Paleokarst in the Lower Ordovician Beekmantown Group, Ottawa Embayment; structural control inboard of the Appalachian Orogen

Two paleokarsts, different in age, character, and origin, occur in dolostones of the Lower Ordovician Beekmantown Group of eastern Ontario, well within the interior of the Laurentian paleoplatform. Surficial to shallow subsurface (<2 m) epikarst formed during the latter stage (late Arenig to Llanvirn) of Sauk platform development in this region, resulting from local change to base level and patterns of meteoric circulation likely initiated by reactivation of shallowly buried Precambrian structures along a cratonic fault system, now defined by the Ottawa-Bonnechere Graben. Faulting was contemporaneous with initiation of the crustal forebulge within the distal, developing Taconic orogen. Mineralized, intrastratal, vuggy to local cavernous porosity composes a prominent second, more regional paleokarst. Dissolution followed burial chemical compaction, but predated a history of further burial and tectonism defined by stages of geopetal cavity-fill sedimentation, cavity-fill mineralization, calcite veining, and renewed stylolitization. Regional paleokarst is interpreted to have arisen from changes in pCO (sub 2) and H (sub 2) S concentrations arising from mixing of continentally derived pore waters with brines derived from dissolution of Beekmantown evaporites. Compared to the region's geologic history, a pre-late Paleozoic age for formation of the regional paleokarst and mineralization is likely. The two platform-interior paleokarsts demonstrate unexpected links between tectonism, changes in paleohydrological patterns, and porosity development well inboard of the Appalachian orogen.