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Master of Science




A. Guy Plint


Late Albian rocks of the Upper Viking, Westgate and lower Fish Scales alloformations were deposited about 101-100 Ma in the Western Canada Foreland Basin. Sediments in the study area span the forebulge, and are dominated by mudstone which forms 5-15 m thick, siltier-or sandier-upward sucessions bounded by marine flooding surfaces. Stratification is on a mm scale and represents storm deposits and fluid muds. Viking allomember VD and Westgate allomembers WA, WB and WC all thin towards the E and S and progressively onlap the forebulge. They record relative sea-level rise and gradual drowning and subsidence of the forebulge. In contrast, Fish Scales allomember FA thins to the west because it is erosionally truncated by surface BFSM. Beveling of FA implies contemporaneous uplift and westward migration of the forebulge, which can be linked to an abrupt reorientation, and increase in subsidence rate of the foredeep to the west.

Summary for Lay Audience

The Rocky Mountains in Western Canada formed about 150 million years ago because of convergence and compression between the North American and Pacific plates. Uplift of the Rockies resulted in a load on the crust, which caused it to bend downward, forming a ‘foreland basin’. Sediment eroded from the mountains was carried eastward by rivers to accumulate in the adjacent basin. Computer models of foreland basins predict greatest subsidence close to the mountains, forming the ‘foredeep’, and an area of subtle uplift- the ‘forebulge’ – located 200-400 km from the edge of the mountains. Models predict that the forebulge will migrate both up and down, and closer and further from the mountains, depending on how fast, and where the mountains are being uplifted.

The behavior of forebulges in ‘real-world’ sedimentary basins is not well known because there are few areas where sedimentary layers can be observed in the necessary detail. The Western Canada foreland basin, because of extensive oil and gas drilling, is an unique natural laboratory in which to investigate forebulge behavior. This study used data from about 3000 boreholes in NE Alberta to map mudstone layers, deposited in a shallow sea during the Cretaceous Period, between about 101 and 100 million years ago. The rocks were divided into 12 packages, defined by erosion surfaces that formed when sea-level rose and fell. After tracing the layers between all the boreholes, it was evident that, during the first part of the study interval, the forebulge migrated gradually to the SE and subsided, and the sea progressively flooded a subtly-uplifted land area. This phase implies a time of diminishing mountain-building in the west. Later, it could be shown that layers were tilted and beveled off by the action of waves in shallow water. This phase of erosion provided evidence that the forebulge had been uplifted and migrated to the west. The new results, when compared with previous UWO studies in the western foredeep showed that diminishing, then accelerating tectonic activity in the west corresponded closely in time with subsidence, and later uplift of the forebulge, thus corroborating the predictions of numerical models.

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