Can. J. Earth Sci. 44: 695–705 (2007) doi:10.1139/E07-007 © 2007 NRC Canada 695 Neomorphism of the Middle Devonian Ratner limestone related to dehydration of gypsum in south-central Saskatchewan Qilong Fu and Hairuo Qing Abstract: The Middle Devonian Ratner Formation in south-central Saskatchewan consists of laminated carbonate and interlaminated to interbedded carbonate and anhydrite, and the Ratner carbonate is preserved as limestone in many cored intervals. Medium to very coarsely crystalline (MVC) limestone (up to 4 m in thickness) is dominantly present in the uppermost part or the upper cycle of the Ratner Formation directly overlain by the Whitkow anhydrite. Gen- erally, both abundance and size of the MVC calcite crystals decrease downward away from contact between the Ratner carbonate and the Whitkow anhydrite. The average 87 Sr/ 86 Sr ratio of the MVC calcite is 0.7078, comparable to that of Middle Devonian seawater. The Ratner MVC limestone is interpreted to result from neomorphism of the precursor microcrystalline lime mudstone in burial environments, and neomorphic fluids might have consisted of gypsum- dehydration water mixing with minor various ratios of formation waters, based on petrographic observation, O- and Sr- isotopic data, and stratigraphic evidence. This research provides a case study in which the neomorphism of limestone is related to gypsum-dehydration water. Résumé : La Formation de Ratner (Dévonien moyen) dans le centre-sud de la Saskatchewan est composée de carbonate laminé et d’anhydrite interlaminée ou interstratifiée; le carbonate de la Formation de Ratner est préservé en tant que calcaire dans des intervalles à multiples noyaux. Un calcaire (atteignant des épaisseurs de 4 m) à cristaux moyens à très grossiers « MVC » domine la partie la plus élevée du cycle supérieur de la Formation de Ratner, laquelle est directe- ment recouverte par l’anhydrite de Whitkow. L’abondance et la dimension des cristaux MVC de calcite décroissent générale- ment vers le bas, en s’éloignant du contact entre le carbonate de Ratner et l’anhydrite de Whitkow. Le rapport moyen 87 Sr/ 86 Sr de la calcite MVC est de 0,7078, ce qui est comparable à l’eau de mer au Dévonien moyen. Selon les inter- prétations, le calcaire MVC de Ratner découlerait du néomorphisme de la calcilutite microcristalline dans un environne- ment d’enfouissement et, selon les observations pétrographiques, des données d’isotopes de l’oxygène et du strontium ainsi que des évidences stratigraphiques, des liquides néomorphiques pouvaient comprendre une eau déshydratant le gypse tout en se mêlant à divers rapports mineurs d’eau de formation. Cette recherche fournit une étude de cas dans laquelle le néomorphisme du calcaire est relié à l’eau de déshydratation du gypse. [Traduit par la Rédaction] Fu and Qing 705 Introduction The term neomorphism was introduced by Folk (1965) to refer to processes of recrystallization and polymorphic trans- formations where there may have been a change of mineralogy. Recrystallization refers to petrographic and geochemical changes without any change of mineralogy. Because carbonate sediments may consist originally of low magnesium calcite, high magnesium calcite, and (or) aragonite and the original mineralogy of limestone is often difficult to determine (Tucker 1990), the term neomorphism is used here. Neomorphic pro- cesses always take place in the medium of water, and neomorphism of limestone is typically associated with crystal growth and (or) transformations of metastable Mg-calcite and aragonite to low-Mg calcite (Bathurst 1975; Tucker 1990). Microspar and pseudospar were interpreted to result from “aggrading neomorphism” (Folk 1965). Lasemi and Sandberg (1984) suggested a one-step process of aragonite calcitization leading to microspar. In some argillaceous carbonates, neo- morphism took place by precipitation of progressively youn- ger calcite on crystals at the growing margins of continuous beds and nodules (Coniglio 1989). A study of neomorphized rocks can provide information of related fluid events. Medium to very coarsely crystalline (MVC) limestone in the Middle Devonian Ratner Formation is interpreted to result from neomorphism of the precursor microcrystalline lime mudstone. This work is focused on the formation of MVC limestone in the Ratner Formation and provides a case study that neomorphism of limestone is greatly influenced by gypsum-dehydration water. Commercial oil has been produced from the Ratner car- bonate reservoirs since 1997, and the Ratner rocks remain underexplored (Kissling and Slingsby 1999; Nimegeers 2005). In some cores of the Ratner Formation, abundant bitumen is preferentially present in the MVC limestone. Investigation of Received 19 June 2006. Accepted 30 January 2007. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 15 June 2007. Paper handled by Associate Editor D. Lavoie. Q. Fu 1 and H. Qing. Department of Geology, University of Regina, Regina, SK S4S 0A2, Canada. 1 Corresponding author (e-mail: fuqilo11@uregina.ca).