| Description |
Thin-bedded, argillaceous, silty and (or) dolomitic limestone, lesser massive micritic limestone, and local chert; rocks are light gray to medium gray and weather light gray to orange to tan. The unit is widely exposed in and adjacent to the York Mountains in the western and central Teller quadrangle, and is at least 350 m thick (Dumoulin and Harris, 1994). Like unit Ol, it contains 8- to 15-m-thick shallowing-upward cycles (Vandervoort, 1985) and locally abundant trace fossils, but Oal is less fossiliferous than Ol and includes quartzose grainstone and ripple marks not seen in Ol. Common rock types in Oal are dolomitic, locally argillaceous lime mudstone and grainstone made up mainly of peloids and intraclasts with lesser bioclasts and ooids. Mud-supported strata are bioturbated, with bedding-plane feeding trails and subvertical burrows. Grain-supported rocks are planar- to cross-bedded with locally well developed oscillation and current ripples. Some grainstones contain 10 to 40 percent fine-sand- to silt-size non-carbonate grains, mainly quartz and lesser feldspar, with trace amounts of pyroxene, zircon, and leucoxene (Sainsbury, 1969b). Most exposures of Oal are fault-bounded, and its original depositional relations with other units in the York Mountains are uncertain. Sainsbury (1969b) reported that Oal conformably underlies Ol, but megafossil and conodont data suggest that the upper part of Oal is coeval with much of Ol. Oal is chiefly of Early Ordovician (Tremadoc-early Arenig) age; the tightest ages are based on conodonts (Table A-1). The oldest conodonts represent the Ro. manitouensis Zone, and are older than any definitively dated faunas known from Ol. Younger collections in Oal, however, include those of Mac. dianae Zone age and overlap ages determined for Ol. Sparse megafossils in Oal include brachiopods, gastropods, and trilobite fragments (Sainsbury, 1969b); echinoderm debris, calcareous sponge spicules, and possible calcispheres were noted in thin sections. Various types of stromatolites occur locally and form biostromes as much as 5 m thick (Sainsbury, 1969b; Vandervoort, 1985). Lithologic and fossil data indicate that Oal was deposited in a range of subtidal to supratidal settings within a deepening-upward regime (Dumoulin and Harris, 1994); overall, Oal appears to have formed in somewhat shallower and more agitated water than Ol. Conodonts in Oal are scarcer and less diverse than in Ol, likely because Oal accumulated more rapidly and (or) suffered more terrigenous input. Conodont assemblages in Oal are mainly cosmopolitan but include a few Laurentian (North American) and Siberian endemic forms (Dumoulin and Harris, 1994; Dumoulin and others, 2002; J.E. Repetski, written commun., 2008). Some rocks presently included in O<l are similar in lithofacies, biofacies, and age to Oal (Vandervoort, 1985; Till and Dumoulin, 1994). Oal also correlates well with 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, 2008b), 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 "Oal" of Sainsbury (1969a, 1969b, 1972) |
