Tunnel valley

Tunnel valleys play a role in identifying oil-rich areas in Arabia and North Africa. The Upper Ordovician–Lower Silurian materials there contain a roughly thick, carbon-rich layer of black shale. Approximately 30% of the world's oil is found in these shale deposits. Although the origin of these deposits is still under study, it has been established that the shale routinely overlies glacial and glacio-marine sediment deposited ~445 million years before the present by the Hirnantian glaciation. The shale has been linked to glacial meltwater nutrient enrichment of the shallow marine environment. Hence the presence of tunnel valleys is an indicator of the presence of oil in these areas. Tunnel valleys represent a substantial fraction of all meltwater drainage from glaciers. Meltwater drainage influences the flow of glacial ice, which is important in understanding of the duration of glacial–interglacial periods, and aids in identifying glacial cyclicity, a problem that is important to palaeoenvironmental investigations. Tunnel valleys are typically eroded into bedrock and filled with glacial debris of varying sizes. This configuration makes them excellent at capturing and storing water. Hence they serve an important role as aquifers across much of Northern Europe, Canada and the United States. Examples include Oak Ridges Moraine Aquifer, Spokane Valley-Rathdrum Prairie Aquifer, Mahomet Aquifer, the Saginaw Lobe Aquifer, and the Corning Aquifer. ==Characteristics==

Buried, open and partially filled Tunnel valleys have been observed as open valleys and as partially or totally buried valleys. If buried they may be partially or totally filled with glacial outwash or other debris. The valleys may be incised in bedrock, sand, silt, or clay. Dimensions They vary in channel depth and width; Danish examples run from wide and from deep. They vary in depth along their course, exhibiting overdeepening; overdeepened sections cut into bedrock and typically are significantly deeper than either upstream or downstream sections of the same tunnel valley. They have steep sides which are frequently asymmetric. They frequently terminate at a recessional moraine. Tunnel valleys from successive glaciations may crosscut one another. Tunnel valleys frequently run along roughly parallel courses. They originate in and run through regions which include clear evidence of glacial erosion through abrasion and may exhibit striations and roche moutonnée. Depositional forms such as terminal moraines and outwash fans are found at their terminal end. glacial period. The water flow was from upper right to lower left. Close examination shows the existence of buried tunnel valleys as well – they can be identified by contrasting vegetation. Tunnel valley channels often start or stop abruptly. They have convex-up longitudinal profiles. They are often occupied by elongated lakes of underfit streams. They frequently show signs of subsequent depositions such as eskers. Evidence of erosion mechanisms Evidence suggests that erosion in a tunnel valley is primarily the result of water flow. They erode by meltwater, which it has been argued, episodically drains in repeated jökulhlaups from subglacial lakes and reservoirs; examples of such motion have been observed in Antarctica. Although there is evidence of ice erosion such as linear striations in the bedrock, these are observed only in the widest valleys, and are believed to have played a secondary role. The subglacial layout of valley tunnels is predominantly oriented parallel to glacial ice flow lines – essentially they stretch from areas of thicker sheet ice toward areas of thinner sheet ice. They can exhibit reverse gradients, which result when pressurized meltwater flows over obstacles such as ridges or hills along the glacier bed. Tunnel valleys can be formed under extremely thick glacial ice – examples have been observed on the bottom of Lake Superior and in the oceans offshore in Antarctica. The course of a tunnel valley typically runs from thickest glacial ice to the glacier margin; as a result the glacial ice pressurizes the water such that it runs uphill toward its end. ==Formation of tunnel valleys==

Although there is agreement on the role of meltwater in creation of tunnel valleys, several theories are still under consideration for the role of that meltwater: :*Steady state theory – Boulton and Hindmarsh propose a steady state theory. They suggest tunnel valleys form in unconsolidated sediment when meltwater flows under pressure through an initially narrow subglacial conduit. With progressive removal of sediment by meltwater, ice deforms under its own weight into the cavity to create a tunnel valley through a positive feedback mechanism. :*Jökulhlaup driven erosion – Piotrowski argues that ice sheets may, in some instances, be cold-based; that is they contact land that is frozen (permafrost) and they freeze to the permafrost. Meltwater builds up behind this frozen ice terminus until it generates sufficient pressure to lift the ice and break the bond, with a catastrophic meltwater release such as is seen with the Icelandic jökulhlaup. As a consequence of this jökulhlaup a tunnel valley is formed. Periodic outbursts of subglacial water have been observed moving subglacial water between subglacial lakes beneath the East Antarctic Ice Sheet. Satellite data recorded a subglacial discharge totaling traveling ~ over a period of less than a year. As the flow subsided, the weight of ice closed the tunnel and sealed the lake again. The water flow was modeled satisfactorily with channeling in ice and in sediment. The analytic model shows that over some regions, the ice-bedrock geometry included sections which would have frozen, blocking off flow, unless erosion of the sedimentary substrate was the means of creating a channel and sustaining the discharge. Hence, combining this data and analysis with Icelandic jökulhlaup observations, there is experimental evidence that some form of the jökulhlaup hypothesis with features of the steady state model is correct. Common features of tunnel valley theories formed in a tunnel valley. Note the variable width and the interruption between segments of the course. There is also evidence of other sediment-filled channels adjacent to this (e.g., two smaller lakes to the right) Subglacial meltwater flow is common to all theories; hence a key to understanding channel formation is an understanding of subglacial meltwater flow. Meltwater may be produced on the glacier surface (supraglacially), below the glacier (basally) or both. Meltwater may flow either supraglacially or basally as well; the signatures of supraglacial and basal water flow differ with the passage zone. Supraglacial flow is similar to stream flow in all surface environments – water flows from higher areas to lower areas under the influence of gravity. Basal flow exhibits significant differences. In basal flow the water, either produced by melting at the base or drawn downward from the surface by gravity, collects at the base of the glacier in ponds and lakes in a pocket overlain by hundreds of meters of ice. If there is no surface drainage path, water from surface melting will flow downward and collect in crevices in the ice, while water from basal melting will collect under the glacier; either source will form a subglacial lake. The hydraulic head of the water collected in a basal lake will increase as water drains through the ice until the pressure grows high enough to either develop a path through the ice or to float the ice above it.) or through channels upward into the glacial ice (called Rothlisberger channels), eventually flowing out at the ice margin. On the simplest level, the tunnel valley can be considered a larger-scale version of these phenomena. Tunnel valleys or tunnel channels are produced by meltwater flows beneath glacial ice. Tunnel valleys are often buried or partially buried by sediment accumulation during periods of ice advance and retreat. Although attractive since it scales up the Nye channel formation which has been observed in sediments, a weakness of the steady state theory is that it requires that tunnel valleys be excavated in unconsolidated sediment, in which meltwater is initially forced through an initially narrow subglacial conduit. With progressive sediment erosion by the meltwater, ice deforms under its own weight into the cavity to creating an ever-larger tunnel valley. However the steady state theory appears not to account for erosion into bedrock, which has been extensively observed. As water collects, additional ice is lifted until a release path is created. If no preexisting channel is present, the water is initially released in a broad-front jökulhlaup which can have a flow front that is tens of kilometers wide, spreading out in a thin front. As the flow continues, it tends to erode the underlying materials and the overlying ice, creating a channel even as the reduced pressure allows most of the glacial ice to settle back to the underlying surface, sealing off the broad front release and channelizing the flow. The direction of the channel is defined primarily by the overlying ice thickness and secondarily by the gradient of the underlying earth, and may be observed to “run uphill” as the pressure of the ice forces the water to areas of lower ice coverage until it emerges at a glacial face. Hence the configuration of the various tunnel valleys formed by a specific glaciation provide a general mapping of the glacier thickness when the tunnel valleys were formed, particularly if the original surface relief under the glacier was limited. The debris found in tunnels and at the mouth of tunnels tends to be coarse rocks and boulders – this is indicative of high flow velocities and an extremely erosive environment. This erosive environment is consistent with creation of tunnels over deep and wide, as have been observed in the Antarctic. Tunnel valleys may remain open, partially filled or filled, as a function of the glacial recession. The filled configuration is significant because filled tunnel valleys become excellent reservoirs for either water (aquifer) or for oil. This results since relatively coarse-grained sandstones are located on the valley floors and valley margins and valley floor because the coarser-grained sediments settle out more easily and accumulate preferentially in the flowing water common to the tunnel valley fill stages. ==Geographic distribution==

in Denmark. Glacially formed tunnel valleys have been identified on every continent. Africa Tunnel valleys associated with the Late Ordovician glaciation have been observed in north African countries, including Libya. These large-scale channel-fill sandstone bodies (tunnel valleys) are a striking sedimentological feature of the glacially related deposits on the old North Gondwanaland margin. They range from in depth, and wide. The tunnel valleys are incised into the bedrock and can be traced for in length. In one example, in Mauritania, in the western Sahara, Late Ordovician siliciclastic glacial features and deposits on the North Gondwana continental shelf include incised channels identified as tunnel valleys. The filled tunnel valley are several kilometers long and several hundred meters wide. Reconstructions conclude that these structures were located in glacier ice-margin regions; the cross-sections of the valleys are comparable to those confirmed to have formed glacially, the valleys end in outwash fans similar to tunnel valleys, and the infill is post-glacial typical of that observed for tunnel valleys. In southern Africa a Permo-Carboniferous tunnel valley system has been identified in northern Cape Province, South Africa. Antarctica The active formation of tunnel valleys is observed in the present period beneath the Antarctic ice. Europe Tunnel valleys and related glacial impacts have been identified in Russia, Belarus, Ukraine, Poland, Germany, Northern France, the Netherlands, Belgium, Great Britain, Finland, Sweden, Denmark and Norway. They have been studied in detail in Denmark, north Germany and north Poland where the thick ice sheet of the Weichsel and earlier Glaciations, having flowed down from the mountains of Scandinavia, began to rise up the north-European slope, driven by the altitude of the glacial ice accumulation over Scandinavia. Their alignment indicates the direction of ice flow at the time of their formation. They are found extensively in the United Kingdom with several examples reported from Cheshire for example. They are also to be found under the North Sea. Examples of lakes formed in tunnel valleys include the Ruppiner See (a lake in Ostprignitz-Ruppin, Brandenburg), the Werbellinsee, and the Schwielochsee, all in Germany. North America Okanagan Lake is a large, deep ribbon lake in the Okanagan Valley of British Columbia which formed in a tunnel valley from the Okanogan lobe of the Cordilleran Ice Sheet. The lake is long, between wide, and has a surface area of . Northern Idaho and Montana show evidence of tunnel valley formation under the Purcell lobe and the Flathead Lobe of the Cordilleran Ice Sheet. Tunnel valleys in southeast Alberta form an interconnected, anabranching network comprising Sage Creek, the Lost River and the Milk River and generally drain southeast. . Examples of bedrock tunnel valleys in Minnesota include River Warren Falls and several valleys which lie deep beneath till deposited by the glaciers which created them, but can be traced in many places by the Chain of Lakes in Minneapolis and lakes and dry valleys in St. Paul. The Kawartha lakes of Ontario formed in the Late Wisconsinan glacial period. Ice melt from the Niagara Escarpment flowed through tunnel valleys beneath the ice expanded to form a west-to-east passage between the main Laurentide Ice Sheet and a mass of ice in the Lake Ontario basin. Cedar Creek Canyon is a tunnel valley located in Allen County, Indiana. It is a very straight, narrow gorge about deep that contains part of the lower segment of Cedar Creek, the largest tributary of the St. Joseph River. In the Laurentian Channel offshore eastern Canada, numerous tunnel valleys have been identified originating from the submerged valley of the St. Lawrence River, which is also of glacial origin. Seismic reflection profiles of the fill of tunnel valleys suggest that they are of various ages, with the youngest dating from shortly after the Late Glacial Maximum. They result from erosion by sub-glacial water crossing the eastern Scotian Shelf off Nova Scotia. They originate from the Laurentian Channel south of the Cabot Strait. Additionally, seismic profiles show deeply buried post-Miocene channels, some of which lie below modern sea level, cutting across the eastern part of the outer Laurentian Channel which have also tentatively been determined to be tunnel valleys. Seismic profiles have also mapped large tunnel valleys on Banquereau Bank and Sable Island Bank. South America The Perito Moreno Glacier is located in the southern Southern Patagonian Ice Field, terminating in Lake Argentino. It divides Lake Argentino into the Los Témpanos channel, and the Rico branch, blocking the channel and forming an ice dam. Lake Argentino periodically breaks through in outburst floods with drainage initially through a tunnel with subsequent roof collapse to form an open channel. ==Temporal distribution==

There have been five known ice ages in the Earth's history; the Earth is experiencing the Quaternary Ice Age during the present time. Tunnel valleys formed during four of the five have been identified. ==See also==