Petrographic and Engineering Properties of Fine Grained Rocks of Central Alberta

The central Alberta Plains are underlain by a gently westward-dipping succession of non-marine detrital strata of Late Cretaceous and Paleocene ages, which merges to the west with folded and faulted retaceous beds of the Rocky Mountain Foothills. Bedrock slumping is specially common along river valleys and man-made slopes in the eastern Plains, decreasing to the west towards the Foothills margin. This change in bedrock 'strength' across the strike of the strata corresponds to an apparent regional increase in a westerly direction of the relative intensity of diagenetic effects, described in terms of compression, recrystallization, cementation, and chemical alteration.

The finer-grained rocks of central Alberta are mainly siltstones and silty claystones containing montmorillonite as the dominant clay material. The strata exhibit extreme variability in texture and microstructures: most are laminated to some degree, but few are issile or exhibit marked grain orientation. Wet-dry cycle tests, an pproximate measure of bond strength and hence the effects of diagenesis, show that the fine-grained strata in the eastern part of the area (Upper Cretaceous) are mainly 'compaction' rocks, whereas hose in the western Plains and adjacent Foothills (Paleocene and Cretaceous, respectively) are mainly 'cementation' rocks. This distinction is supported by bulk density determinations of outcrop nd corehole samples, which increase in value from east to west, although this trend is masked to some extent by local variations in mineral composition, especially montmorillonite content.

Most of the fine-grained rocks from the central Alberta Plains are characterized by properties intermediate between those of true 'soil' and true 'rock', i.e. they are 'soft rocks.' Drained direct shear tests on corehole samples from the Plains area show that three strength parameters exhibit high correlations with clay and ontmorillonite contents, bulk density, wet-dry cycle ratings, and distance from the Foothills margin. The fourth parameter shows low or nonexistent correlations with other rock properties. Multiple regression analyses of the data indicate that the major casual factors associated with strength parameters are: clay content, density and distance.