Buried valley

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A buried valley is an ancient river or stream valley that has been filled with glacial or unconsolidated sediment. [1] This sediment is made up of predominantly gravel and sand, with some silt and clay. These types of sediments can often store and transmit large amounts of groundwater and act as a local aquifer.

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Buried valleys may have been created by glacial lake runoff prior to the last major advance and retreat of continental glaciation. These valleys often have no surface expression, but constitute a major source of groundwater in the glaciated mid-continent region of North America [2] and Northern Europe. [3] Recently, research has been focused on understanding the sedimentology of these formations in an effort to determine the safety of continued use of the aquifers which are often found in them. [4]

Overview

Buried valleys are created when ancient river or stream valleys are present that predate the most recent glaciation, and since have been filled with glacial till and/or outwash. [5] In the Pleistocene, advancement and retreat of glaciers carved out the preexisting valleys, and deposited the material that had accumulated in the glacier by the melting of either the glacial outwash or the melting of the ice that composed the glacier itself. Buried valleys are traditionally V- or U-shaped due to the natural shape of valleys, but buried valleys can exhibit different shapes if there were any sort of erosional events after the glacier had finished retreating. [6] The ability to hold groundwater comes from the makeup of the glacial outwash deposits. The glacial outwash deposits of these preexisting valleys mainly consist of coarser materials, such as sand and gravel. Since these materials are coarser, when there is a soil that is almost purely made of these materials, the pore space of the soil increases. This increased pore space creates more voids for water, especially when compared to silt or clay rich soil. After these valleys are formed and filled in, a layer of finer sediment such as silts and clays covers the top of the valley, burying the valley. Buried valleys are best known as being aquifers, and are often used to supply humans with potable water, as well as supply the agriculture and industrial fields with water.

Mapping buried valleys

Buried valleys are difficult to measure and create models for, as they are normally buried deeply beneath the earth's surface in geologically complex areas. There are several different methods that are used to identify buried valleys, including determining depth to bedrock, the drilling/boring in to the earth's crust to analyze soil makeup, and utilizing existing water wells, although it is said that these methods alone are barely adequate. [7] In order to map the buried valleys, dense data coverage is essential. Typically, the borehole density is relatively small and especially deep boreholes are sparse. [8] It is important that the borehole density is large throughout the area being mapped so that a higher level of accuracy can be attained. Since the buried valleys are carved out by glaciers, topography can be highly variable. The more borings/ cores that are taken, the more accurate the map will be. Based on depth to bedrock reported in water well observations, the electrical conductivity of bedrock appears largely separable from that of the overlying glacial sediments due to the higher conductivity of the shale present in some soils. In one study done in Denmark, a series of braided buried valleys were found anywhere from 10 m to 300 m below the surface using drilling/boring. [9] In order to map the buried valleys, dense data coverage is essential. Although there are no published 3D models of buried models, 2D models have been published many times.

Buried valleys as aquifers

Perhaps what buried valleys are most known for is their ability to become aquifers that store groundwater. The main form of recharge found in buried valley aquifers occurs is the percolation of groundwater through glacial tills and upper intertill aquifers. [10] In fact, many communities in the Midwest get their water from aquifers that were created from these buried valleys, such as Miami, Ohio. [11] In some prairie buried valleys in Canada, the underlying material is made of dense shale, while the covering over the buried valley is made up of such a dense composition Quaternary fill that it vastly limits recharge of the buried valley aquifer, almost to the point of restricting it completely. [12] Recharge is very important, as many different cities all over the world use buried valleys as their main water source. Often, the reason buried valleys are mapped is due to the ability to utilize previously deeply drilled wells that have been supplying water to communities. [13] Since buried valleys were once valleys on the earth surface, they have a slope or gradient to them. Due to this slope within the valleys, the water will move according to gravity, and produce a flow of water from the higher elevation areas to the lower elevation areas.

See also

Related Research Articles

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A drumlin, from the Irish word droimnín, first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine. Clusters of drumlins create a landscape which is often described as having a 'basket of eggs topography'.

Aquifer Underground layer of water-bearing permeable rock

An aquifer is an underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials. Groundwater can be extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude, which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could create a confined aquifer.

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Groundwater Water located beneath the ground surface

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A kettle is a depression/hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating glaciers, which become surrounded by sediment deposited by meltwater streams as there is increased friction. The ice becomes buried in the sediment and when the ice melts, a depression is left called a kettle hole, creating a dimpled appearance on the outwash plain. Lakes often fill these kettles; these are called kettle hole lakes. Another source is the sudden drainage of an ice-dammed lake. When the block melts, the hole it leaves behind is a kettle. As the ice melts, ramparts can form around the edge of the kettle hole. The lakes that fill these holes are seldom more than 10 m (33 ft) deep and eventually become filled with sediment. In acid conditions, a kettle bog may form but in alkaline conditions, it will be kettle peatland.

Outwash plain Plain formed from glacier sediment that was transported by meltwater.

An outwash plain, also called a sandur, sandr or sandar, is a plain formed of glacial sediments deposited by meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and carries the debris along. The meltwater at the snout of the glacier deposits its load of sediment over the outwash plain, with larger boulders being deposited near the terminal moraine, and smaller particles travelling further before being deposited. Sandurs are common in Iceland where geothermal activity accelerates the melting of ice flows and the deposition of sediment by meltwater.

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References

  1. ↑ Ohio DNR, Well Construction in a Buried Valley
  2. ↑ Kehew, Alan E.; Boettger, William M. (Nov–Dec 1986). "Depositional Environments of Buried-Valley Aquifers in North Dakota". Groundwater. 24 (6): 728–734. doi:10.1111/j.1745-6584.1986.tb01688.x.
  3. ↑ Jorgensen, F; Sandersen, P (2006). "Buried and open tunnel valleys in Denmark - erosion beneath multiple ice sheets". Quaternary Science Reviews . 25 (11–12): 1339–1363. Bibcode:2006QSRv...25.1339J. doi:10.1016/j.quascirev.2005.11.006.
  4. ↑ Smith, LN (2004). "Late Pleistocene stratigraphy and implications for deglaciation and subglacial processes of the Flathead Lobe of the Cordilleran Ice Sheet, Flathead Valley, Montana, USA". Sedimentary Geology . 165 (3–4): 295–332. Bibcode:2004SedG..165..295S. doi:10.1016/j.sedgeo.2003.11.013.
  5. ↑ Seyoum, W. , & Eckstein, Y. (2014). Hydraulic relationships between buried valley sediments of the glacial drift and adjacent bedrock formations in northeastern Ohio, USA. Hydrogeology Journal, 22(5), 1193–1206.
  6. ↑ Metzen, J. (2012). Discovery of an e-w trending Pleistocene buried valley in the German bight, southern North Sea. Quaternary International, 279–280, 325.
  7. ↑ Oldenborger, G. , Logan, C. , Hinton, M. , Pugin, A. , Sapia, V. , et al. (2016). Bedrock mapping of buried valley networks using seismic reflection and airborne electromagnetic data. Journal of Applied Geophysics, 128, 191–201.
  8. ↑ Høyer, A. , Jørgensen, F. , Sandersen, P. , Viezzoli, A. , & Møller, I. (2015). 3d geological modelling of a complex buried-valley network delineated from borehole and aem data. Journal of Applied Geophysics, 122, 94–102.
  9. ↑ He, X. , Sonnenborg, T. , Jørgensen, F. , & Jensen, K. (2017). Modelling a real-world buried valley system with vertical non-stationarity using multiple-point statistics. Hydrogeology Journal, 25(2), 359–370.
  10. ↑ Dragon, K. (2008). The influence of anthropogenic contamination on the groundwater chemistry of a semi-confined aquifer (the wielkopolska buried valley aquifer, Poland). Water Resources Management, 22(3), 343–355.
  11. ↑ "Miami Conservation District. (2009). Water in the Great Miami River Watershed". Archived from the original on 2016-11-11. Retrieved 2017-05-08.
  12. ↑ Buried-valley aquifers in the Canadian prairies: Geology, hydrogeology, and origin 1 1 earth science sector (ess) contribution 20120131. (2012). Canadian Journal of Earth Sciences, 49(9), 987–1004.
  13. ↑ Kluiving, S. , Aleid Bosch, J. , Ebbing, J. , Mesdag, C. , & Westerhoff, R. (2003). Onshore and offshore seismic and lithostratigraphic analysis of a deeply incised Quaternary buried valley system in the northern Netherlands. Journal of Applied Geophysics, 53(4), 249–271.

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