Nonhydrothermal Saddle Dolomite: Petrological and Geochemical Evidence from the Ordovician Yeoman Formation, Southeastern Saskatchewan, Canada

Key words: 
• saddle dolomite

Abstract: Minor saddle dolomite cements are identified in the Upper Ordovician Yeoman carbonate rocks in southeastern Saskatchewan, Canada. These saddle dolomite cements are restricted to upper part of the Yeoman Formation, in a 20~30 m zone of dolomite rocks, and they are conspicuously absent both in the overlying and underlain carbonate strata, suggesting their precipitation in a relatively closed system. These Ordovician saddle dolomite cements are characterized by δ13C values (0.2‰ to 0.9 ‰ PDB) and Sr isotopic ratios (0.7082 to 0.7090) similar to those of their host replacement dolomites, indicating the sources of carbon and strontium for the saddle dolomite cements were mostly from precursor host dolomite rocks via pressure dissolution. In addition, the measured homogenization temperatures from the saddle dolomite cements vary from 99 to 105℃, which could be accounted for by the normal burial temperatures in the region. Based on these data and observations, the saddle dolomite cements in the Upper Ordovician Yeoman Formation in southeastern Saskatchewan are interpreted to be related to cannibalization of earlier replacement dolomite through chemical compaction in a relatively closed system during burial, rather than related to hydrothermal activities documented elsewhere in the Western Canada Sedimentary Basin. The occurrences of saddle dolomite, therefore, are not necessarily indicative of hydrothermal activity or fluids; nor are all saddle dolomites definitely related to hydrothermal fluids.