Dolomitization of the Leduc Formation (Upper Devonian), Southern Rimbey-Meadowbrook Reef Trend, Alberta

Abstract

ABSTRACT Upper Devonian Leduc carbonate buildups along the southern part of the Rimbey-Meadowbrook reef trend in Alberta (buried at depths of 2300-3500 m) have been extensively dolomitized. Dolomitization occurred first by pervasive replacement and later by minor cementation. In general, replacement dolomites are composed of fabric-destructive, fine to medium (60-250 µm) planar subhedral to anhedral crystals. Replacement dolomitization postdates submarine cementation and deposition of shales and carbonates from the overlying Ireton Formation, and overlaps stylolitization. It predates dolomite cementation that is interpreted to have originated from hydrothermal fluids in a shallow burial environment prior to basin tilting. This, combined with the d18O (mean = -4.5 ± 0.4 PDB) and 87Sr/86Sr (mean = 0.70853) values in the replacement dolomites, suggests that replacement dolomitization occurred also prior to basin tilting, probably during the Antler Orogeny in the subsurface at a temperature of about 45°C and a depth of about 500 m, from slightly modified Late Devonian seawater. Two generations of dolomite cements postdate replacement dolomites: a coarse and planar-e(s) cement (Dolomite-C1), and a later coarser nonplanar cement (Dolomite-C2). Dolomite-C1 has oxygen (mean d18O = -4.73 PDB) and strontium (mean 87Sr/86Sr = 0.70855) isotopic compositions similar to replacement dolomites. These similarities, combined with the local association of Dolomite-C1 with stylolites, suggest that some Dolomite-C1 precipitated from fluids whose composition reflects pressure solution of the preexisting replacement dolomites. The composition of fluid inclusions in Dolomite-C1 (mean Th = 117°C; mean salinity = 19 wt % aCl equivalent) indicates that most Dolomite-C1 precipitated from warm hydrothermal brines. Little variation in fluid-inclusion homogenization temperatures occur with burial depth, suggesting that Dolomite-C1 precipitated prior to basin tilting, and may be related to Antler orogenic events. Dolomite-C2 postdates Dolomite-C1, is relatively more depleted in 18O (mean d18O = -9.06 PDB), and has a more radiogenic 87Sr/86Sr ratio (0.71087). Preliminary fluid-inclusion data from the Crimson Field suggest precipitation temperatures of about 150°C. Thus, Dolomite-C2 originated from fluids of higher temperatures and different composition than Do omite-C1. The petrographic, paragenetic, and geochemical characteristics of replacement dolomites in this study are similar to those of other Leduc buildups along the reef trend and of other Devonian dolostones of western Canada, supporting a model of large-scale basinwide fluid flow. The pervasive distribution of replacement dolomites along the Rimbey-Meadowbrook reef trend, and the similar petrographic and geochemical character of dolomites from both the Leduc and Cooking Lake Formations, is consistent with the hypothesis that the underlying Cooking Lake platform acted as a subsurface conduit system for the dolomitizing fluids. A potential regional fluid source may have been formation fluids (including residual evaporite brines) expelled tectonically during the Antler orogeny.