PETROGRAPHIC STUDY OF MARGINAL SEDIMENTS FROM THE ELK POINT GROUP OF NORTHWESTERN ALBERTA, CANADA

Primary mineralogy of the sandstones in the Devonian Elk Point Group w as dictated by a predominantly granitic source area with minor metamor phic and volcanic components. Extensive diagenesis involved dissolutio n of feldspars and lithoclasts, precipitation of quartz, and a second phase of dissolution of feldspar, lithoclasts and quartz. Analysis of the clay fraction shows an absence of significant amounts of kaolinite but an abundance of illite and chlorite. Later, authigenic calcite an d dolomite precipitated in Elk Point sandstones, which were subsequent ly further cemented by anhydrite. Cementation, especially by quartz, h as considerably reduced the porosity in many places. Regional variatio n in porosity is marked by high variability on meso- and micro-scales. Authigenic calcite and dolomite are significant components but were s ubjected to dissolution before a phase of anhydrite growth. Partial di ssolution of anhydrite is evidence of the last event. Periods of flush ing by meteoric-water affected early stages of diagensis with evaporit ic brines prominent in mesogenesis.