Paleoenvironmental Reconstruction of an Upper Ordovician Rocky Shoreline: The Lindsay Formation of Heywood and Partridge Islands, Ontario

The islands of Heywood and Partridge, located northeast of Manitoulin Island in Georgian Bay, Ontario, preserve a major unconformity between the Paleoproterozoic Bar River and Lorrain Formation quartz arenite and Upper Ordovician Lindsay Formation carbonate and mudrock. The Lindsay Formation strata record two transgressive pulses of an epeiric sea that flooded the North American continent during the Late Ordovician. During this time, mountain building processes associated with the Taconic Orogeny were taking place to the present east, and the tectonic effects of this orogeny resulted in subsidence of the Michigan Basin. The Lindsay Formation of the Trenton Group was deposited in this basin during a prolonged period of low siliciclastic input. Based on outcrop investigations, the Lindsay Formation in the Manitoulin Island area is divided into six distinct facies: (1) quartz clast dolostone, (2) lower crinoidal dolostone, (3) brachiopod dolostone, (4) stromatoporoid dolostone, (5) upper crinoidal dolostone, and (6) shale-mudstone. These facies differ from those identified in cores extracted from Manitoulin Island and the Bruce Peninsula in that they have been dolomitized and they contain less of a mud component. This difference is attributed to the development of a carbonate ramp dipping to the present south, which would have allowed for greater accumulation of muds in slightly deeper water. Major and trace element geochemistry of the Lindsay Formation mudrocks suggests that the detritus was derived from a mixed mafic-felsic source. The felsic component was eroded from quartz-rich Paleoproterozoic basement, whereas the mafic material may have been eroded from a distal Taconic arc or more proximal volcanic rocks of the Canadian Shield. The paleodepositional model for the Lindsay Formation on the islands of Heywood and Partridge involves a tropical homoclinal ramp passing seaward from a rocky shoreline to an outer ramp setting. The presence of hummocky cross-stratification, tempestites, large-scale wave ripples, and fragmented and disarticulated fossils, combined with the abundance of quartz clasts in the carbonate and mudstone units indicates frequent influence of storm activity during deposition