Sabkha and Burrow-Mediated Dolomitization in the Mississippian Debolt Formation, Northwestern Alberta, Canada

Dolomitized burrows in the Mississippian (Visean) Debolt Formation of northwestern Alberta, Canada form the primary reservoir intervals in the Dunvegan gas field. Sedimentological and ichnological analyses suggest a carbonate ramp setting that includes subenvironments such as sabkhas, hypersaline lagoons, restricted subtidal lagoons, intertidal mud flats, and peloidal shoals. Dolomitization occurs primarily within oxidized muds and highly bioturbated sediments, with the primary mode being sabkha-associated precipitation. In this context, dolomitization within the burrows also appears to be mediated by sulfate-reducing bacteria. delta O-18 values for dolomite within burrows (mean 2.4%) are enriched by 1.3% relative to calcite values (mean 1.1%) within the burrows. This degree of fractionation is similar for dolomite and calcite that have precipitated from the same solution. It is therefore suggested that the protodolomite precipitated in equilibrium with calcite rather than by replacement of preexisting calcite. Isotopic values of delta C-13 measured for dolomite associated with burrows (mean 3.4%) and matrix (mean 3.5%) is slightly enriched relative to measured calcite values (mean 3.2% for matrix; mean 3.1% for burrows). These isotopic trends are common for modern dolomite that has precipitated in equilibrium with seawater where concomitant sulfate reduction and organic carbon-oxidation is inferred to occur near the surface.