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Geology and hydrogeology of prairie buried valleys

Buried valleys (BVs) in Canadian Prairies are sediment-filled incised valleys eroded into low-permeability, poorly-consolidated bedrock, typically Cretaceous shale, and blanketed by low-permeability, mud-rich glacial diamicton. Many contain aquifers and they are important sources of
groundwater for drinking, agriculture, mining, and sustaining surface-water fluxes. The origin of BVs is inferred as either; 1) glaciofluvial erosion by meltwater flowing beneath or in front of glaciers, or, 2) fluvial erosion during the late Tertiary (i.e., prior to Quaternary glaciation) by rivers
flowing north-eastward from Rocky Mountains via Hudson Bay to the Labrador Sea. It is possible that many larger valleys are not the result of a single formative process. Buried valleys occur across the Prairies, with lengths ~1- 300 km, widths ~0.5-30 km and depths ~10-180 m. Inferred glaciofluvial
BVs have width-to-depth ratios of less than 10:1, whereas inferred pre-glacial (fluvial) BVs generally have width-to-depth ratios greater than 10:1. Sparse regional data suggest that Prairie BVs slope 0.5 to ~5 m per kilometre along their lengths, commonly parallel to regional bedrock slope.
However, large Spiritwood and Hatfield buried valleys, two important Prairie aquifer systems, exhibit no obvious slope over hundreds of kilometres using a modern day datum. Prairie buried-valley fills tend to be heterogeneous and display complex architectures. Sand, gravel, mud and diamicton are
common components and any one can dominate the fill sequence. Variability in BV fill and geometry, and irregularity of the valley floor may result in flow barriers, which complicate resource assessment and sustainable development. Prairie BVs have distinct geological characteristics that can be
related to their hydrogeological properties. Low permeability shale tends to underlie buried-valley aquifers and thick (many 10s of m) low-permeability till and glacial lake mud tend to overlie them. This reduces or prevents recharge to buried-valley aquifers, thus protecting groundwater resources
from contamination and from hydrologic stress. Low-permeability settings tend to lead to groundwater chemistries that are intermediate between those of groundwater in bedrock and till.

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