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Doctor of Philosophy




Plint, A. Guy


The marine mudstone-dominated, Muskiki and Marshybank members of the Wapiabi Formation occupy the central foredeep of the Western Canada Foreland Basin. These Coniacian (86.3-89.8 Ma) rocks form a ~100 m thick, overall shallowing-upward depositional succession that records a basin-scale transgressive-regressive event following an early Coniacian lowstand. The strata, deposited on a low-gradient storm- dominated marine ramp, contain numerous well-preserved molluscan fossils and comprise many upward-coarsening successions (5-15 m thick). Detailed correlation integrates biostratigraphic and lithologic data from 14 cores and 13 outcrops into a regional (~45,000 km2) allostratigraphic framework constructed from 1020 wireline logs. This integrated stratigraphy provides a basis for an investigation of regional stratal relationships, the stratigraphic distribution of ammonites and inoceramid bivalves, and the completeness of the carbon-isotope record. This examination is part of a larger, basin- scale study that embraces Upper Cretaceous rocks within Alberta, British Columbia, and northern Montana.

Allostratigraphic correlation of flooding surfaces reveals 4 subtle, regional disconformities that package metre-scale, sheet-like allomembers into 5 ‘tectono- stratigraphic’ units. Collectively, the Muskiki-Marshybank package forms a westward- thickening wedge, readily explicable in terms of flexural subsidence within a foreland basin. Each tectono-stratigraphic unit has a broadly sheet- to wedge-like geometry, and successive isopach maps show that depocentres shifted abruptly, with sediment accumulating in shallow sags, and being eroded over subtle up-warps. These patterns are thought to be the result of subtle warping of the Late Cretaceous seafloor, perhaps attributable to differential movement between deep-seated fault-bounded blocks. Within the limited, available public-domain seismic and potential field datasets, faults and/or structures that underlie the current study are recognizable; however, it is difficult to find features that correlate with the observed stratigraphic patterns. This could be attributed to a lack of resolution or a physical property variation that is insufficient to be measured with these geophysical methods.

This study demonstrates that it may be naïve to assume that seemingly monotonous successions of offshore mudstones preserve a complete stratigraphic record. Where comparable mudstone units have reservoir properties, complex depositional patterns and internal stratigraphy could affect attempts to exploit resources.

Summary for Lay Audience

In Western Canada, the abundance of outcrop exposures within the Rocky Mountain Foothills and the subsurface data from borehole logs permit detailed investigations that further our current understanding of the geologic development mechanisms and physical landscapes that existed in the past. In this study of the Upper Cretaceous (Coniacian age) Muskiki and Marshybank members (Wapiabi Formation), a stratigraphic approach (allostratigraphy) was used to create a proxy time framework in which the sedimentary and subsidence history of the Western Interior Basin could be reconstructed. These marine, predominantly mud-rich rocks are found to record an overall geometry that corresponded to the predicted regional subsidence pattern for the Cretaceous Western Interior Basin (foreland basin) – a wedge of sediment that thickens towards the mountain belt, the result of simple asymmetric flexure of the tectonic plate underlying the foreland basin. When investigated at a smaller-scale, the overall Muskiki-Marshybank rock package was found to contain several sub-units that exhibited more complex depositional geometries – interpreted to be the result of local, short-lived tectonically-controlled uplift and subsidence of the seafloor, and sea level changes.

The depositional complexity observed within the Coniacian Muskiki and Marshybank members emphasizes that future investigations of similar, apparently monotonous mudstones should be conducted at a smaller-scale in order to reduce the risk of oversimplification. Since this investigation completes an investigation of the Muskiki and Marshybank members within Alberta, British Columbia, and northern Montana, it is now possible to investigate the Coniacian Western Canada Foreland Basin at a basin-scale, and further the current understanding of the dynamics of foreland basins and the timescales at which tectonic and sea level changes occur. In addition, the large suite of Coniacian fossils (inoceramid bivalves and scaphitid ammonites) that were collected in association with this project can be used for future long-distance correlation (e.g., from Canada to the United States and Europe).

Available for download on Friday, January 01, 2021

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