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
Symbol Ol
Unit name Limestone, York terrane
Description Mainly 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 in the western and central Teller quadrangle. Ol is at least 450 m thick (Dumoulin and Harris, 1994); it resembles unit Oal in containing 8- to 15-m-thick shallowing-upward cycles (Vandervoort, 1985) and locally abundant trace fossils, but differs in containing more megafossils and lacking quartzose grainstone and ripple marks. Common rock types include lime mudstone, bioclastic wackestone, and fine to very fine grained peloid and intraclast grainstone. The upper 70 m of the unit is a distinctive blue-gray-weathering, white to pinkish-gray lime mudstone with rare trilobite fragments. Most exposures of Ol are bounded by faults, but at a few localities it appears to grade upward into Olsh. Sainsbury (1969b) suggested that Ol conformably overlies Oal, but megafossil and conodont data suggest that the lower part of Ol may be coeval with much of Oal. Ol is chiefly of Early Ordovician (early and middle Arenig) age. The tightest ages are based on conodonts and include collections restricted to the Mac. dianae, Ac. deltatus-On. costatus, lower Oe. communis, and Re. andinus-T. laevis Zones (Table A-1); the youngest of these collections are from near the top of Ol and are definitively younger than any faunas recovered from Oal. Megafossils in Ol include brachiopods, cephalopods, echinoderm debris, gastropods, and trilobites (Ross, 1965; Flower, 1968; Sainsbury, 1969b). Graptolitic shale of early Arenig age (T. fruticosus Zone; C. Carter, 1994, unpublished fossil report) forms local lenses in Ol. Lithofacies and biofacies indicate that Ol accumulated in a range of subtidal to supratidal environments within a deepening-upward regime (Dumoulin and Harris, 1994); overall, Ol appears to have formed in somewhat deeper water than Oal. Conodont assemblages in Ol include both Siberian and Laurentian (North American) endemic forms (Dumoulin and Harris, 1994; Dumoulin and others, 2002); trilobites have Siberian affinities (Ormiston and Ross, 1979). Ol correlates well with older parts of unit Od in the Nome Complex, the Baird Group (Tailleur and others, 1967; Dumoulin and Harris, 1994) in the western Brooks Range (map unit "DOb" of Till and others, 2008), and the Novi Mountain Formation, lower Telsitna Formation, and related rocks in the Farewell terrane of interior Alaska (Dumoulin and Harris, 1994; Dumoulin and others, 2002). Equivalent to "Ol" and "Olu" of Sainsbury (1969a, 1969b, 1972)
Lithology Sedimentary

Correlated geologic units

Label Ol
Description Limestone
Geologic age Ordovician to Early-Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Calcareous Major
Chert < Chemical < Sedimentary Bed Incidental