Early Paleozoic Surface and Subsurface Paleokarst: Middle Ordovician Carbonates, Mingan Islands, Québec

Middle Ordovician platform carbonates on the Mingan Islands are characterized by two major paleokarst unconformities and numerous local paleokarst surfaces which are today exposed on extensive bedding planes.

The lower paleokarst unconformity was developed on Lower Ordovician dolomites (Canadian to earliest Whiterockian) of the Romaine Formation. It corresponds to the Knox or Beekmantown unconformity present elsewhere in eastern North America. The post-Romaine unconformity represents an extensive karst plain that was veneered by eolian sediments and exhibits a variety of surface solution features ranging from karren to small dolines. Subsurface features are local in the form of small caves with collapse breccias.

The upper paleokarst unconformity, within the Middle Ordovician limestones (Chazyan) of the Mingan Formation, displays substantial karst relief which was modified by intertidal erosion preceding renewed submergence. The intra-Mingan unconformity, at a smaller scale, is sculpted into a variety of sharp-crested karren and basin features.

These two unconformities developed during uplift caused by Taconic Orogenesis some 500 km to the east. Eustatic sealevel changes also occurred but were probably less important in the formation of these unconformities.

Local paleokarst horizons also separate meter-scale shallowing-upward calcarenite cycles. The short-lived exposure events resulted in lithification followed by karst erosion forming sharp karren. If exposure was prolonged, the karren progressively widened and eventually developed into extensive planar surfaces. The origin of these cycles is explained by shoaling with the paleokarst surfaces, capping the cycles, formed during periods of relative sealevel fall.