A Tale of Two Saddle Dolomites: Research Continued from Earlier Work with Professor Mountjoy

Hairuo Qing
Dept. of Geology, University of Regina, Regina, SK, Canada

Saddle dolomite can be associated with hydrothermal fluids as illustrated in the Middle Devonian Pres’quile barrier reef complex, which was carried out as my PhD project under supervision of Professor Mountjoy. In this case, saddle dolomite and associated dolomitization are interpreted to be formed by hydrothermal fluids expelled by tectonic compression and sedimentary loading along the western margin of the Western Canada Sedimentary Basin. This interpretation is supported by the regional occurrence of saddle dolomite along the barrier and the corresponding trend of Sr isotopes (0.7081 to 0.7106), O isotopes (-7‰ to -16‰ PDB) and homogenization temperatures of fluid inclusions (92 to 178^oC) that exceed the ambient regional burial temperatures (60 to 160 ^oC).

Saddle dolomite cement in the Ordovician Red River carbonate southeastern Saskatchewan, however, (1) is restricted to the host replacement dolomites; (2) has C isotope values (-0.2 to 0.9 ‰ PDB) overlapping with those of their host dolomite, all of which fall within the range of the expected value of Ordovician seawater; (3) produces Sr isotopic ratios (0.7082 to 0.7090) similar to those of its host replacement dolomites; and (4) displays relatively low homogenization temperatures (about 100 ^oC) that can be related to the normal burial temperatures in the region. In this case, saddle dolomite cement is interpreted to be related to cannibalization of earlier replacement dolomite through chemical compaction in a relatively closed system during burial. The occurrence of saddle dolomite, therefore, is not necessarily indicative for hydrothermal activity or fluid flow.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas