Difference between revisions of "OR/12/032 Introduction"
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|Hobbs, P R N, Entwisle, D C, Northmore, K J, Sumbler, M G, Jones, L D, Kemp, S, Self, S, Barron, M, and Meakin, J L. 2012. Engineering Geology of British rocks and soils - Lias Group. British Geological Survey, Nottingham, UK. (OR/12/032).|
Lias is a name, which dates back to some of the oldest works of geology, signifying the earliest of the three sub-divisions of the Jurassic Period; the subsequent ones being the ‘dogger’ and the ‘malm’. William Smith’s (and the world’s) first geology map of 1799 depicts the outcrop of the Lias to the northwest of Bath (Phillips, 2003). The Lias probably takes its name from the Old French word ‘liais’ referring to hard layered limestone (Webster, 1913). The Lias is characterised by mudstones interlayered with limestones.
In Britain, the Lias Group encompasses an important group of geological materials. It has been the working medium of major engineering projects, industrial exploitation, and scientific research. It comprises those predominantly clay-rich sediments of latest Triassic and Early Jurassic age found in and around the United Kingdom, deposited between 180 and 205 million years ago. The outcrop of the Lias extends in a continuous band from the coast of Dorset in a north-north-easterly direction to Yorkshire, with outlying areas in Somerset and South Wales (Figure 1.1). Significant outcrops are found in the high sea-cliffs on the coasts of Dorset, South Wales, and Yorkshire. This report is the result of an in-depth study of data generated by, and deposited with, the British Geological Survey over many decades. The report describes the geological, lithological, mineralogical and geotechnical features of the major Formations within the Lias Group; each aspect being dealt with in a separate chapter. However, in the geotechnical chapter not all formations are assessed due to lack of data. As a result there tends to be a bias in this chapter towards either clay-rich formations or the clay-rich or sand-rich members of a formation. The processes and effects of weathering are also considered in detail and are dealt with in a separate chapter.
The Lias Group rocks were deposited in a Mediterranean-like climate in warm, shallow seas. Carbonates were deposited as limestone beds and nodules, particularly in the lower part of the succession, to form the Blue Lias Formation. The Lias Group is very fossiliferous, and ammonites in particular have been used to sub-divide the group into twenty bio-zones. Traditionally, the group has been divided into the Lower, Middle, and Upper Lias, but the succession is better described in terms of the formations, as defined by Cox et al. (1999). Of these, the main clay-bearing formations are the Blue Lias (Ambrose, 2001), Charmouth Mudstone, and Whitby Mudstone Formations. The sole major sand formation is the Bridport Sand, while the Dyrham Formation consists of siltstones and mudstones. Other formations, though much thinner and less well represented in the database, provide topographic features and have been important in terms of industrial exploitation. The Lias Group is best seen in coastal cliff sections in Dorset between Lyme Regis and Bridport, and also in Yorkshire between Robin Hood’s Bay and Redcar where large-scale alteration of the landscape has been the legacy of a once thriving alum quarrying, production, and shipping industry (alum was used as a dyeing and tanning fixative). Slopes within the Lias Group have been subject to extensive landsliding and cambering, particularly where overlain by Inferior Oolite limestones, for example in the Cotswolds where both movement processes may be seen within the same valley. Shales within the Lias Group are often highly bituminous and lignitic. Lias Group shales are believed to be the source rock for oil reserves discovered within the Great Oolite at Basingstoke, Hampshire in 1980 (Duff & Smith, 1992). Ironstone bands are also found, for example within the Cleveland Ironstone Formation, and have been exploited for iron ore in the past.
The principal sedimentary basins in which the Lias was deposited are the Wessex, Bristol Channel, Worcester and Cleveland Basins (Figure 2.1). These had developed during the Triassic Period as a result of crustal tension and subsidence. During the succeeding Jurassic Period marine deposition and basin subsidence continued, building up great thicknesses of the Lias Group, with 400 to 500 m in the Wessex, Worcester and Cleveland basins, and some 1300 m near Llanbedr on the margin of the mainly offshore Cardigan Bay Basin, the thickest Lias succession proved in Britain (Woodland, 1971). Because subsidence was less rapid on the inter-basin ‘highs’ than in the adjoining basins, the successions there are substantially thinner and less complete, with the older parts of the Lias missing. Thus the base of the Lias tends to be younger when traced from the basins onto the highs (e.g. Donovan et al, 1979). The Lias Group is overlain non-sequentially by younger beds throughout southern and eastern England.
The Lias has undergone different amounts of induration and over-consolidation from one basin to another. This is largely determined by the thicknesses of sediment (Jurassic to Quaternary) that may once have overlain a particular site, and the amount of erosion that has subsequently taken place. For example, a maximum previous burial depth of the order of 2 km is indicated for the Cleveland Basin. This has resulted in greater strength (‘strong’ to ‘very strong’) and durability overall in the case of the northern Lias mudstones, compared with ‘weak’ to ‘moderately strong’ in the south. However, this trend is reversed for plasticity. Merriman & Kemp (1996) indicated burial depths of <4 km based on clay crystallite size. Fresh Lias mudstones tend to be strong and durable but, in common with other Jurassic clay-rich formations, undergo considerable deterioration of most engineering properties following stress relief and weathering (Cripps & Taylor, 1981). Weathering by oxidation, resulting in a colour change from grey to brown, typically extends to depths of about 5 m below ground level. This is accompanied by a significant increase in water content, resulting in altered engineering properties (Chandler, 2000). Shales are inherently weaker and less durable due to the marked anisotropy associated with this lithology. Weak horizons within the Whitby Mudstone have contributed to the processes of cambering, valley bulging, and landsliding, for example in Northamptonshire, Rutland (e.g. Empingham), and the Cotswold escarpment (e.g. Breedon Hill, Broadway Hill, Leckhampton Hill) where it is overlain by Inferior Oolite limestones. Reworking of Lias mudstones imparts considerable loss of strength and trafficability. This is seen in quarries, particularly within the Charmouth Mudstone, after periods of rain. A good negative correlation has been established between strength and water content (Cripps & Taylor, 1987). Somerset’s famous landmark, Glastonbury Tor, is made up of Bridport Sand Formation on Beacon Limestone Formation on Dyrham Formation; the latter also forming Wearyall Hill west of the Tor. Brent Knoll, adjacent to the M5, also consists of Inferior Oolite overlying Lias Group rocks. These locations have also been subject to recent shallow landslides.
The most impressive exposures of Lias Group rocks can be seen in coastal sections where they are often affected by slope movement processes and/or modified by past industrial activity, for example:
North Yorkshire coast
The Lias Group rocks occur in the dales of the North York Moors, and intersect the North Yorkshire coast between Redcar and Scalby Bay. The cliffs here are very steep and in places exceed 200 m in height. In areas where the Lias Group rocks are overlain by significant thicknesses of superficial deposits, active coastal landsliding has taken place; for example, at Runswick Bay, Robin Hood’s Bay, Ravenscar, and Filey Bay. For over two hundred years from the mid 17th century a thriving alum quarrying and processing industry existed on the cliff tops between Saltburn and Ravenscar. The alum was taken from the Alum Shale Member of the Whitby Mudstone Formation. This industry has resulted in a distinctly man-made profile to many cliff sections in the area; for example the upper cliffs at Kettleness, Ravenscar, and Sandsend. Kettleness is also famous due to the discoveries, dating back to the 19th century but also very recently, of large marine reptile fossils, such as plesiosaurs, at the former alum workings. The Cleveland iron industry, the largest in Britain in its heyday, had mines and infrastructure dotted across East Cleveland, Rosedale, and Eskdale. On the coast, mining from the Cleveland Ironstone Formation was centred on Skinningrove and Port Mulgrave; at the latter location, the ore was transported from the top of the cliff to a small harbour at the bottom via an inclined tunnel within the cliff (Osborne & Bowden, 2001). A much smaller industry developed, in response to Victorian fashions, to extract ‘jet’ from lenses within the lower part of the Mulgrave Shale Member (Jet Rock).
West Dorset coast
The Lias Group rocks intersect the coastline between Lyme Regis and WestBay, Dorset and, although not at their thickest here, are well exposed in the cliffs. This scenic stretch of coast is noted for both its abundance of fossils and its extensive and active landslide terrains. Perhaps the best known of the landslides are ‘Black Ven’ and ‘Stonebarrow Hill’, situated to west and east of Charmouth, respectively, and ‘Golden Cap’ further to the east. These are probably the largest coastal landslide features in England, and are also National Trust and World Heritage coastal sites. Black Ven and Stonebarrow have been active in recent times, affecting properties and services in Charmouth and Lyme Regis. The area is also famous for its fossil finds, including large marine reptiles such as ichthyosaurs, dating back to the mid 19th century, which have contributed to the foundation of palaeontology. The cliffs reach a height of 170 m in the central parts of the landslides. There have been many publications dealing with the landslides (Brunsden, 1969; Conway, 1974, Conway, 1976; Brunsden & Jones, 1976; Brunsden & Goudie, 1981; Gibson, 2005).
Whilst not having undergone the levels of industrial exploitation found in North Yorkshire, the West Dorset coast has been influenced by man’s activities. These include the construction of jetties, sea walls, rock-armour, and groynes, ‘quarrying’ of rock and gravel from the shore platform, and beach nourishment (Brunsden & Goudie, 1981). Some of these have had an adverse effect on beach volumes and on slope stability. Due to the prevailing southwesterly winds, the overall direction of sediment transport is to the east, though there have been sustained reversals of this trend. Jetties such as the Cobb at Lyme Regis, interrupt this process and allow depletion of beaches to the east of them. The landslide complexes of West Dorset, in particular Black Ven and Stonebarrow, supply large amounts of material to the shore. Black Ven, which has an estimated landslide volume about double that of Stonebarrow, nevertheless supplies over ten times the annual amount of gravel.
Although not described in detail in this report because of the absence of geotechnical data in the area, Lias Group rocks outcrop at many locations within the Inner Hebrides, including Raasay (P002828), Skye, and Mull. Here they mainly consist of pale-coloured sandstones and shales. These are frequently faulted, landslipped, and riven and altered by dikes. The Fiurnean Shale, Raasay Ironstone, and Portree Shale Formations (Toarcian, Upper Lias) outcrop in eastern Skye and Raasay. These are frequently obscured by landslides. Layers of jet are found in the soft, micaceous shales of the Portree Shale Formation, and are believed to be equivalent to the Mulgrave Shale Member (Whitby Mudstone Formation) of North Yorkshire. The Scalpa Sandstone (Middle Lias) is found underlying the shales, particularly on Skye and Scalpa, and consists of hard, calcareous, micaceous sandstone with sandy shale layers and doggers (Anderson & Dunham, 1966). This is in turn underlain by the limestones, sandstones, and mudstones of the Breakish Formation. Ironstone has been mined on a small scale from the Raasay Ironstone (Upper Lias) on the isle of Raasay.
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