Origin of Molds in Dolostones Formed by the Dissolution of Calcitic Grains: Evidence from the Devonian Swan Hills Formation in West Central Alberta
Geological Survey of Canada, Calgary, AB
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 near the center 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 molds in dolostones rather than in limestones provides evidence to interpret that dissolution of calcitic grains occurred during the dolomitization process.
Investigation of pores in a core (well 10-3-61-24W5) within the zone of transition from limestone to dolostone in 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 this zone of transition, 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 interval 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 walls of stromatoporoids and chamber-filling equant calcite cements are completely dissolved, creating macro partial-molds.
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.