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 Graptolitic shale of early Arenig age (T. fruticosus Zone; C. Carter, 1994, unpublished fossil report) forms local lenses in Ol.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) |