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. FIG. 8.-Isopach map of the BAT1 sequence. Contours (10-m interval; legend lower left) represent valley-fill thickness, which is a surrogate for depth of incision. Dashed line indicates fault; U ϭ upthrown side; D ϭ downdropped side. Inset map provides a simplified sketch of the valley pattern showing the inferred stream orders for the various valleys.

Sediment Transport, Bed Morphology and the Sedimentology of River Channel Confluences

“…This figure matches well with past studies examining the depth of scour within modern channels (Figure 4,2 and references therein). Ardies et al (2002) highlight that these localized scour fills may be excellent targets for hydrocarbon exploration. The presence/absence of confluence scour and its depth may also be a considerable aid to deciphering the nature of autocyclic and allocyclic scour and identifying the controls on fluvial deposition (Salter, 1993;Best and Ashworth, 1997;Fielding, 2007); however, the scour zones need to migrate spatially through time to produce widespread erosion surfaces (see Roy and Sinha, 2005, for an account of migration of confluences of the Ganga-Ramganga-Garra rivers in India).…”

“…For instance, Ardies et al (2002) show that in the Lower Cretaceous incised valleys of Western Canada, enhanced erosion at tributary junctions has produced regions, approximately 2-3 km in diameter, where the valley fill is up to five times thicker and much coarser grained than in the deposits of the adjacent valleys. This figure matches well with past studies examining the depth of scour within modern channels (Figure 4,2 and references therein).…”

A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

“…The thickness variations that do exist in Lower Cretaceous deposits are significantly influenced by differential erosion on the angular sub-Cretaceous unconformity, with thicker sediment accumulations occupying paleovalley systems incised into Jurassic and pre-foreland sedimentary rocks (Jackson, 1984;Hayes, 1986;Ranger and Pemberton, 1988;Wightman and Pemberton, 1997;Ardies et al, 2002;Zaitlin et al, 2002).…”

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

“…Within this zone of reworking, the associated confluence scours are each likely to occupy zones up to 8-10 km long and up to 5 km wide. Over longer time periods, these are likely to form continuous composite scour surfaces, perhaps similar to the Lower Cretaceous tributary scour surfaces reconstructed by Ardies et al (2002). Confluence evolution in response to channel movement can also be seen in meandering rivers, as illustrated by the junction of the Paraguay and Bermejo rivers in Argentina (Fig.…”

“…Since the depth of junction scour and mobility of the confluence are determined by flow processes in the confluence hydrodynamic zone (Best and Rhoads, 2008), it can be argued that differing junction dynamics may produce a range of characteristic confluence zone sedimentology from sandy bar development to mudfilled scours. Furthermore, understanding the planform mobility of confluences, and thus the potential spatial extent of basal scour surfaces, particularly in large rivers, is key to interpreting alluvial stratigraphy and discriminating between autocyclic and allocyclic scour surfaces (Best and Ashworth, 1997;Fielding, 2008), reconstructing palaeohydraulics and channel sedimentary architecture (Bristow et al, 1993;Siegenthaler and Huggenberger, 1993;Miall and Jones, 2003;Davies and Gibling, 2011), as well as identifying potential sites for hydrocarbon exploration (Ardies et al, 2002).…”