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 Sainsbury, C.L., 1972, Geologic map of the Teller quadrangle, western Seward Peninsula, Alaska: U.S. Geological Survey Geologic Miscellaneous Investigations Map I-685, 4 p., 1 sheet, scale 1:250,000.
Symbol Ol
Unit name Limestone and argillaceous limestone
Description Massive to thick-bedded micritic limestone containing interbeds of argillaceous limestone; sparse chert nodules in thick-bedded limestones which are entirely calcitic. Sparse fossils include cephalopod siphuncles and brachiopods of late Canadian age (Sainsbury, (1965)). Upper 200 feet is distinctly white to pinkish gray, weathering blue gray, and contains trilobites of late Canadian age (Ross, 1965).
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