Argillaceous limestone and limestone

Unit symbol: Oyl
Age range Middle and Lower Ordovician (485.4 to 458.4 Ma)
Lithology: Sedimentary
Group name: Sedimentary and metasedimentary rocks of York terrane
Sainsbury (1972) described this unit as “thin-bedded ruditic argillaceous and silty limestone and dolomite limestone, carbonaceous limestone, and subordinate massive micritic limestone which locally contains chert; abundant ripple marks, swash marks, casts of worm tubes, crossbedding, and limestone clasts. Local stromatolites in massive beds.” Till and others (2011) reported that the unit is at least 350 m thick (Dumoulin and Harris, 1994) and contains 8- to 15-m-thick shallowing-upward cycles (Vandervoort, 1985) and locally abundant trace fossils, but is less fossiliferous than unit SOyld and includes quartzose grainstone and ripple marks not seen in SOyld. “Common rock types * * * 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 sub-vertical 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, 1969). Most exposures * * * are fault-bounded, and its original depositional relations with other units in the York Mountains are uncertain. * * * Lithologic and fossil data indicate that [the unit] * * * was deposited in a range of subtidal to supratidal settings within a deepening-upward regime (Dumoulin and Harris, 1994); overall, * * * appears to have formed in somewhat shallower and more agitated water than Ol [unit SOyld here]. * * * Conodont assemblages * * * 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)” (Till and others, 2011). According to Till and others (2011), rocks of similar lithofacies, biofacies, and age are found in unit O<pt of this map. Oyl 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, 2008a), 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) [see unit DZwp here]

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 Oal
Unit name Argillaceous limestone and limestone
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)
Lithology Sedimentary

Correlated geologic units

Label Oal
Description Argillaceous limestone and limestone (Oal, TE002; TE004)
Geologic age Early-Ordovician to Dapingian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Dolostone < Carbonate < Sedimentary Bed Minor
Chert < Chemical < Sedimentary Bed Incidental