ABSTRACT: Diagenetic Evolution of Cambro-Ordovician Clastic Sedimentary Rocks, Southwestern Ontario

M. A. Wadleigh, F. J. Longstaffe, S. K. Frape, R. H. McNutt, C. McKenna

Cambrian Mount Simon/Eau Claire interbedded sandstones and sandy dolomites and Ordovician Shadow Lake argillaceous sandstones are stratigraphically situated on Precambrian basement at the interface between the Appalachian and Michigan basins. This unique location provides an opportunity to examine diagenetic effects resulting from water-rock interaction involving multiple fluid sources, possibly including Precambrian basement brines.

A striking feature of the sandstones is the abundance of K-feldspar (to the virtual exclusion of other feldspars). K-feldspar occurs as detrital grains, overgrowths and euhedral (authigenic?) crystals. Diagenetic phases are enriched in K and depleted in Na and Ba relative to their detrital equivalents. Widespread potassic alteration has been recognized elsewhere in midcontinental North America. Similar, if not the same, formation waters may have entered the Cambrian section in southwestern Ontario.

A variety of other diagenetic minerals are observed. Early diagenetic glauconite with a chemical composition midway in the typical range is found near the top of the Cambrian section and within the Ordovician sandstones. Fine-grained clay, mostly illite ± chlorite, partially fills pores between detrital quartz and feldspar grains. It appears to predate dolomite and calcite cements and perhaps quartz/feldspar overgrowths.

Dolomite is the most common carbonate cement. It occurs as euhedral rhombs within pores, as a replacement of K-feldspar and as a massive pore filler. Some varieties exhibit regular chemical zonation of Fe and Mn; others display complex intergrowths of Fe+Mn-rich and Fe+Mn-poor phases. Dolomite crystallization appears to postdate formation of diagenetic pyrite, matrix illite, and most calcite cement. Calcite occurs as scattered inclusions within dolomite, as cores to dolomite rhombs, and as a massive pore filler.

The diagenetic mineralogy of the sandy dolomites is similar to that of the sandstones except that a minor amount of smectitic clay occurs together with illite in the <2 µm size fraction. The preservation of smectitic clay in the sandy dolomite suggests isolation of that portion of the pore system during the major episode of potassic diagenesis.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990