Geology of the Leicester district : sheet description of the British Geological Survey 1:50 000 series Sheet 156 Leicester (England and Wales)

Carney, J.N.; Ambrose, K.; Cheney, C.S.; Hobbs, P.R.N.. 2009 Geology of the Leicester district : sheet description of the British Geological Survey 1:50 000 series Sheet 156 Leicester (England and Wales). Nottingham, UK, British Geological Survey, 110pp.

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This Sheet Description provides a brief account of the geology of the district covered by Geological Sheet 156 Leicester. The district encompasses the whole of the Leicester conurbation and extends eastwards to Whissendine and Horninghold, northwards to Mountsorrel and southwards to Whetstone. It includes the eastern fringe of Charnwood Forest, a large tract of rural Leicestershire and, in the east, part of Rutland. The oldest rocks crop out close to the western edge of the sheet, as small inliers of predominantly fine-grained volcaniclastic strata belonging to the Charnian Supergroup, of Precambrian (Neoproterozoic III) age. They are overlain by Cambrian mudrocks, which are only patchily exposed, but which evidently include Ordovician (Tremadoc) representatives encountered in deep boreholes. In late Ordovician (Caradoc) times, a series of small granodiorite batholiths was emplaced. Some of these are exposed whereas others, concealed by younger strata, have been proved by drilling or geophysical investigations. A phase of folding and mild metamorphism occurred in latest Silurian times, towards the end of the Caledonian orogeny, imposing a highly penetrative cleavage on the more argillaceous basement rocks. The Mesozoic rocks comprise a strongly unconformable ‘cover’ to the basement, and both dip and ‘young’ eastwards. Arid conditions prevailed throughout much of the Triassic, when the distinctive red-beds of the Mercia Mudstone Group were deposited. The base-Triassic unconformity is particularly irregular around Mountsorrel and Charnwood Forest, which formed dissected hill ranges at the end of the Permian Period, but by latest Triassic times sedimentation had largely buried these upstanding areas. A shallow sea subsequently covered most of the region and strata of the Penarth Group accumulated, although at this early stage of the transgression the highest parts of Charnwood Forest may have remained emergent. Quiescent, warm tropical marine conditions became established during deposition of the overlying Lias Group. Such environments persisted throughout the Early Jurassic, although there was an episode in which water depths had shallowed significantly enough to usher in the high-energy, nearshore conditions that resulted in deposition of ooidal ironstones of the Marlstone Rock Formation. The uppermost part of the outcropping sequence occurs mainly as erosional outliers capped by Middle Jurassic strata of the Northampton Sand Formation, part of the Inferior Oolite Group and representing a second shallowing cycle of the Jurassic sea. Quaternary geological processes chiefly moulded the landforms and drainage systems seen today. About two million years ago, in earliest Quaternary times, the landscape was dominated by the major north-east flowing Bytham River, which deposited sands and gravels. This drainage system was subsequently over-run by the ice sheets of the Middle Pleistocene glaciation, which left behind superficial deposits consisting of glaciofluvial outwash, glaciolacustrine clay, and till. These deposits now form a veneer covering the bedrock or hill-top capping, but in places are more thickly developed within palaeovalleys. Development of the modern landscape followed the retreat of the ice sheets, about 430 000 years ago. The various cycles of drainage development involved valley incision and aggradation, with further slope modification resulting from periglacial processes, particularly during Devensian times. A major economic legacy of these geomorphological cycles was the formation of five seperate generations of sandy and gravelly river terrace deposits in the trunk valleys of the Soar and Wreake rivers. Industrial development has been centred on Leicester, which was well served by canals and railways developed during the 19th century. The district was never particularly rich in minerals and although brick-clay, limestone, sandstone and ironstone were quarried in the past, these resources are no longer exploited. Current economic value comes from a thriving aggregate industry based on the quarrying of ‘hard rock’ (Ordovician granodiorite) and sand and gravel (Quaternary alluvium and river terrace deposits). Basal Triassic sandstones constitute a major aquifer in the general region, and there are other, minor aquifers from which groundwater has been abstracted. However, the district relies mostly on water supplied from elsewhere. Much of the Leicester city area is undergoing regeneration and here geology has an important role to play in evaluating natural resource potential and predicting the constraints and opportunities that the rocks and superficial deposits of the district present to further development.

Item Type: Publication - Book
Programmes: BGS Programmes 2009 > Geology and Landscape England
ISBN: 9780852725733
Additional Information. Not used in RCUK Gateway to Research.: Available from the BGS Sales Desk Tel: 0115 936 3241 Fax: 0115 936 3488 email
NORA Subject Terms: Earth Sciences
Date made live: 01 Dec 2009 11:48 +0 (UTC)

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