Hampshire Basin
The Hampshire Basin is a sedimentary basin in southern England . Its sediment filling is palaeogenic and consists mainly of sand and clays .
geography
The basin extends over large parts of the counties of Hampshire , from which it is named, Dorset and Sussex , and also touches the north of the Isle of Wight . It forms the northern, inland part of the Hampshire-Dieppe basin in the English Channel . The Hampshire Basin can be traced along the southern coast of England from the Dorchester area in the west to Beachy Head in the east over a distance of around 160 kilometers. To the north it is framed by the chalk layers of the South Downs , Salisbury Plain and Cranborne Chase . Its southern limit is the Purbeck Monocline , which runs from Old Harry Rocks to The Needles and the central ridge of the Isle of Wight. It then continues under the sea in the English Channel as a Wight Bray monocline . The layer package was folded monoclinally , with the layers now partially vertical, as seen in the Purbeck Hills . The rather asymmetrical (with shallow dipping north wing and steep south wing), east-west trending basin reaches 30 miles at its widest point between Salisbury and Newport .
In the basin there are many forest and heathland areas such as the Wareham Forest and the New Forest . Major coastal cities are Bournemouth , Southampton and Portsmouth . The many drowned river valleys ( rias ) such as B. Solent , Southampton Water , Cowes and Bembridge with the ports of Poole Harbor , Portsmouth Harbor , Chichester Harbor , Langstone Harbor , Pagham Harbor and Yarmouth .
The Hampshire Basin has no dominant receiving waters. The basin was previously drained by the Frome River and the Solent in a west-east direction, the branches of which were oriented north-south. This network was suspended at the end of the last ice age by rising sea levels and the former land connection to the Isle of Wight was flooded. The western part of the basin is now drained into Poole Harbor via the Frome and the River Piddle . The central part lies in the catchment area of the Solent (with the rivers Lymington River , Test , Itchen , Meon , Hamble , Western Yar , Medina and Eastern Yar ), which is connected to the English Channel either directly or via Southampton Water. The eastern part of the basin consists of a very narrow coastal plain , which is crossed by many small watercourses. The somewhat larger ones like the River Arun and the River Adur have their origin in the Weald .
Geological structure
The Hampshire Basin is an asymmetrical syncline that is framed by layers of chalk from the Upper Cretaceous. At the northern edge of the basin, the layers from Salisbury Plain dip flat to the south, but are monoclinally steep at the southern edge. The boundary surface with the Upper Cretaceous lies west of Winchester at 170 meters above sea level, whereas in Newport it is already 600 meters below ground. It then rises steeply and reaches a height of almost 200 meters in the central hills of the Isle of Wight.
This synlinal structure, which is quite simple in and of itself, is somewhat complicated by several minor anomalies. To the southeast of Salisbury, for example, there is a small syncline of Paleocene and Eocene layers. An anticline north of Portsmouth is Portsdown Hill ; at its core, chalk is found amid younger sediments. The structure between Bognor Regis and Worthing is also comparable , which is separated from the chalk layers of the South Downs by a band of Reading Beds and London Clay (runs from Havant to Lancing ).
The chalk layers represent a rather thin Venier that has laid over older and relatively rigid crustal blocks of the variscus . During the Alpine Orogeny , these blocks of crust experienced vertical displacements that were interrupted in the sedimentary shell.
stratigraphy
The geology of the deeper subsurface is complex. The individual crustal blocks are bounded by northwest-southeast trending faults over which a west-east oriented fracture system has laid. The individual blocks moved against each other in the late Paleozoic and again in the Mesozoic , so that the formations deposited on them before the Cretaceous show very considerable fluctuations in thickness. The Wytch Farm - oil field has been, for example, jammed in one of these blocks. The Permian , Triassic and Jurassic to Lower Cretaceous ( oolite limestone of the Great Oolite from the Middle Jurassic ( Bathonian ) to Weald Clay from the Hauterivian ) sedimented on the Paleozoic crustal blocks . The crustal area of the Hampshire Basin then experienced an expansion phase in the Lower Cretaceous, with the crust blocks slipping southwards on Listrian faults ( rift phase ). There was a down to the Aptian permanent hiatus under formation of an erosion surface. In the Lower Aptium, the Lower Greensand transgressed over this area. The following chalk layers show strong fluctuations in thickness, as they transgressed from the east towards Dorset. There are indications that the Upper Greensand, set in the west, is the same age as the first chalk deposits on the Isle of Wight ( Upper Albium / Cenomanium ).
The sediment filling of the actual Hampshire Basin comes from the Paleogene and ranges from the Thanetian ( Paleocene ) to the Rupelian ( Oligocene ). It is therefore younger than the comparable sequence in the London Basin . The sediment sequence is composed as follows (from hanging to lying):
Lambeth group
The basal, about 30 meters thick Lambeth group comes from the uppermost Thanetium and consists essentially of the Reading Formation (or Reading Beds ). It begins with thin, scree-bearing sea sands, followed by fluvial sands and lagoon, red-speckled clays. The place of origin of the sands is the Armorican massif . The Woolwich Formation is a special facies that occurs only offshore in the southeast.
Thames group
The following, about 100 meters thick, shallow marine Thames group in the Hampshire Basin consists only of the London Clay Formation of the Ypresium . It shows sequences with increasing grain size towards the hanging wall. The sequences end in diagonally layered sands, which indicate tidal channels near the beach. At the west end of the basin, the final Christchurch member even has paleo soils with remains of roots; this suggests a seaward expansion of the tidal valleys.
The Thanet and Thames groups are open in a relatively narrow band that frames the northern edge of the basin. This begins on the coast at Studland , frames the Dorset Heath, goes north around Romsey , then turns south-east towards Eastleigh and then runs east over Chichester , Worthing and Shoreham-by-Sea . The following groups occupy the inside of the pool.
Bracklesham group
The 140 meter thick Bracklesham Group from the Upper Ypresian and Lutetian is divided into two facies areas. In the east (Hampshire) it is made up of the following formations (from young to old):
The western part in Dorset consists of the basal Poole Formation and the Branksome Sand Formation above ; the latter is equivalent to the Selsey formation.
All these formations generally document a complex interlocking of marine, estuarine , lagoon and fluvial sedimentation areas and thus a frequent change of position of the beach line. Open marine sediments only make up the Earnley Formation and the Selsey Formation ( glauconitic sands, silts and clays). Their depths of deposit may have fluctuated between 10 and 100 meters. However, the Poole Formation in Dorset already consists of fairly coarse-grained, sloping river sands .
Barton group
The Barton group of the Upper Lutetian and Bartonian reaches 120 meters in thickness. It is built up from the following formations (from young to old):
The basal Barton Clay Formation, stratotype for the Bartonium , is replaced to the west by the Boscombe Sand Formation . In general, the grain size of the Barton group increases towards the hanging wall, ie the marine clays at the base merge into sand near the beach, which is then closed off by a paleo-soil horizon. This is interpreted to mean that towards the end of the Eocene a coastal plain had progressed far into the basin.
Solent group
The 40 meter thick Solent group from the Priabonium and Rupelium is composed as follows (from hanging wall to lying):
The group started with the Headon Hill Formation, a brackish water formation that was deposited behind a sand bar in lagoons, rivers and lakes . In its central part, it experiences a brief marine incursion. The following Bembridge Limestone is a freshwater limestone with a terrestrial mollusc fauna and mammal remains . The final Bouldnor formation from the Rupel is the youngest formation in the Hampshire Basin. It was created near the coast, recognizable by the alternation of faunas in the fresh and brackish water area with marine elements.
Heavy mineral analyzes from the Hampshire Basin show that the fully marine formations obtained their sediment load from the Scottish area . At relatively high standing sea level to the East Coast parallel ocean currents had ( engl. Longshore currents sediments) transported from the north. The deposits near the coast suggest, however, the much closer areas of origin of the Armorican massif and Cornwall .
tectonics
After the settling of the last formation (Bouldnor formation) in the Hampshire basin, it silted up and was tectonically raised, clearly recognizable by the almost vertical collapse of the Portland-Wight monocline . This event is usually attributed to a late Alpine, compressive phase in the Miocene . The Listrian fault areas from the Mesozoic rift phase were reactivated as deferrals. The relatively thin sediment skin that had accumulated during the post-drifting period was pressed open on them and deformed into steep-winged, east-west stroking monoclines. In this way the Portsdown Antikline , the already mentioned Portland-Wight-Monocline, the south-east pivoting axis of the Weald-Artois-Uplift and the south-east trending Start-Cotentin-Ridge were created .
However, there are reprocessed clasts and fossils in the Selsey Formation on the Isle of Wight, which indicate that parts of the Portland Wight high had already emerged in the Lower Lutetian. This process continued during the deposition of the Barton Clay into the Lower Bartonium. From this it follows that the uplifting process of the high structures lasted over the period from the Middle Eocene to the Miocene and possibly took place in several pulse-like bursts.
Final consideration
The filling of the Hampshire basin with sediments from the Upper Thanetium to the Rupelium was preceded by an intensive phase of uplift and erosion, which lasted from the end of the Cretaceous to the Paleocene and resulted in a leveling of the Upper Cretaceous. During the Thanetium, a shelf sea transgressed . Due to global sea level changes, however, there was a frequent change from marine facies to adjacent coastal and river facies in the sedimentation area. The marine sediments were delivered via the North Sea from the north of Scotland, whereas the marginal facial sediments, along with local components , are likely to come mainly from the closer delivery areas of the English Channel - Armorika and Cornwall.
Already towards the end of the Lutetium, the first localized tectonic movements ( inversion tectonics with the formation of anticline structures) occurred . These movements, which are causally linked to orogeny in the Alps and Pyrenees, intensified from the late Oligocene and lasted into the Miocene.
It is very likely that the Hampshire Basin and the London Basin were once a cohesive sedimentation area that encompassed the entire south-east of England in the Paleocene. The two basins were separated from each other during the Eocene by the gradual emergence of the Weald-Artois anticline . At the beginning of the Oligocene the London Basin had already dried up and only the central part of the Hampshire Basin around the Solent was still covered by the sea (sedimentation of the Bouldnor Formation).
See also
swell
literature
- Woodcock, N. & Strachan, N .: Geological History of Britain and Ireland . Blackwell Science Ltd, 2000, ISBN 0-632-03656-7 .
Individual evidence
- ^ Bird, E.: The Shaping of the Isle of White . Ex Libris Press, Bradford on Avon 1997, ISBN 0-948578-83-1 .
- ↑ Melville, RV & Freshney EC: The Hampshire Basin and adjoining areas . In: British Regional Geology series, Institute of Geological Sciences, London: HMSO . 1982, ISBN 0-11-884203-X .
- ↑ Jackson, AA: Chilterns. Sheet 51N 02W Solid Geology . In: British Geological Survey (Ed.): 1: 250,000 Geological map series, Keyworth . 1982, ISBN 978-0-7518-1900-7 .
- ^ Gibbard P. & Lewin J .: History of the major rivers of southern Britain during the Tertiary . Ed .: Quaternary Palaeoenvironments Group. 2007.