57-575: The London Basin is an elongated, roughly triangular sedimentary basin approximately 250 kilometres (160 mi) long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea . The basin formed as a result of compressional tectonics related to the Alpine orogeny during the Palaeogene period and was mainly active between 40 and 60 million years ago. The generally accepted boundaries are
114-463: A continental environment, such as mudcracks and -in some rare cases- dinosaur footprints. Taxa included in the table below have an uncertain provenance and cannot be placed into one of the constituent formations, thus they are placed here. [REDACTED] The term "Wealden" and "Wealden facies" has been applied to other Lower Cretaceous sequences in Europe, including the "German Wealden", comprising
171-422: A convergent plate tectonic boundary in the gap between an active volcanic arc and the associated trench , thus above the subducting oceanic plate. The formation of a forearc basin is often created by the vertical growth of an accretionary wedge that acts as a linear dam, parallel to the volcanic arc, creating a depression in which sediments can accumulate. Trench basins are deep linear depressions formed where
228-514: A high probability of preservation. In contrast, sedimentary basins formed on oceanic crust are likely to be destroyed by subduction . Continental margins formed when new ocean basins like the Atlantic are created as continents rift apart are likely to have lifespans of hundreds of millions of years, but may be only partially preserved when those ocean basins close as continents collide. Sedimentary basins are of great economic importance. Almost all
285-445: A load is placed on the lithosphere, it will tend to flex in the manner of an elastic plate. The magnitude of the lithospheric flexure is a function of the imposed load and the flexural rigidity of the lithosphere, and the wavelength of flexure is a function of flexural rigidity of the lithospheric plate. Flexural rigidity is in itself, a function of the lithospheric mineral composition, thermal regime, and effective elastic thickness of
342-606: A million, and their sedimentary fills range from one to almost twenty kilometers in thickness. A dozen or so common types of sedimentary basins are widely recognized and several classification schemes are proposed, however no single classification scheme is recognized as the standard. Most sedimentary basin classification schemes are based on one or more of these interrelated criteria: Although no one basin classification scheme has been widely adopted, several common types of sedimentary basins are widely accepted and well understood as distinct types. Over its complete lifespan
399-665: A result of isostasy . The long-term preserved geologic record of a sedimentary basin is a large scale contiguous three-dimensional package of sedimentary rocks created during a particular period of geologic time, a 'stratigraphic succession', that geologists continue to refer to as a sedimentary basin even if it is no longer a bathymetric or topographic depression. The Williston Basin , Molasse basin and Magallanes Basin are examples of sedimentary basins that are no longer depressions. Basins formed in different tectonic regimes vary in their preservation potential . Intracratonic basins, which form on highly-stable continental interiors, have
456-658: A result of the closing of a major ocean through continental collision resulting from plate tectonics. As a result the sedimentary record of inactive passive margins often are found as thick sedimentary sequences in mountain belts. For example the passive margins of the ancient Tethys Ocean are found in the mountain belts of the Alps and Himalayas that formed when the Tethys closed. Many authors recognize two subtypes of foreland basins: Peripheral foreland basins Retroarc foreland basins A sedimentary basin formed in association with
513-575: A single sedimentary basin can go through multiple phases and evolve from one of these types to another, such as a rift process going to completion to form a passive margin. In this case the sedimentary rocks of the rift basin phase are overlain by those rocks deposited during the passive margin phase. Hybrid basins where a single regional basin results from the processes that are characteristic of multiple of these types are also possible. Terrestrial rift valleys Proto-oceanic rift troughs Passive margins are long-lived and generally become inactive only as
570-404: A subducting oceanic plate descends into the mantle, beneath the overriding continental (Andean type) or oceanic plate (Mariana type). Trenches form in the deep ocean but, particularly where the overriding plate is continental crust they can accumulate thick sequences of sediments from eroding coastal mountains. Smaller 'trench slope basins' can form in association with a trench can form directly atop
627-419: A thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock . They form when long-term subsidence creates a regional depression that provides accommodation space for accumulation of sediments. Over millions or tens or hundreds of millions of years the deposition of sediment , primarily gravity-driven transportation of water-borne eroded material, acts to fill
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#1732765191260684-525: Is a piece of rubber, which thins in the middle when stretched.) An example of a basin caused by lithospheric stretching is the North Sea – also an important location for significant hydrocarbon reserves. Another such feature is the Basin and Range Province which covers most of Nevada, forming a series of horst and graben structures. Tectonic extension at divergent boundaries where continental rifting
741-745: Is also composed of shallow marine (near-shore) and coastal facies rocks. It can be up to 140 metres thick and is of late Ypresian to Lutetian age. The basin is mainly drained by the River Thames , but does not coincide with the Thames drainage basin . The upper Thames cuts through the Chilterns via the Goring Gap , and consequently the Thames drains parts of the Cotswolds , Vale of White Horse and Vale of Aylesbury . The main headstream within
798-591: Is large enough and long-lived enough to create a sedimentary basin often called a pull-apart basin or strike-slip basin. These basins are often roughly rhombohedral in shape and may be called a rhombochasm . A classic rhombochasm is illustrated by the Dead Sea rift, where northward movement of the Arabian Plate relative to the Anatolian Plate has created a strike slip basin. The opposite effect
855-553: Is occurring can create a nascent ocean basin leading to either an ocean or the failure of the rift zone . Another expression of lithospheric stretching results in the formation of ocean basins with central ridges. The Red Sea is in fact an incipient ocean, in a plate tectonic context. The mouth of the Red Sea is also a tectonic triple junction where the Indian Ocean Ridge, Red Sea Rift and East African Rift meet. This
912-686: Is only found as an outlier on top of hills and only consists of a single formation, the Whitchurch Sand Formation . In Yorkshire, the equivalently aged Speeton Clay Formation , a marine unit, is present. On top of the Wealden Group is the Lower Greensand Group . The difference between these two groups has been formed by a major eustatic (global) transgression of the sea. The Greensand ( Aptian / Albian in age) consists of marine deposits. The sequence in
969-458: Is particularly measurable and observable with oceanic crust, as there is a well-established correlation between the age of the underlying crust and depth of the ocean . As newly-formed oceanic crust cools over a period of tens of millions of years. This is an important contribution to subsidence in rift basins, backarc basins and passive margins where they are underlain by newly-formed oceanic crust. In strike-slip tectonic settings, deformation of
1026-553: Is that of transpression , where converging movement of a curved fault plane causes collision of the opposing sides of the fault. An example is the San Bernardino Mountains north of Los Angeles, which result from convergence along a curve in the San Andreas Fault system. The Northridge earthquake was caused by vertical movement along local thrust and reverse faults "bunching up" against the bend in
1083-416: Is the only place on the planet where such a triple junction in oceanic crust is exposed subaerially . This is due to a high thermal buoyancy ( thermal subsidence ) of the junction, and also to a local crumpled zone of seafloor crust acting as a dam against the Red Sea. Lithospheric flexure is another geodynamic mechanism that can cause regional subsidence resulting in the creation of a sedimentary basin. If
1140-466: Is thus an important area of study for purely scientific and academic reasons. There are however important economic incentives as well for understanding the processes of sedimentary basin formation and evolution because almost all of the world's fossil fuel reserves were formed in sedimentary basins. All of these perspectives on the history of a particular region are based on the study of a large three-dimensional body of sedimentary rocks that resulted from
1197-979: The Harwich Formation and the London Clay Formation (famous for its fossils). They were deposited in shallow marine, estuarine or coastal plain ( paralic ) environments and were deposited during the Ypresian age (55.8 – 48.6 million years ago). On top of the Thames Group is the Bracklesham Group , which consists of the Bagshot Formation , the Windlesham Formation and the Camberley Sand Formation . The Bracklesham Group
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#17327651912601254-828: The Late Cretaceous Chalk Group 's escarpments of the Chilterns and Marlborough Downs to the north and the North Downs and Berkshire Downs to the south. To the south lie the Weald and Salisbury Plain and to the north is the Vale of Aylesbury . The approximate western limit is in the Marlborough area of Wiltshire . The eastern end merges with the North Sea Basin , extending on land along
1311-571: The Wealden Supergroup , is a group (a sequence of rock strata ) in the lithostratigraphy of southern England . The Wealden group consists of paralic to continental (freshwater) facies sedimentary rocks of Berriasian to Aptian age and thus forms part of the English Lower Cretaceous . It is composed of alternating sands and clays . The sandy units were deposited in a flood plain of braided rivers ,
1368-896: The Wessex Formation and overlying Vectis Formation . In the Weald Basin , the Wealden Group consists of four formations: the Ashdown Formation , the Wadhurst Clay Formation , the Tunbridge Wells Sand Formation and the Weald Clay Formation . The lower three formations are sometimes collectively referred to as the Hastings Beds . In Oxfordshire , Buckinghamshire and Wiltshire , the Wealden Group
1425-914: The Chalk. It is composed dominantly of shallow marine sands. On top of the Thanet Formation lies the Lambeth Group , which was deposited in a coastal plain and/or adjacent shallow sea water during the late Thanetian and early Ypresian ages . The Lambeth Group consists of the Upnor Formation , the Woolwich Formation and the Reading Formation . The Lambeth Group is overlain by the Thames Group , which consists of two formations . From old to young these are
1482-791: The Late Cretaceous, though the Chalk contains levels indicating periods of erosion. During the Palaeogene the south-east of England was affected by movements related to the formation of the Alps (the Alpine Orogeny ). One consequence of this was relative vertical movements, with the eastern part of the Wessex Basin being uplifted as the Weald-Artois Anticline and the London Platform subsiding to form
1539-809: The London Basin proper is the Kennet , which flows along the axis from the Marlborough area, joining the Thames at Reading . To the south rivers such as the Mole and Medway , draining from the Weald, cut through the North Downs into the basin; these are presumed to date from before the erosion of the Weald dome. During the Pleistocene ice age the lower Thames is believed to have been diverted well to
1596-498: The London Basin. Up to 320 metres (1,050 ft) of Palaeocene and Eocene sediments were deposited in the basin. The Pleistocene saw the sea retreat from the basin as global sea-level fell due to accumulation of ice sheets. The London Basin is mainly filled with Palaeogene sediments . The lowest (oldest) unit is the Thanet Formation (of Thanetian age, 56–58 million years old), which rests unconformably on top of
1653-666: The Weald Basin has also been described as a supergroup , containing the Weald Clay Group and Hastings Group. The Wealden Group forms outcrops covering a large part of south and south-eastern England including the Isle of Wight . It takes its name from the Weald region of Kent , Sussex , Surrey and Hampshire . It has yielded many fossils, including dinosaurs like Iguanodon and Hypsilophodon . Apart from fossils, it shows many other signs of being deposited in
1710-579: The Wealden Group can reach a thickness of 700 metres. The terms Wealden and Wealden facies are also used as generic terms referring to Early Cretaceous non-marine sequences elsewhere in Europe. The Wealden Group lies stratigraphically on top of the Purbeck Group , which spans the Jurassic - Cretaceous boundary. Within the Wessex Basin , the Wealden Group consists of two formations :
1767-482: The associated accretionary prism as it grows and changes shape creating ponded basins. Pull-apart basins is are created along major strike-slip faults where a bend in the fault geometry or the splitting of the fault into two or more faults creates tensional forces that cause crustal thinning or stretching due to extension, creating a regional depression. Frequently, the basins are rhombic, S-like or Z-like in shape. A broad comparatively shallow basin formed far from
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1824-476: The centre of the basin the Chalk is mainly covered by Palaeocene , Eocene and younger rocks, though the chalk is also brought to the surface by localised folds and faults , for example at Windsor Castle , Lewisham and Purfleet . The Chalk forms an artesian basin , with fresh water springs emerging on the bed of the Thames . In the greater part of the basin the surface rock is Eocene London Clay , flanked at
1881-570: The clays mostly in a lagoonal coastal plain . The Wealden Group can be found in almost all Early Cretaceous basins of England: its outcrops curve from the Wessex Basin in the south to the Cleveland Basin in the northeast. It is not found in northwest England and Wales , areas which were at the time tectonic highs where no deposition took place. The same is true for the London Platform around London and Essex . Offshore,
1938-443: The depression. As the sediments are buried, they are subject to increasing pressure and begin the processes of compaction and lithification that transform them into sedimentary rock . Sedimentary basins are created by deformation of Earth's lithosphere in diverse geological settings, usually as a result of plate tectonic activity. Mechanisms of crustal deformation that lead to subsidence and sedimentary basin formation include
1995-401: The earth's surface over time. Regional study of these rocks can be used as the primary record for different kinds of scientific investigation aimed at understanding and reconstructing the earth's past plate tectonics (paleotectonics), geography ( paleogeography , climate ( paleoclimatology ), oceans ( paleoceanography ), habitats ( paleoecology and paleobiogeography ). Sedimentary basin analysis
2052-598: The east coast between the estuaries of the Crouch and the Blackwater . Though north of the current mouth of the Thames, this line is well to the south of the centre-line of the basin which is asymmetric, its southern limb dipping more steeply than the northern. The Basin formed on top of the Late Cretaceous Chalk Group , which is exposed on the dip slopes of the Chilterns and North Downs. Within
2109-425: The edge of a continental craton as a result of prolonged, broadly distributed but slow subsidence of the continental lithosphere relative to the surrounding area. They are sometimes referred to as intracratonic sag basins. They tend to be subcircular in shape and are commonly filled with shallow water marine or terrestrial sedimentary rocks that remain flat-lying and relatively undeformed over long periods of time due to
2166-439: The effect is believed to be twofold. The lower, hotter part of the lithosphere will "flow" slowly away from the main area being stretched, whilst the upper, cooler and more brittle crust will tend to fault (crack) and fracture. The combined effect of these two mechanisms is for Earth's surface in the area of extension to subside, creating a geographical depression which is then often infilled with water and/or sediments. (An analogy
2223-405: The fill of one or more sedimentary basins over time. The scientific studies of stratigraphy and in recent decades sequence stratigraphy are focused on understanding the three-dimensional architecture, packaging and layering of this body of sedimentary rocks as a record resulting from sedimentary processes acting over time, influenced by global sea level change and regional plate tectonics. Where
2280-425: The lithosphere occurs primarily in the plane of Earth as a result of near horizontal maximum and minimum principal stresses . Faults associated with these plate boundaries are primarily vertical. Wherever these vertical fault planes encounter bends, movement along the fault can create local areas of compression or tension. When the curve in the fault plane moves apart, a region of transtension occurs and sometimes
2337-405: The lithosphere. Plate tectonic processes that can create sufficient loads on the lithosphere to induce basin-forming processes include: After any kind of sedimentary basin has begun to form, the load created by the water and sediments filling the basin creates additional load, thus causing additional lithospheric flexure and amplifying the original subsidence that created the basin, regardless of
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2394-478: The long-lived tectonic stability of the underlying craton. The geodynamic forces that create them remain poorly understood. Sedimentary basins form as a result of regional subsidence of the lithosphere, mostly as a result of a few geodynamic processes. If the lithosphere is caused to stretch horizontally, by mechanisms such as rifting (which is associated with divergent plate boundaries) or ridge-push or trench-pull (associated with convergent boundaries),
2451-694: The margins by older deposits such as the Reading Beds . In large areas towards the western end the London Clay is overlain by rather younger deposits of the Bagshot Beds etc., forming sandy heaths. Most of the basin is underlain at depth by a block of Palaeozoic rocks known as the London Platform . This is separated by faulting from the Early Cretaceous Wessex Basin (underlying the Weald and Hampshire Basin ) to
2508-554: The north Kent coast to Reculver and up the east coast of Essex and into Suffolk , where it is overlain by Pleistocene 'Crag' deposits which cover much of eastern Suffolk and Norfolk and are better considered as part of the North Sea Basin. The axis of the basin runs west–east from Marlborough and Newbury (Berkshire) to Chertsey (Surrey) before swinging slightly north of east through Westminster , passing midway between Chelmsford and Southend-on-Sea (Essex) to
2565-535: The north and northwest of London including Bishops Stortford , Harlow , Hertford , Stevenage , Harpenden , Hatfield , Welwyn Garden City , Potters Bar , St Albans , Luton and High Wycombe . 51°48′N 0°24′E / 51.8°N 0.4°E / 51.8; 0.4 Sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and
2622-471: The original cause of basin inception. Cooling of a lithospheric plate, particularly young oceanic crust or recently stretched continental crust, causes thermal subsidence . As the plate cools it shrinks and becomes denser through thermal contraction . Analogous to a solid floating in a liquid, as the lithospheric plate gets denser it sinks because it displaces more of the underlying mantle through an equilibrium process known as isostasy . Thermal subsidence
2679-413: The otherwise strike-slip fault environment. The study of sedimentary basins as entities unto themselves is often referred to as sedimentary basin analysis . Study involving quantitative modeling of the dynamic geologic processes by which they evolved is called basin modelling . The sedimentary rocks comprising the fill of sedimentary basins hold the most complete historical record of the evolution of
2736-429: The rocks directly and also very importantly allow paleontologists to study the microfossils they contain ( micropaleontology ). At the time they are being drilled, boreholes are also surveyed by pulling electronic instruments along the length of the borehole in a process known as well logging . Well logging, which is sometimes appropriately called borehole geophysics , uses electromagnetic and radioactive properties of
2793-505: The rocks surrounding the borehole, as well as their interaction with the fluids used in the process of drilling the borehole, to create a continuous record of the rocks along the length of the borehole, displayed as of a family of curves. Comparison of well log curves between multiple boreholes can be used to understand the stratigraphy of a sedimentary basin, particularly if used in conjunction with seismic stratigraphy. Wealden Group The Wealden Group, occasionally also referred to as
2850-486: The sedimentary rocks comprising a sedimentary basin's fill are exposed at the earth's surface, traditional field geology and aerial photography techniques as well as satellite imagery can be used in the study of sedimentary basins. Much of a sedimentary basin's fill often remains buried below the surface, often submerged in the ocean, and thus cannot be studied directly. Acoustic imaging using seismic reflection acquired through seismic data acquisition and studied through
2907-523: The south and the Worcester Basin to the west. Vertical movements of this block have affected both deposition and structure, with the result that many of the Mesozoic rocks underlying the neighbouring Weald are not present, or are very much thinner. Rocks of Triassic age are absent under London, occurring at depth only at the western edge of the basin. Jurassic rocks occur over a wider area to
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#17327651912602964-623: The south of its earlier course closer to the main axis of the basin, probably by the Anglian Stage . The north eastern part of the basin is now drained to the North Sea by rivers including the Crouch , Blackwater , Stour and Orwell . Much of the basin is urbanised. Besides the whole Greater London Urban Area of over 9.8 million people, the basin contains most of the M4 Corridor including Newbury , Reading , Bracknell , Wokingham , Finchampstead , Maidenhead and Slough ;
3021-400: The specific sub-discipline of seismic stratigraphy is the primary means of understanding the three-dimensional architecture of the basin's fill through remote sensing . Direct sampling of the rocks themselves is accomplished via the drilling of boreholes and the retrieval of rock samples in the form of both core samples and drill cuttings . These allow geologists to study small samples of
3078-402: The thinning of underlying crust; depression of the crust by sedimentary, tectonic or volcanic loading; or changes in the thickness or density of underlying or adjacent lithosphere . Once the process of basin formation has begun, the weight of the sediments being deposited in the basin adds a further load on the underlying crust that accentuates subsidence and thus amplifies basin development as
3135-481: The urbanised area to the south-west of London ( Camberley , Farnborough , Aldershot and Guildford ); north and south Thames-side ( Thurrock , Tilbury , Basildon , Southend-on-Sea , Northfleet , Gravesend etc.); north Kent towns including Chatham , Rochester , Gillingham , Sittingbourne , Faversham , Whitstable , Herne Bay and Canterbury ; Essex towns including Brentwood , Chelmsford , Braintree , Colchester , Clacton-on-Sea and Harwich ; and towns to
3192-527: The west and south but are also absent under London itself. During Early Cretaceous times the platform was uplifted and eroded, with the erosional material deposited to the south as the Wealden Group . Sea level rose during deposition of the Lower Greensand Group , but only completely covered the platform during deposition of the Gault Formation . The whole area remained submerged during much of
3249-546: The world's natural gas and petroleum and all of its coal are found in sedimentary rock. Many metal ores are found in sedimentary rocks formed in particular sedimentary environments. Sedimentary basins are also important from a purely scientific perspective because their sedimentary fill provides a record of Earth's history during the time in which the basin was actively receiving sediment. More than six hundred sedimentary basins have been identified worldwide. They range in areal size from tens of square kilometers to well over
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