ABSTRACT

Middle Devonian Keg River and Muskeg pools along the northern margin of Rainbow Basin in northern Alberta produce mainly from secondary porosity created by dissolution of replacement dolomites and cements, including saddle dolomites. Resultant pores are predominantly vug and moldic, and commonly microporous. Coarse, vuggy porosity is also developed in Zebra Dolomites.

Several relationships confirm the burial dissolution of dolomites in these sequences. First, dolomite crystals leached are those that replaced stylolitized sediment, including saddle dolomites, as shown by fluorescence microscopy. Second, high amounts of secondary porosity are commonly preserved in highly stylolitic dolomites, commonly directly adjacent to pressure solution seams. Third, pressure solution seams terminate directly into secondary porosity. And fourth, fractures, both open and healed, commonly terminate directly into secondary pores.

These petrographic relationships are consistent only if dolomite dissolution and secondary porosity development were contemporaneous with, or postdated, pressure solution. The spatial juxtaposition of fractures and stylolites with secondary porosity implies these diagenetic fabrics served as pathways for burial fluids promoting dolomite dissolution.

Given the regional extent of pervasive dolomite dissolution, dolomite stability in these sequences is not a function of the type or relative age of dolomite crystals precipitated. Rather, pore-fluid chemistry appears to be the major control. Calcium-rich burial fluids, responsible for emplacement of burial anhydrites and fluorite cements, are thought to promote the deep-burial dolomite dissolution and secondary porosity development in these pools.

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