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Micro-Textural Analysis of a Mudstone-Dominated Succession: Insights from the Upper Cretaceous Puskwaskau Formation of North-Central Alberta


The Upper Cretaceous Puskwaskau Formation is an extensive syn-tectonic wedge that spans a large area of the Western Canada Foreland Basin. This regionally-extensive, mudstone-dominated formation provides an opportunity to investigate mudstone sedimentology and regional, basin-scale stratigraphy of fine-grained shelf deposits in a distal foreland basin setting (> 200 km offshore). Mudstone-dominated successions of the Western Canada Foreland Basin have the potential to be used as analogues for shale reservoirs in the Appalachian Foreland Basin. The studied interval is exposed at a location in northern Alberta. At that locality, a suite of 19 oriented block samples were obtained. In the laboratory, samples were impregnated with resin that yielded a solid block of stabilized mud about 5 × 5 × 8 cm. These samples were cut into slabs from which were prepared, over-sized thin sections, with the original field orientation retained. Thin-section observations show that these mudstones are heterogeneous at scales ranging from 10-2 to 10-5 m. Samples of the Puskwaskau Formation examined with SEM in backscatter mode reveal a wealth of micron-scale fabrics. There are few studies that have attempted to understand the sedimentology of mudrocks at the micron-scale, or the transport mechanisms that distributed fine-grained sediment over dip and strike directions of hundreds of kilometers in the Western Canada Foreland Basin. An organic-rich, condensed section is recognized near the middle of the Puskwaskau Formation and represents the maximum flooding of the Santonian Western Interior Seaway. This ‘hot shale’ facies consists of calcareous, bentonitic mudstone and is an important source-rock (TOC up to 2.9%) that is recognized as being equivalent to the Niobrara Formation, a hot shale gas play that has received much attention in recent years.