Windermere Supergroup

the Windermere group in North America

The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~, 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. 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 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, 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, ). During the Llandovery, the Stockdale Supergroup 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, ), when it increased greatly, eventually reaching over 1000 m/Ma when the record is terminated by erosion in the Pridoli (terminal Silurian, ). 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 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 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 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.