ABSTRACT

Despite the widespread extent of the Winnipeg Formation in the Williston Basin and its importance as a source of silica sand in outcrop and a potential oil and gas reservoir in the subsurface, surprisingly little is known of the petrography and diagenesis of this Ordovician siliciclastic unit. The Black Island Member, the lower coarse-grained clastic unit of the formation, was sampled from subsurface core and outcrop in Manitoba, and examined using thin sections, X-ray diffraction, and scanning electron microscopy. A surprisingly diverse suite of authigenic minerals exists, including: quartz, K-feldspar, Na-Ca plagioclase, Ca-plagioclase, anhydrite, calcite, dolomite, ankerite, halite, illite, kaolinite, and pyrite. Although there is considerable regional variation in southern Manitoba, the generalized paragenetic sequence recognized is: 1) Quartz formation as encrusting and terminated (euhedral) overgrowths; 2) Formation of anhydrite cement and neoformed crystals; 3) Precipitation of euhedral, 5 to 50 Jim-sized feldspars; 4) Formation of anhydrite cement and neoformed crystals; 5) Dolomite precipitation; 6)Dedolomitization and formation of calcite crystals and cement; 7)Feldspar dissolution and clay mineral authigenesis; and 8) Formation of pyrite. These results show that the history of pore water solutions leading to the development of this diagenetic mineral suite is complex.