The role of salt tectonics, glacioeustatic variations, and high pH evaporitic groundwater in the development of synsedimentary paleokarst within Carboniferous polymictic fanglomerate at Hopewell Cape, Atlantic Canada

A succession of Lower Carboniferous polymictic conglomerate in eastern Canada is truncated by synsedimentary endokarst conduits, which are clogged by lithified karst infill. Although such rocks do not usually host karst because of their polymineralic and poorly soluble contents, there are contextual and geochemical lines of evidence suggesting that groundwater conditions may have been highly alkaline at the time of karst formation, thus substantially increasing the solubility of common silicate minerals. The succession was deposited in a hyperarid climate and in close proximity to an evaporitic marine body. Because of a combination of high subsidence rates along a basin-bounding normal fault and entrapment within a salt expulsion minibasin, the evaporitic body of water was forced to prograde toward the source area of the sedimentary basin. This restricted sea was also responding to ongoing glacioeustatic variations at the time, which are interpreted to have generated a cyclic progradation of evaporitic groundwater into basin margin fanglomerates, thus favoring their early cementation by calcite. Karstification of the siliciclastic material is interpreted to have occurred during cyclic retreats toward lowstands, when groundwater alkalinity may have been highest.