Abstract

The Scollard Formation sandstones in the Red Deer Valley region of central Alberta are composed of lithic arenites and sublitharenites, which accumulated in braided to meandering fluvial environments. The early diagenesis of the Scollard sandstones is characterized by initial mechanical compaction; calcite cementation; dissolution of detrital feldspars along with the dissolution of volcanic fragments; authigenic quartz overgrowth; and formation of authigenic clay minerals as coatings, rims, pore-linings, and pore-fills. Late diagenesis is dominated by continued precipitation of authigenic quartz and pore-filling kaolinite; dissolution of feldspar grains; corrosion and dissolution of quartz grains and authigenic silica; alteration of mica, biotite and iron-bearing minerals; dissolution of calcite cement; and precipitation of hematite cement. The limited conversion of smectite to illite, as well as the weak albitization of K-feldspar imply maximum burial temperatures of less than 120°C.

The detrital and authigenic mineral composition coupled with oxygen and carbon isotope analyses indicate that significant climatic fluctuations between relatively wetter and drier conditions appear to have occurred during the accumulation of the Scollard Formation. Wetter climatic conditions are interpreted for the arenites with higher amounts of mono- and/or polycrystalline quartz, kaolinite, and quartz overgrowth. Drier climatic conditions are inferred for the arenites with lesser percentages of quartz but higher amounts of authigenic smectite. Since neither the petrographic (framework grains) nor the SEM-XRD (authigenic clay mineralogy) data sets can address the issue of paleoclimate in an univocal manner, the key for such interpretations is the correlation between the detrital and authigenic mineral constituents. The good correlation between the two constituent types suggests that the precipitation of authigenic clay minerals took place relatively soon after burial, which is in agreement with the meteoric origin of the diagenetic fluids that is indicated by the δ18O stable isotope data.

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