Ordovician paleotopography as evidenced from original dips and differential compaction of dolostone and shale unconformably overlying Precambrian basement on Manitoulin Island, Canada

The Middle to basal Upper Ordovician Lindsay Formation unconformably overlies the Paleoproterozoic Bar River and Lorrain formations on Manitoulin Island (Ontario, Canada), representing a depositional hiatus of at least 1.7 billion years. The Paleoproterozoic inliers currently exposed as basement highs in the Sheguiandah region comprised a 14 km2 area of quartz arenite archipelago during the Middle to Late Ordovician inland sea transgression of North America. The quartz arenite outcrop collectively covers an area of approximately 1.8 km2. High-energy transport and reworking during wave and tropical storm activity along a high relief rocky shoreline is indicated by dolostone containing 2.5 m-size rounded to subrounded quartz arenite boulders with percussion marks, up to 4.5 m-size subangular quartz arenite blocks consistent with mechanical breakage along joints, fractures or bedding planes, lenticular conglomerate beds, and cracks in quartz arenite basement containing mixtures of dolostone, quartz granules and fossiliferous debris. With continued transgression, organic-rich carbonate muds were deposited under low energy conditions either on carbonate strata or directly on quartz arenite basement in restricted regions protected from the open sea. Clast size distribution in the dolostone, clast rounding, and the location of the shale/quartz arenite unconformity relative to basement highs are all consistent with a longshore current running from the present northeast to southwest, with the leeward sides of rocky islands protected from high wave and storm energy processes. Subsequent deposition of Upper Ordovician and Silurian strata resulted in burial and differential compaction of the Lindsay Formation units. The compaction may have been associated with fractures through which Mg-rich fluids migrated, locally altering the lowermost Lindsay Formation to dolostone. Evidence for differential compaction is provided by bed thickening down-dip of basement highs and boulder flanks, and higher regional dips in shale beds proximal to basement highs compared with dolostone strata. The relatively high-angle strata of the Lindsay Formation are a result of the combined effects of paleotopography, a high relief rocky shoreline, and differential compaction.