Characterization of Cenomanian-early Turonian bentonites in southwestern Saskatchewan through lithostratigraphy and chemostratigraphy analysis
Volcanic eruptions can disperse tephra over a wide area, and may result in deposition of ash far from their sources. For example, ash from the 1980 eruption of Mt. St. Helens volcano in Washington State was observed falling upon Regina, Saskatchewan. Bentonite is a name for widely distributed beds of altered ash that have the potential to be used as isochronous horizons in lithostratigraphic correlation, much in the same way that tephrochronology is applied to young volcanic deposits. Numerous bentonite horizons are preserved within Late Cretaceous strata of western Canada and northern USA, and several widespread layers delineate unit or formation boundaries (e.g., the Clay Spur bentonite, X-bentonite). This thesis outlines studies of Cenomanian-early Turonian bentonite across southwestern Saskatchewan, with an aim not only to document these, but to rigorously test whether bentonite can help in refining the regional stratigraphic framework of an area. Specifically, bentonite occurring within the Belle Fourche and Second White Specks formations of southwestern Saskatchewan were investigated using lithological, well log and mineral chemistry data. As evidenced in previous research, near pristine magmatic mineral grains can be preserved in bentonite, despite the changes that accompanied its burial and lithification. Fresh grains of mica and feldspar were identified in bentonite from the study area. Analysis of these crystals record specific and unique compositions for each layer and can provide strong discriminants to identify or correlate layers of bentonite. Moreover, the character of determined mica and feldspar compositions furnish clues to their magmatic sources. Three regional cross-sections are presented herein, highlighting lithological and stratigraphic associations for bentonite within and across the study area. These derive from the examination of 6 principal lithologies, from across 13 wells and aided by detailed studies and interpretation of 56 geophysical wireline logs in Geoscout software. Fourteen bentonites were recorded from the Late Cretaceous strata, with a subset of 35 samples characterized in terms of well log responses, lithological characteristics, and mineral compositions. The 14 layers may be subdivided into four compositional groups, reflecting their volcanic sources (i.e., calc-alkaline, alkaline, peraluminous and sub-alkaline group). Also provided is a database of bentonite, that summarizes key characteristics of each bed to be used as a reference in future studies or for a regional stratigraphic correlation. The results of this study show that bentonite lithostratigraphy and chemostratigraphy have great promise in local and regional correlation and that bentonite is readily identifiable through a combination of core examination and geophysical well log studies. Analysis of crystals within can provide a fingerprint for each bentonite that may help to refine local stratigraphy, especially useful when other methods are inconclusive; for example, a lack of microfossils or facies changes across an area.
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Faculty of Graduate Studies and Research, University of Regina