User:Mikenorton/sandbox/Geology of the Lake District

The Geology of the Lake District is dominated by sedimentary and volcanic rocks of mainly Ordovician age underpinned by large granitic intrusions. Younger sedimentary sequences outcrop on the edges of the Lake District area, with Silurian to the south, Carboniferous to the north, east and west and Permo-Triassic to the west and east. The entire area was covered by a Mesozoic sequence that was eroded off during Paleogene uplift related to the opening of the North Atlantic. During the Quaternary the area was affected by repeated glaciations, which sculpted the current mountainous landscape.

Early Paleozoic

In early Paleozoic times the area was on the northern margin of the microcontinent of Avalonia, which was moving northwards as the Iapetus Ocean began to close. Initial deepwater sedimentation recorded by the Skiddaw Group was followed by the development of a continental arc caused by the subduction of Iapetus oceanic crust beneath the Avalonian margin. This magmatism produced both the Borrowdale Volcanic Group sequence and the older granitic intrusions, which are the preserved magma chambers to the volcanics. Continental collision began near the end of the Ordovician and the area became part of a foreland basin in which the Windermere Supergroup sediments were deposited initially interfingering with the volcanics and then overlying them. The collision reached its end during the Early Devonian, part of the Acadian Orogeny with the final closure of Iapetus and the intrusion of late orogenic granites.

Skiddaw Group

Main article: Skiddaw Group

The Skiddaw Group are the oldest rocks known from the Lake District. They are mainly Ordovician in age, from possibly Cambrian up to Llanvirn (upper middle Ordovician). The sequence, which is up to about 5 km in thickness, consist mainly of mudstones and siltstones, with lesser amounts of sandstone. Following deformation and low-grade metamorphism during the Acadian Orogeny, they now have a well-developed slaty cleavage, giving rise to their common name, the Skiddaw Slates.

Eycott Volcanic Group

Main article: Eycott Volcanic Group

The Eycott Volcanic Group forms the youngest part of the preserved Ordovician sequence in the northernmost part of the Lake District. This set of mainly andesitic lavas and tuffs, with related minor intrusions is of Caradocian (Sandbian to Early Katian) age.

Borrowdale Volcanic Group

Main article: Borrowdale Volcanic Group

The Borrowdale Volcanic Group (BVG) lies unconformably above the Skiddaw Group, and is of Caradocian age. It is similar to, but distinct from, the Eycott Volcanic Group. It consists of a thick sequence of basaltic, andesitic, dacitic and rhyolitic lavas and pyroclastic rocks with interbedded volcaniclastic sediments. It is subdivided informally into two parts known as the Lower Borrowdale Volcanic Group and Upper Borrowdale Volcanic groups. It is further subdivided into many formations, of which only a few have regional extent.

Lower Borrowdale Volcanic Group

The lower part of the succession is dominantly andesitic. Locally in the western part, the lowest formation is the non-volcanic sandstones of the Latterbarrow Formation. In the Furness Inlier the Greenscoe Tuff Formation is the lowermost unit. In the southwest at Millom Park the Whinny Bank Tuff Formation is the lowermost unit, overlain by the Po House Tuff Formation. The Birker Fell Andesite Formation makes up most of the lower BVG, varying in thickness from 980 m to 2700 m.

Upper Borrowdale Volcanic Group

The upper part of the group consist of mainly intermediate to acidic pyroclastic rocks and interbedded volcaniclastic sediments. There are four main successions recognised at the base of the upper sequence: the Duddon Basin, the Scafell Caldera, the Haweswater Caldera and the Kentmere succession. These are all followed by volcaniclastic sandstone of the Seathwaite Fell Sandstone Formation, which is developed over almost the whole outcrop, varying in thickness from 30 m to >1100 m. This is followed by the equally extensive Lincombe Tarns Tuff Formation, which is an ignimbrite. The uppermost part is the Helvellyn Basin succession, consisting of volcaniclastic sandstones, dacitic lavas, andesitic lavas and tuffs, and dacitic ignimbrites.

Windermere Supergroup

Main article: Windermere Supergroup

The transition from the top of the Borrowdale Volcanics into the overlying Dent Group is marked by an unconformity. The Dent Group is of latest Ordovician age and forms the lowermost part of the Windermere Supergroup. It was deposited in shallow marine conditions, consisting mainly of calcareous mudstones, siltstones, limestones and shales. There is evidence of continued volcanism, with tuffs locally developed at various levels within the group. The Dent Group is overlain unconformably by the Stockdale Group, a sequence of mudstones and siltstones deposited during the latest Ordovician to earliest Silurian. The overlying Sheinwoodian to Gorstian Tranearth Group consists mainly of hemipelagite (clays and silts) with some turbiditic sandstones. This is succeeded by turbiditic sandstones, siltstones and hemipelagites of the Gorstian Coniston Group. The youngest unit is the Ludlow to Pridoli Kendal Group, which consists of couplets of graded siltstone and mudstone, locally with thick turbiditic sandstones.

Granitic intrusions

The Lake District was affected by two stages of granitic intrusion. The earlier Ordovician granites were intruded at the same time as the BVG, representing the solidified magma chambers related to the volcanics, which later uplift has now exposed at the surface. Intrusions of this age include the Ennerdale Granophyre and the Eskdale Granite.

The later granites were intruded during the Early Devonian as part of group of intrusions found within a SW-NE trending belt that straddles the Iapetus Suture. Granites of this age include the Shap Granite, the Skiddaw Granite and the unexposed granite responsible for the Crummock Water aureole.

As a result of these two separate magmatic events, the Lake District is underpinned by a large batholith with an area of about 1500 km². Most of the batholith is thought to be Late Ordovician in age but with some Early Devonian contributions.

Late Paleozoic

After the end of the Acadian collision, Northern England was affected by a phase of Early Carboniferous extension, possibly related to the subduction of Rheic Ocean crust beneath Gondwana. Preserved Lower Carbonifeous rocks are found to the west, north and east of the Lake District in the East Irish Sea, Solway and Vale of Eden Basins. The sequence consists of limestones of the Carboniferous Limestone Supergroup, overlain by sandstones of the Millstone Grit Group and deltaic sandstones, shales and coals of the Coal Measures Group.

The only Permian rocks known from the Lake District area are from the upper part of the sequence and form part of the early syn-rift associated with the mainly Triassic rifting event. This sequence consists of the Appleby Group, dominantly sandstones, and the Cumbrian Coast Group consisting mainly of siltstones and dolomites overlain by evaporites and red shales (locally with the development of halite). On the western edge of the Lake District and in the Vale of Eden the Appleby Group includes thick breccias, known as the Brockram.

Mesozoic

The Mesozoic saw the first stages of break-up of the supercontinent of Pangaea. Rifting began at the end of the Permian, with development of a series of rift basins over much of the British Isles. The rifts became more localised during the Jurassic and by the Cretaceous most of the rifting has stopped, apart from the area south of the Variscan Front, where rifting continued into the Early Cretaceous. The Late Cretaceous saw the inundation of almost all parts of the British Isles by the "chalk sea".

Triassic age rocks of the New Red Sandstone outcrop to the northeast and southwest of the Lake District massif. To the southwest they were deposited at the eastern edge of the East Irish Sea Basin. To the northeast they were deposited in the Vale of Eden Basin. The Triassic sequences consist mainly of continental sandstones. At the end of the Triassic, shallow seas transgressed over all of the Triassic basins. There are no rocks of Jurassic or Cretaceous age preserved anywhere near the Lake District. However, based on the analysis of apatite fission tracks (AFTA) and estimates from neighbouring preserved sequences, it is thought that the Lake District was covered by between about 700 to 1750 m of Triassic, Jurassic and Cretaceous sediments by the Palaeocene that has since been removed by erosion.

Cenozoic

During the Paleogene, the Lake District, like most of the northern and western British Isles was affected by the break-up of the Atlantic at the end of the Paleocene, including the formation of the North Atlantic Igneous Province. The timing of the exhumation that removed the Mesozoic section from the top of the Lake District massif remains uncertain, although a significant part of the uplift is thought to have been caused by underplating at the end of the Paleocene. The amount of Neogene exhumation is particularly poorly constrained.

Quaternary

The Quaternary saw repeated glaciations of the Lake District area with the development of an icecap, although the current landscape is the result of the Last glacial period, which occurred over the period 115,000 to 11,700 years ago. As the Lake District was an upland area it was unaffected by the main ice sheets.

External links

• Lake District National Park Authority's Geology Factsheet