Origin of molds in dolostones formed by the dissolution of calcitic grains: Evidence from the Swan Hills Formation in west-central Alberta and other Devonian formations in Alberta and northeastern British Columbia

Abstract

Abstract Molds or partial-molds formed by the dissolution of calcitic grains selectively occur in dolostones and are virtually ubiquitous in Phanerozoic strata throughout the world. The preferential position of partial-molds the centre of originally calcitic grains in pure dolostones is direct evidence of mineralogically controlled dissolution of calcite either during or following dolomitization. Furthermore, the selective occurrence of these calcitic grain partial-molds or molds in dolostones rather than in limestones or partially dolomitized limestones and their occurrence in the middle of thick dolostone successions provides good evidence to support the interpretation that the dissolution of calcitic grains to form these pores occurred both as part of and during the dolomitization process. Investigation of pores within the zone of transition from limestone to dolostone in a core (well 10-3-61-24 W5) from the Swan Hills Formation of west-central Alberta links the evolution of micro-pores of leached origin in undolomitized portions of stromatoporoids in partially dolomitized limestones to the origin of macro partial-molds in pure dolostones. In the zone of transition from limestone to dolostone, micro-pores of leached origin in the undolomitized walls of stromatoporoids increase in abundance, with the advancing degree of dolomitization, toward the pure dolostone. In the transition between extensively dolomitized limestone and pure dolostone, both the undolomitized walls of the stromatoporoids and chamber-filling equant calcite cements are extensively dissolved, producing a near partial-mold. In the pure dolostone, the undolomitized walls of stromatoporoids and chamber-filling equant calcite cements are completely dissolved, creating macro partial-molds. I interpret this distribution of porosity and pore types to be a snapshot of that immediately following the dolomitization process. Because dolomite replacement occurred as a dolomite front advanced through time, it is reasonable to assume that a similar distribution of porosity and pore types existed at earlier phases of dolomitization, when the pure dolostone bodies were significantly smaller. From this, I conclude that macro-moldic pores in pure dolostones formed by the formation and coalescence of micro-pores of leached origin along the peripheral halo of a migrating dolomite front. Dissolution of calcite is interpreted to have occurred throughout the duration of the dolomitization process, with increasing degrees in any given position as the dolomite front advanced through. Speculative models to account for the dissolution of calcite during dolomitization are offered.