The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~ 450 million years ago , and exposed in northwest England , including the Pennines and correlates along its strike, in the Isle of Man and Ireland, and down-dip in the Southern Uplands and Welsh Borderlands. It underlies much of north England's younger cover, extending south to East Anglia. It formed as a foreland basin , in a similar setting to the modern Ganges basin , fronting the continent of Avalonia as the remains of the attached Iapetus ocean subducted under Laurentia .
28-595: The supergroup comprises the Dent Group of turbiditic limestones, and the overlying series of shales , grits and greywackes of the Stockdale Group , Tranearth Group , Coniston Group and Kendal Group . Compression from the south east during the later Acadian orogeny (probably caused by the closure of the Rheic ocean) buckled the strata into anticlines and synclines and caused slaty cleavage in some sediment beds. To
56-399: A sedimentary basin . Thermal subsidence can occur anywhere in which a temperature differential exists between a section of the lithosphere and its surroundings. There are a variety of contributing factors that can initiate thermal subsidence or affect the process as it is ongoing. As endogenous and exogenous processes cause denudation of the earth's surface, lower, warmer sections of
84-457: A higher relative geothermal gradient . This gradient can cause metamorphism in rocks, as seen in South Australia. Eustasy refers to a change in the relative sea level. It can have effects on the thermal subsidence during the formation of geological features such as mountain ranges . Sea level often changes in response to the formation of glaciers on land; the weight of these glaciers or
112-403: A significant source of molasse sediments, with turbidites and deltas prograding through the basin. The basin is eventually filled, and covered with fluvial and alluvial molasse (stage 4). Stage 1 is hard to spot in the sedimentary record. The Dent Group, the oldest part of the supergroup, is a good match for the carbonate facies expected in the shallow waters of stages 2–3; accommodation space
140-695: Is exposed in four areas, the southern Lake District as a narrow strip across the whole width of the outcrop, in the Cautley and Dent inliers , the Cross Fell inlier and the Craven inliers. In the main Lake District outcrop, the group consists of calcareous siltstones and mudstones of the basal Kirkley Bank Formation, micritic limestones of the Broughton Moor Formation and dark blue-gray shales of
168-480: Is marked by a number of oxic- anoxic transitions, with black shales corresponding to transgressions - these may have helped to mitigate a runaway greenhouse effect . The rate of sediment accumulation accelerated with time; it held fairly steady at a low ~50 metres per million years (m/Ma) until the Wenlock (mid Silurian, 424 million years ago ), when it increased greatly, eventually reaching over 1000 m/Ma when
196-599: Is represented by the Coniston Group, a series of sandy turbidites, with sediment supply from the north east (and controlled by basement faulting). The group is subdivided into formations, which each represent a turbidite lobe, and are separated by anoxic background sedimentation. The Kendal Group covers the transition into stage four, with pronounced shallowing up; turbidites become thinner-bedded, and anoxic hemipelagics give way to oxygenated sediments, with storm beds becoming more and more common, and intertidal sediments topping
224-525: The Devonian Acadian orogeny , with the resulting continental collision causing deformation of the Ordovician rocks, including folding , thrusting and cleavage development. The Dent Group was deposited in a shallow water environment. Thermal subsidence In geology and geophysics , thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens
252-607: The Windermere Supergroup , which was deposited in the foreland basin formed during the collision between Laurentia and Avalonia . It lies unconformably on the Borrowdale Volcanic Group . This unit was previously known as the Coniston Limestone Group or Coniston Limestone Formation and should not be confused with the significantly younger (uppermost Silurian ) Coniston Group . The Dent Group
280-436: The lithosphere and causes it to decrease in elevation. This is because of thermal expansion : as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes denser than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming
308-824: The Ashgill Formation. Locally volcanic formations are developed, including the Yarlside Volcanic Formation at the base of the Kirkley Bank Formation and the High Haume Tuff and Appletreeworth Formations at the base of the Ashgill Formations. In the Cautley and Dent inliers, rocks of the Dent Group occur in two fault bounded strips next to the Dent Fault . In these inliers, calcareous mudstones of
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#1732844943862336-697: The Cautley Mudstone Formation are overlain by the Ashgill Formation. The Cautley Volcanic Member is a unit of volcaniclastic rocks , within the Cautley Mudstone Formation. In the Cross Fell inlier, the basal Dufton Shale Formation is followed by the Swindale Limestone Formation and the Ashgill Formation. In the Keisley area, the uppermost part of the Dent Group is represented by the highly fossiliferous Keisley Limestone Formation, exposed in
364-729: The Southern Uplands accretionary prism , flanking the edge of the Laurentian continent, was advancing towards Avalonia. The load of the mountains formed during this collision weighed down the Avalonian plate, causing the development of accommodation space . Sedimentation began in the Caradoc (upper Ordovician, 455 million years ago ). During the Llandovery, the Stockdale Supergroup
392-413: The actual process of thermal subsidence is governed by the loss of heat via thermal conduction . Contact with surrounding rock or the surface causes heat to leach out of a section of the lithosphere. As the lithosphere cools, it causes the rock to contract. When conduction causes a section of the lithosphere to contract and increase in density, it does not directly add mass to the rock. Instead, it causes
420-543: The basin's state. Instead of being underfilled, and trapping all sediment that flowed into it, it became overfilled. This was reflected by a shallowing of water depth, as the basin silted up. This culminated with a transition to terrestrial conditions in the Přídolí. Analysis of the clay mineral illite from a section across the Windermere Supergroup permits an estimate to be made of its maximum burial depth. Deposits at
448-462: The bulge moves backwards, it leaves shallow waters in its wake, which can be filled with carbonates, while hemipelagic sediments and turbidites continue to fill the deeper parts of the basin, leaving a "trinity" of facies --- this is stage 2. At a certain point, the deep water basin changes from an underfilled state, where accommodation space is created as fast as it is filled with flysch , to an overfilled one (stage 3). The orogenic wedge then provides
476-474: The convergence and eventual collision of the Avalonia microcontinent with the continent of Laurentia. The underlying Borrowdale Volcanic Group formed in an island arc , caused by the southerly subduction of the crust of the now vanished Iapetus Ocean . At the end of active arc volcanism, the area began to subside, allowing deposition of the shallow marine Dent Group. The final closure of Iapetus occurred during
504-590: The core of an anticline . The Craven inliers are found to the north of the North Craven Fault . In these inliers, calcareous siltstones and muddy limestones of the Norber Formation are overlain by siltstones and mudstones of the Sowerthwaite Formation. The Dam House Bridge Tuff Member forms the basal part of the Sowerthwaite Formation. The early Paleozoic geology of northern England records
532-424: The countervailing isostatic uplift is eliminated, causing them to subside and create an atoll . Thermal subsidence can drive metamorphism in rocks. The conduction of heat out of a section of lithosphere causes the rock to thicken and become more insulated to heat flowing in from the mantle; as this thicker section is buried by the descending column of the lithosphere, it descends into surrounding rock layers with
560-468: The group. The missing Old Red Sandstone mentioned above formed the molasse sediments of stage 4. Throughout the Silurian, until the beginning of Old Red Sandstone deposition, sedimentation rate increases steadily, reaching a peak of 1 mm a. Dent Group The Dent Group is a group of Upper Ordovician sedimentary and volcanic rocks in north-west England . It is the lowermost part of
588-446: The lithosphere and increase the total mass of the section per unit area, causing it to sink further. Thermal subsidence can have an effect on island formation. Isostatic uplift can be balanced with thermal subsidence in response to erosion on islands without barrier reefs , which sink only when subjected to wave erosion . However, volcanic islands and seamounts with barrier reefs are shielded from wave and stream erosion, and thus
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#1732844943862616-464: The lithosphere are exposed to relative differences in weight and density. This relative difference creates buoyancy . Isostatic uplift can then further expose the lithosphere to conductive cooling, causing a “rise and fall” phenomenon as warmer, less dense rock layers are pushed or buoyed up, then cooled, causing it to contract and sink back down. The conditions to create thermal subsidence can be initiated by various forms of uplift and denudation, but
644-589: The north west of the unit lies the Cambro-Ordovician Skiddaw Group , a sequence that formed on the Avalonian continental margin, composed mainly of turbidites. Between the two, the Borrowdale Volcanic Group consists of tuffs erupted by an underlying calc-alkaline volcanic arc, active during the subduction of the Iapetus ocean crust. The Windermere basin was formed by flexure. Prior to its formation,
672-502: The record is terminated by erosion in the Pridoli (terminal Silurian, 419 million years ago ). This sudden upturn in deposition rate is a result of the increasing proximity of the Avalon mountain belt, which started to depress the plate from the Ordovician, but was not close enough to increase the sedimentary input until the Silurian. The latest phase of sedimentation reflected a change in
700-480: The state of the art for over a decade, and his four-stage model provides a good match for the Windermere supergroup. In the first stage, an orogenic wedge (here, the mountains of the Southern Uplands accretionary wedge) loads a passive margin, causing flexural subsidence and providing accommodation space. A "forebulge", caused by the rigidity of the crust flexing up behind the load, causes uplift and permits erosion. As
728-404: The surface today were once covered with 5–6 km of sediment; some of this would belong to faulted Windermere deposits, but it is postulated that the bulk of it was Old Red Sandstone , including molasse deposits laid down by alluvial fans on the flanks of the mountain belt, and probably lower energy fluvial (river) or aeolian (dune) deposits. Sinclair's model of foreland basins has remained
756-410: The volume to decrease, increasing the mass of the section for a given area. The lithosphere is isostatic with the mantle ; its weight is supported by the relative density of the surrounding rock. When a section cools and its density increases, it sinks, causing the relative elevation to decrease. This can create a basin in which sediments are deposited, which adds weight on top of the sinking section of
784-469: Was created through thermal subsidence . The deeper water deposits of stage three are represented by the Stockdale and Tranearth groups, which display a steady deepening, as expected of sediments deposited in an underfilled basin, with a sedimentation rate high enough to preserve annual variation in places. (This signal is complicated by an overprinted signature of eustatic glaciations.) The end of stage three
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