300 m thick (Dumoulin and Harris, 1994), and it appears to contain the Early-Middle Ordovician boundary near its base (Dutro in Ross and others, 1982). The unit has produced pandemic, cool-water conodonts of earliest Middle Ordovician (latest Arenig and earliest Llanvirn) age (Table A-1; Dumoulin and Harris, 1994), as well as cephalopods, graptolites, gastropods, and trilobites (Ross, 1967; Sainsbury, 1969b; Dutro in Ross and others, 1982). The youngest beds in Ols occur in a klippe west of the Mint River (unit "Odl" of Sainsbury, 1969b) and consist of medium-bedded, mottled to laminated, gray to brown lime mudstone and dolostone that contain conodonts, corals, and cephalopods of late Llanvirn age (Flower, 1968; Sainsbury, 1969b; Dumoulin and Harris, 1994). Bedforms, sedimentary structures, and faunas indicate that unit Ols was deposited in a shallowing-upward, basin to peri-platform environment; the youngest strata accumulated in a mid- to outer-platform setting (Dumoulin and Harris, 1994). Lithofacies and biofacies of Ols correlate well with those of deeper water Middle Ordovician strata in the western and central Brooks Range (map unit "O<c" of Till and others, 2008b; Dumoulin and Harris, 1994). Equivalent to "Os" and "Oshd" of Sainsbury (1969a), "Olsh" and "Odl" of Sainsbury (1969b), and "Oshl" and "Oshd" of Sainsbury (1972)
Lithology Sedimentary

Correlated geologic units

Label Oshd
Description Limestone and shale
Geologic age Middle-Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Black-shale < Shale < Mudstone < Clastic < Sedimentary Bed Major