Diagenetic History of Givetian Winnipegosis Formation in Southeastern Saskatchewan

Robin L. Stanford, Brian Jones

A paragenetic sequence, established for the Winnipegosis Formation in the Tableland area of southeastern Saskatchewan, indicates that the main phases of diagenesis occurred in early marine and hypersaline fluids and late pressure-solution fluids.

Initially, marine and possibly meteoric fluids precipitated several generations of fibrous-isopachous and equant-mosaic cements into primary porosity. Prior to extensive compaction and lithification of the sediments, increasingly hypersaline pore fluids resulted in the formation of CaSO₄ nodules and laminations and subsequently, dolomitization. The examination of incompletely dolomitized locations indicates that dolomitizing fluids came from above since dolostones overlie limestones. The dolostones are associated with common-to-abundant, early CaSO₄ precipitation. In addition, dolomitization and lithification of the sediments must have occurred prior to compaction since early CaSO₄ laminations, isolated in mudstones, are unbroken.

Secondary porosity creation accompanied and postdated early dolomitization. Leaching was related to variations in the solubility of the substrate and, possibly, freshening of the pore fluids. Locally, leaching was accompanied by partial replacement of earlier dolomites by poorly consolidated very finely crystalline sucrosic dolomites.

Pressure solutioning led to the precipitation of zoned overgrowth cements into secondary and remnant primary porosity. The zoned cements are most abundant in lithologies rich in stylolites, and the cement composition varies with the substrate composition (i.e., calcite in limestones and dolomite in dolostones).

Pressure solution also caused local obliterative recrystallization and, possibly, a minor phase of cannibalization-dolomitization. The late dolomites replace less than 5% to 20% of pelmatozoan-rich limestones and occur primarily within patches of black residue associated with unusually highly corroded pelmatozoans.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988