Systematic ichnology of the Late Ordovician Georgian Bay Formation of southern Ontario, eastern Canada

The Late Ordovician Georgian Bay Formation of southern Ontario comprises between 127 and 177 m of alternating predominantly grey shales and grey calcareous sandstones, and is informally subdivided into lower shale-dominated and upper sandstone-dominated members. The succession is interpreted as a storm-dominated shelf sequence; many individual sandstone-shale couplets display all or part of a characteristic internal sequence encompassing a basal lag zone overlain by hummocky cross-stratification overlain by horizontal-laminated and cross-laminated deposits, all overlain by shale. While the upper member, as exposed extensively on Manitoulin Island, contains few and only poorly preserved trace fossils, the lower member has revealed an abundant ich- nofauna characterized by 26 ichnogenera represented by at least 47 ichnospecies, namely, Arenicolites isp.; Arenituba verso (Chamberlain, 1971a); Arthraria antiquata Billings, 1872; Aulichnites parkerensis Fenton and Fenton, 1937; Chondrites isp.; Cochlichnus anguineus Hitchcock, 1858; Cochlichnus n. isp.; Cruziana cf. lobosa Seilacher, 1970; Cruziana problematica (Schindewolf, 1921); Cruziana quadrata Seilacher, 1970; Cruziana ispp.; Curvolithus multiplex Fritsch, 1908; Didymaulichnus lyelli (Rouault, 1850); Diplocraterion cf. biclavatum (Miller, 1875); Diplocraterion helmerseni (Opik, 1929); Diplocraterion parallelum Torell, 1870; Fustiglyphus annulatus Vialov, 1971; Gordia marina Emmons, 1844; Gyrochorte comosa Heer, 1865; Helminthopsis hieroglyphica Wetzel and Bromley, 1996; cf. Lingulichnus verticalis Hakes, 1976; Lockeia siliquaria James, 1879; cf. Monocraterion tentaculatum Torell, 1870; Monomorphichnus bilinearis Crimes, 1970; Monomorphichnus lineatus Crimes, Legg, Marcos and Arboleya, 1977; cf. Palaeophycus crenulatus Buckman, 1995; Palaeophycus heberti (de Saporta, 1872); Palaeophycus striatus Hall, 1852; Palaeophycus tubularis Hall, 1847; Paleodictyon ispp. a-b; Phycodes flabellus (Miller and Dyer, 1878a); Phycodes palmatus (Hall, 1852); Planolites annularius Walcott, 1890; Planolites beverleyensis (Billings, 1862); Planolites constriannulatus Stanley and Pickerill, 1994; Protovirgularia rugosa (Miller and Dyer, 1878a); Protovirgularia isp.; Rusophycus carbonarius Dawson, 1864; Rusophycus cryptolithi Osgood, 1970; Rusophycus osgoodii n. isp.; Rusophycus polonicus Orlowski, Radwahski and Roniewicz, 1970; Rusophycus pudicus Hall, 1852; Skolithos magnus Howell, 1944; Skolithos verticalis (Hall, 1843); Trichophycus lanosus Miller and Dyer, 1878a; and Trichophycus venosus Miller, 1 879. This association of ichnotaxa contains elements of the Cruziana ichnofacies, indicative of a subtidal environment below fair-weather wave base, but above storm wave base. In overall taxonomic composition the ichnofaunal assemblage compares favourably to that from coeval and palaeoenvironmentally similar strata previously documented from carbonates of the Cincinnatian Series of Ohio.