Controls on the Geometry of Incised Valleys in the Basal Quartz Unit (Lower Cretaceous), Western Canada Sedimentary Basin

Abstract

In southern Alberta, multiple episodes of fluvial erosion occurred during Lower Mannville, Basal Quartz time (Early Cretaceous), creating a series of incised-valley systems. This study examines two of these valley networks, in order to illustrate the geomorphic and genetic complexity of incised-valley systems. Both valley networks are filled by a transgressive succession of fluvial to estuarine deposits. Isopach mapping of these deposits over an area of approximately 10,000 km 2 in southern Alberta shows that both systems are dendritic to rectilinear in character, with the trunk valley flowing from south to north along the length of the Western Canada Sedimentary Basin. These systems were fed from the west, south, and east by a network of tributaries. Eustatic sea-level changes may have played a role in initiating river incision, but regional tectonic movements had an important influence on the history of accommodation change. The periods of erosion appear to have been relatively brief because the valleys are narrow, although confinement within cohesive, mudstone-rich paleosols of the underlying incised-valley deposits may have slowed valley widening. A relatively wet paleoclimate may also have contributed to the formation of narrow valleys by favoring the existence of single-channel rivers. The general morphology of the Alberta foreland basin, together with the distribution of unfilled relief associated with a preceding valley system and/or of less resistant, slightly older deposits, determined the location of the trunk valleys. Uplift of the Bow Island Arch (Sweetgrass Arch) to the east may have been responsible for a shift in the valley headwaters between the formation of the two valley systems. Synerosional block faulting in response to flexural loading had a significant influence on the geometry of the valley network through its control on the location and geometry of major tributary valleys. The trunk valley is also significantly narrower and more deeply incised where it crosses the most pronounced uplifted block. Enhanced erosion at tributary junctions and valley bends produced localized areas (ca. 2-3 km in diameter) where the valley fill is up to five times thicker and considerably coarser grained than the adjacent valley deposits. Such localized scour fills represent prime petroleum-exploration targets.