Hampshire Basin and adjacent areas - Geology: Difference between revisions

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Throughout the region, there are widespread but rather patchy surface deposits of relatively recent origin, formed over the past 2 to 3 million years and spanning the Ice Ages and Interglacial periods. These are known as superficial deposits, the most common of which are associated with the major river valleys and the coast of the Solent. These superficial deposits mainly comprise thin clays, silts, peats, sands and gravels. The deposits are generally between 5 and 10 m thick. Over the highest Chalk downlands stony clay cover, usually less than 5 m thick, is also present. This region lay beyond the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2-3 million years.
Throughout the region, there are widespread but rather patchy surface deposits of relatively recent origin, formed over the past 2 to 3 million years and spanning the ice ages and interglacial periods. These are known as superficial deposits, the most common of which are associated with the major river valleys and the coast of the Solent. These superficial deposits mainly comprise thin clays, silts, peats, sands and gravels. The deposits are generally between 5 and 10 m thick. Over the highest Chalk downlands stony clay cover, usually less than 5 m thick, is also present. This region lay beyond the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2 to 3 million years.


==Geology at depth==
==Geology at depth==
Beneath the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks which geologists broadly split into two distinct types:
Beneath the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks, which geologists broadly split into two distinct types:


* The sedimentary bedrock geology is composed of quite hard rocks which were originally deposited a few hundred to tens of millions of years ago as layers of sediments in shallow seas, deserts and vast river systems in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
* The sedimentary bedrock geology is composed of quite hard rocks which were originally deposited a few hundred to tens of millions of years ago as layers of sediments in shallow seas, deserts and vast river systems in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
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Although basement rocks underlie the whole region at depth, the very deepest boreholes have only been drilled deep enough to penetrate the older sedimentary bedrock similar to that encountered at the surface within the adjacent regions. The older sedimentary bedrock was laid down between 400 and 290 million years ago and underlies the whole of this region at depths of between 1.5 and 3.0 km. Knowledge of these rocks and their distribution is only known in outline; but the depth at which they occur means that they are not considered further in this account, which focuses on the rocks in the top 1 km.
Although basement rocks underlie the whole region at depth, the very deepest boreholes have only been drilled deep enough to penetrate the older sedimentary bedrock similar to that encountered at the surface within the adjacent regions. The older sedimentary bedrock was laid down between 400 and 290 million years ago and underlies the whole of this region at depths of between 1.5 and 3.0 km. Knowledge of these rocks and their distribution is only known in outline; but the depth at which they occur means that they are not considered further in this account, which focuses on the rocks in the top 1 km.


[[Image:P902258.jpg|thumb|300px|left|Schematic cross-section of the underlying geology from the Vale of Pewsey in NW Wiltshire toward the Isle of Wight. The line of section can be seen on Figure P902257. P902258.]][[Image:P902260.jpg|thumb|300px|Schematic cross-section of the underlying geology from Lyme Bay in Dorset through the New Forest and on to NE Hampshire. The line of section can be seen on Figure P902257. P902260.]]Figures P902258 and P902260 are vertical sections through the geology, referred to as geological cross-sections, which illustrate the variations across the region.
[[Image:P902258.jpg|thumb|300px|left|Schematic cross-section of the underlying geology from the Vale of Pewsey in north-west Wiltshire toward the Isle of Wight. The line of section can be seen on '''Figure P902257'''. P902258.]][[Image:P902260.jpg|thumb|300px|Schematic cross-section of the underlying geology from Lyme Bay in Dorset through the New Forest and on to north-east Hampshire. The line of section can be seen on '''Figure P902257'''. P902260.]]'''Figures P902258''' and '''P902260''' are vertical sections through the geology, referred to as geological cross-sections, which illustrate the variations across the region.


In overall form the region is a large down-fold (syncline) that forms the Hampshire Basin. Erosion of this down-fold has resulted in the oldest rocks being found around the margins and the youngest lying in the centre of the basin. Along the northern, eastern and western flanks of this down-fold the sedimentary layers are gently inclined inwards but on the southern edge the basin is bounded by a narrow zone in which the rocks layers are steeply inclined. This is shown in the spectacular cliffs on the Isle of Wight and westwards into the Isle of Purbeck, Lulworth Cove and on to Durdle Door. At the very northern edge of the region another narrow zone of steeply folded rocks is present in the Vale of Pewsey area (Figure P902257).
In overall form the region is a large down-fold (syncline) that forms the Hampshire Basin. Erosion of this down-fold has resulted in the oldest rocks being found around the margins and the youngest lying in the centre of the basin. Along the northern, eastern and western flanks of this down-fold the sedimentary layers are gently inclined inwards but on the southern edge the basin is bounded by a narrow zone in which the rocks layers are steeply inclined. This is shown in the spectacular cliffs on the Isle of Wight and westwards into the Isle of Purbeck, Lulworth Cove and on to Durdle Door. At the very northern edge of the region another narrow zone of steeply folded rocks is present in the Vale of Pewsey area ('''Figure P902257''').


The interpretation of the boreholes, exploration survey data and surface outcrop shows that, from the surface the region is underlain by a vast thickness of younger sedimentary bedrock comprising five major layers related to stages in the infill of a deep sedimentary basin (Wessex Basin). This layered sequence is present across the region with the older lower layers tending to be more heavily folded and faulted. To distinguish between the two basins that have been mentioned, the Wessex Basin is a large deeply buried basin covering southern England that has sunk and filled with sediment over a very long period whereas the Hampshire Basin is a major down-fold (syncline) of these sedimentary layers that happened much later and is expressed at the surface and confined to this region. The principal layers comprising the younger sedimentary bedrock are now described for this region as a whole.
The interpretation of the boreholes, exploration survey data and surface outcrop shows that, from the surface the region is underlain by a vast thickness of younger sedimentary bedrock comprising five major layers related to stages in the infill of a deep sedimentary basin (Wessex Basin). This layered sequence is present across the region with the older lower layers tending to be more heavily folded and faulted. To distinguish between the two basins that have been mentioned, the Wessex Basin is a large deeply buried basin covering southern England that has sunk and filled with sediment over a very long period whereas the Hampshire Basin is a major down-fold (syncline) of these sedimentary layers that happened much later and is expressed at the surface and confined to this region. The principal layers comprising the younger sedimentary bedrock are now described for this region as a whole.


==Younger Sedimentary Bedrock==
==Younger sedimentary bedrock==
[[Image:P902434.jpg|thumb|400px|A general view of the Palaeogene sediments at Alum Bay on the Isle of Wight, looking east from the Needles. P902434.]]
[[Image:P902434.jpg|thumb|400px|A general view of the Palaeogene sediments at Alum Bay on the Isle of Wight, looking east from the Needles. P902434.]]
[[Image:P700021.jpg|thumb|400px|The Needles composed of steeply inclined Chalk seen from Scratchell’s Bay. P700021.]]
[[Image:P700021.jpg|thumb|400px|The Needles composed of steeply inclined Chalk seen from Scratchell’s Bay. P700021.]]
The youngest part of the sedimentary bedrock is composed of sediments laid down between 60 and 35 million years ago in shallow seas and around their margins. These (Palaeogene) sediments are preserved in the core of the down-fold of the Hampshire Basin. Their distribution is restricted to the northern half of the Isle of Wight (Plate P902434) and parts of the adjacent mainland comprising an area running from Dorchester, through Ringwood to Salisbury and then to the area north of Southampton and Portsmouth on to Bognor Regis in the east. They comprise up to 700 m of generally soft alternating layers of clay, silt and sand with thin limestones in the deepest parts of the down-fold on the Isle of Wight. To the north the layers thin rapidly towards the margin of the basin. These deposits rest on the Chalk throughout. The next major layer is the Chalk (Plate P700021), which is a very fine-grained white or pale grey limestone, which often contains nodules of flint, a very hard form of silica. The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it. These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. The Chalk has the largest surface outcrop of any of the layers within the region, including the Dorset Downs, through to Salisbury Plain, the Andover or north Hampshire Downs, the area around Alton and Petersfield, Ports Down and the westernmost part of the South Downs. The Chalk has a maximum thickness of around 400 m in the core of the Hampshire Basin down-fold where it lies beneath the overlying sediments.
The youngest part of the sedimentary bedrock is composed of sediments laid down between 60 and 35 million years ago in shallow seas and around their margins. These (Palaeogene) sediments are preserved in the core of the down-fold of the Hampshire Basin. Their distribution is restricted to the northern half of the Isle of Wight ('''Plate P902434''') and parts of the adjacent mainland comprising an area running from Dorchester, through Ringwood to Salisbury and then to the area north of Southampton and Portsmouth on to Bognor Regis in the east. They comprise up to 700 m of generally soft alternating layers of clay, silt and sand with thin limestones in the deepest parts of the down-fold on the Isle of Wight. To the north, the layers thin rapidly towards the margin of the basin. These deposits rest on the Chalk throughout. The next major layer is the Chalk ('''Plate P700021'''), which is a very fine-grained white or pale grey limestone, which often contains nodules of flint, a very hard form of silica. The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it. These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. The Chalk has the largest surface outcrop of any of the layers within the region, including the Dorset Downs, through to Salisbury Plain, the Andover or north Hampshire Downs, the area around Alton and Petersfield, Ports Down and the westernmost part of the South Downs. The Chalk has a maximum thickness of around 400 m in the core of the Hampshire Basin down-fold where it lies beneath the overlying sediments.


[[Image:P005800.jpg|thumb|300px|left|Stair Hole, adjacent to Lulworth Cove in Dorset . looking towards the east, On the right at the foot of the cliff Portland Stone and Purbeck Limestone have been breached by the sea which is making a cove. P005800.]]
[[Image:P005800.jpg|thumb|300px|left|Stair Hole, adjacent to Lulworth Cove in Dorset looking towards the east. On the right at the foot of the cliff, Portland Stone and Purbeck Limestone have been breached by the sea which is making a cove. P005800.]]
[[Image:P774554.jpg|thumb|300px|left|A panorama of the Lower Greensand Group (Lower Cretaceous Sediments) in Chale Bay on the southwest coast of the Isle of Wight looking southeastward towards St Catherine’s Point. P774554.]]
[[Image:P774554.jpg|thumb|300px|left|A panorama of the Lower Greensand Group (Lower Cretaceous sediments) in Chale Bay on the south-west coast of the Isle of Wight looking south-eastward towards St Catherine’s Point. P774554.]]
Beneath the Chalk are sedimentary rocks deposited between 145 and 100 million years ago in shallow sea and lagoon environments (Lower Cretaceous sediments). The layers include limestones, mudstones and beds of mineral salts. The sequence is thin in the west and north, with as little as 100 m present from surface within the cores of the Kingsclere, Shalbourne and Vale of Pewsey up-folds and from Devizes as a narrowing strip southward through Warminster to the margins of the Dorset Downs. Small detached patches occur north of Lyme Regis. These layers are much thicker (where the Lower Greensand, Wealden and Purbeck groups are also present) between Lulworth (Plate P005800) and Swanage and in the southern part of the Isle of Wight (Plate P774554), up to 1100 m have been encountered in boreholes. These rocks are not seen at the surface in the east of the region but similar thick sequences are known to be represented beneath the Chalk.
Beneath the Chalk are sedimentary rocks deposited between 145 and 100 million years ago in shallow sea and lagoon environments (Lower Cretaceous sediments). The layers include limestones, mudstones and beds of mineral salts. The sequence is thin in the west and north, with as little as 100 m present from surface within the cores of the Kingsclere, Shalbourne and Vale of Pewsey up-folds and from Devizes as a narrowing strip southward through Warminster to the margins of the Dorset Downs. Small detached patches occur north of Lyme Regis. These layers are much thicker (where the Lower Greensand, Wealden and Purbeck groups are also present) between Lulworth ('''Plate P005800''') and Swanage and in the southern part of the Isle of Wight ('''Plate P774554'''), up to 1100 m have been encountered in boreholes. These rocks are not seen at the surface in the east of the region but similar thick sequences are known to be represented beneath the Chalk.


The next major layer is made of three parts each comprising thick mudstones containing thin limestones and sandstones overlain by shelly and sandy limestones. These rocks are seen at the surface along the Jurassic Coast World Heritage Site in the west of the region between Portland and Lyme Regis and inland west of Yeovil and through Wincanton, Frome and Trowbridge to Chippenham. This sequence was deposited between 205 and 145 million years ago, the mudstones being laid down in deep seas whilst the limestones were deposited in shallow tropical reef environments. The mudstones include the Lias, Oxford and Kimmeridge clays and are each between 250 and 400 m thick. The intervening three limestone units are the Oolite, Corallian and Portland groups each about 100 m thick. In total the sequence is up to 1400 m thick beneath south Dorset and the western parts of the region and becomes thinner beneath the younger units towards the north and east. For example, about 600 m is present in the north of the Isle of Wight where many of the upper layers were removed by erosion before the overlying rocks were deposited.
The next major layer is made of three parts each comprising thick mudstones containing thin limestones and sandstones overlain by shelly and sandy limestones. These rocks are seen at the surface along the Jurassic Coast World Heritage Site in the west of the region between Portland and Lyme Regis and inland west of Yeovil and through Wincanton, Frome and Trowbridge to Chippenham. This sequence was deposited between 205 and 145 million years ago, the mudstones being laid down in deep seas whilst the limestones were deposited in shallow tropical reef environments. The mudstones include the Lias, Oxford and Kimmeridge clays and are each between 250 and 400 m thick. The intervening three limestone units are the Oolite, Corallian and Portland groups each about 100 m thick. In total the sequence is up to 1400 m thick beneath south Dorset and the western parts of the region and becomes thinner beneath the younger units towards the north and east. For example, about 600 m is present in the north of the Isle of Wight where many of the upper layers were removed by erosion before the overlying rocks were deposited.
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[[Image:P579296.jpg|thumb|300px|The cliffs east of Sidmouth, south Devon showing the Sherwood Sandstone Group in the lower half of the cliff and the overlying Mercia Mudstone Group. P579296.]]
[[Image:P579296.jpg|thumb|300px|The cliffs east of Sidmouth, south Devon showing the Sherwood Sandstone Group in the lower half of the cliff and the overlying Mercia Mudstone Group. P579296.]]
The oldest rocks of the infill of the Wessex Basin are between 290 and 205 million years old (Triassic sediments) and were deposited on an extensive desert landscape at or close to the equator, they are predominantly red in colour (Plate P579296). These rocks do not occur at surface in the region but they are known from deep boreholes throughout. A maximum of about 1600 m of this sequence is encountered in the vicinity of Lyme Regis, in Dorset. To the north and east these layers are much thinner and occur at depths in excess of 1500 m beneath north and east Hampshire.
The oldest rocks of the infill of the Wessex Basin are between 290 and 205 million years old (Triassic sediments) and were deposited on an extensive desert landscape at or close to the equator, they are predominantly red in colour ('''Plate P579296'''). These rocks do not occur at surface in the region but they are known from deep boreholes throughout. A maximum of about 1600 m of this sequence is encountered in the vicinity of Lyme Regis, in Dorset. To the north and east these layers are much thinner and occur at depths in excess of 1500 m beneath north and east Hampshire.


These red rocks can be divided into three units:
These red rocks can be divided into three units:
* The most widespread and youngest is a series of red mudstones, called the Mercia Mudstone, which was deposited in shallow seasonal lagoons and on broad river floodplains in a desert landscape. They contain significant beds of rock salt. The maximum thickness of these mudstones of about 450 m is proven in boreholes in south Dorset but they are known to thin to about 100 m over much of the remainder of the region.
* The most widespread and youngest is a series of red mudstones, called the Mercia Mudstone, which was deposited in shallow seasonal lagoons and on broad river floodplains in a desert landscape. They contain significant beds of rock salt. The maximum thickness of these mudstones of about 450 m is proven in boreholes in south Dorset but they are known to thin to about 100 m over much of the remainder of the region.


* Beneath the mudstones lies a layer of sandstones, the Sherwood Sandstone, up to 250 m thick, that was deposited by large seasonal rivers crossing an arid desert plain. They are encountered at depth south of a line from Wincanton to Portsmouth and are thickest beneath the southwest of the region. They form the major oil and gas reservoir which is exploited at Wytch Farm south of Poole. Towards the west they are also a major aquifer.
* Beneath the mudstones lies a layer of sandstones, the Sherwood Sandstone, up to 250 m thick, that was deposited by large seasonal rivers crossing an arid desert plain. They are encountered at depth south of a line from Wincanton to Portsmouth and are thickest beneath the south-west of the region. They form the major oil and gas reservoir which is exploited at Wytch Farm south of Poole. Towards the west they are also a major aquifer.


* The lowest and oldest unit (Aylesbeare Group) is only known from boreholes in west and south Dorset and was deposited mainly as large sand river channels and flood deposits and rock screes in a desert. The rocks comprise thin red sandstones and thick mudstones with significant layers rich in pebbles and boulders. In total this unit is up to 800 m thick in south Dorset around Lyme Regis and it is also found in the northwest of the region beneath the area round Devizes and Frome.
* The lowest and oldest unit (Aylesbeare Group) is only known from boreholes in west and south Dorset and was deposited mainly as large sand river channels and flood deposits and rock screes in a desert. The rocks comprise thin red sandstones and thick mudstones with significant layers rich in pebbles and boulders. In total this unit is up to 800 m thick in south Dorset around Lyme Regis and it is also found in the north-west of the region beneath the area round Devizes and Frome.





Revision as of 11:20, 3 November 2014

Throughout the region, there are widespread but rather patchy surface deposits of relatively recent origin, formed over the past 2 to 3 million years and spanning the ice ages and interglacial periods. These are known as superficial deposits, the most common of which are associated with the major river valleys and the coast of the Solent. These superficial deposits mainly comprise thin clays, silts, peats, sands and gravels. The deposits are generally between 5 and 10 m thick. Over the highest Chalk downlands stony clay cover, usually less than 5 m thick, is also present. This region lay beyond the extent of the ice-sheets that have dominated the evolution of the British landscape over the past 2 to 3 million years.

Geology at depth

Beneath the superficial deposits, or with just a cover of soil where such deposits are absent, are older rocks, which geologists broadly split into two distinct types:

  • The sedimentary bedrock geology is composed of quite hard rocks which were originally deposited a few hundred to tens of millions of years ago as layers of sediments in shallow seas, deserts and vast river systems in times when Britain lay closer to the Equator and the climate and landscape were very different from those of today.
  • The basement geology, which underlies the bedrock, is over 400 million years old and mainly comprises harder, denser rocks which have been strongly compacted fractured and folded. They include both rocks originally deposited as sediments and others that are products of volcanic activity. Basement rocks do not occur within the top 1 km in this region and so are not considered further in this account.

Although basement rocks underlie the whole region at depth, the very deepest boreholes have only been drilled deep enough to penetrate the older sedimentary bedrock similar to that encountered at the surface within the adjacent regions. The older sedimentary bedrock was laid down between 400 and 290 million years ago and underlies the whole of this region at depths of between 1.5 and 3.0 km. Knowledge of these rocks and their distribution is only known in outline; but the depth at which they occur means that they are not considered further in this account, which focuses on the rocks in the top 1 km.

Schematic cross-section of the underlying geology from the Vale of Pewsey in north-west Wiltshire toward the Isle of Wight. The line of section can be seen on Figure P902257. P902258.
Schematic cross-section of the underlying geology from Lyme Bay in Dorset through the New Forest and on to north-east Hampshire. The line of section can be seen on Figure P902257. P902260.

Figures P902258 and P902260 are vertical sections through the geology, referred to as geological cross-sections, which illustrate the variations across the region.

In overall form the region is a large down-fold (syncline) that forms the Hampshire Basin. Erosion of this down-fold has resulted in the oldest rocks being found around the margins and the youngest lying in the centre of the basin. Along the northern, eastern and western flanks of this down-fold the sedimentary layers are gently inclined inwards but on the southern edge the basin is bounded by a narrow zone in which the rocks layers are steeply inclined. This is shown in the spectacular cliffs on the Isle of Wight and westwards into the Isle of Purbeck, Lulworth Cove and on to Durdle Door. At the very northern edge of the region another narrow zone of steeply folded rocks is present in the Vale of Pewsey area (Figure P902257).

The interpretation of the boreholes, exploration survey data and surface outcrop shows that, from the surface the region is underlain by a vast thickness of younger sedimentary bedrock comprising five major layers related to stages in the infill of a deep sedimentary basin (Wessex Basin). This layered sequence is present across the region with the older lower layers tending to be more heavily folded and faulted. To distinguish between the two basins that have been mentioned, the Wessex Basin is a large deeply buried basin covering southern England that has sunk and filled with sediment over a very long period whereas the Hampshire Basin is a major down-fold (syncline) of these sedimentary layers that happened much later and is expressed at the surface and confined to this region. The principal layers comprising the younger sedimentary bedrock are now described for this region as a whole.

Younger sedimentary bedrock

A general view of the Palaeogene sediments at Alum Bay on the Isle of Wight, looking east from the Needles. P902434.
The Needles composed of steeply inclined Chalk seen from Scratchell’s Bay. P700021.

The youngest part of the sedimentary bedrock is composed of sediments laid down between 60 and 35 million years ago in shallow seas and around their margins. These (Palaeogene) sediments are preserved in the core of the down-fold of the Hampshire Basin. Their distribution is restricted to the northern half of the Isle of Wight (Plate P902434) and parts of the adjacent mainland comprising an area running from Dorchester, through Ringwood to Salisbury and then to the area north of Southampton and Portsmouth on to Bognor Regis in the east. They comprise up to 700 m of generally soft alternating layers of clay, silt and sand with thin limestones in the deepest parts of the down-fold on the Isle of Wight. To the north, the layers thin rapidly towards the margin of the basin. These deposits rest on the Chalk throughout. The next major layer is the Chalk (Plate P700021), which is a very fine-grained white or pale grey limestone, which often contains nodules of flint, a very hard form of silica. The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it. These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk. The Chalk has the largest surface outcrop of any of the layers within the region, including the Dorset Downs, through to Salisbury Plain, the Andover or north Hampshire Downs, the area around Alton and Petersfield, Ports Down and the westernmost part of the South Downs. The Chalk has a maximum thickness of around 400 m in the core of the Hampshire Basin down-fold where it lies beneath the overlying sediments.

Stair Hole, adjacent to Lulworth Cove in Dorset looking towards the east. On the right at the foot of the cliff, Portland Stone and Purbeck Limestone have been breached by the sea which is making a cove. P005800.
A panorama of the Lower Greensand Group (Lower Cretaceous sediments) in Chale Bay on the south-west coast of the Isle of Wight looking south-eastward towards St Catherine’s Point. P774554.

Beneath the Chalk are sedimentary rocks deposited between 145 and 100 million years ago in shallow sea and lagoon environments (Lower Cretaceous sediments). The layers include limestones, mudstones and beds of mineral salts. The sequence is thin in the west and north, with as little as 100 m present from surface within the cores of the Kingsclere, Shalbourne and Vale of Pewsey up-folds and from Devizes as a narrowing strip southward through Warminster to the margins of the Dorset Downs. Small detached patches occur north of Lyme Regis. These layers are much thicker (where the Lower Greensand, Wealden and Purbeck groups are also present) between Lulworth (Plate P005800) and Swanage and in the southern part of the Isle of Wight (Plate P774554), up to 1100 m have been encountered in boreholes. These rocks are not seen at the surface in the east of the region but similar thick sequences are known to be represented beneath the Chalk.

The next major layer is made of three parts each comprising thick mudstones containing thin limestones and sandstones overlain by shelly and sandy limestones. These rocks are seen at the surface along the Jurassic Coast World Heritage Site in the west of the region between Portland and Lyme Regis and inland west of Yeovil and through Wincanton, Frome and Trowbridge to Chippenham. This sequence was deposited between 205 and 145 million years ago, the mudstones being laid down in deep seas whilst the limestones were deposited in shallow tropical reef environments. The mudstones include the Lias, Oxford and Kimmeridge clays and are each between 250 and 400 m thick. The intervening three limestone units are the Oolite, Corallian and Portland groups each about 100 m thick. In total the sequence is up to 1400 m thick beneath south Dorset and the western parts of the region and becomes thinner beneath the younger units towards the north and east. For example, about 600 m is present in the north of the Isle of Wight where many of the upper layers were removed by erosion before the overlying rocks were deposited.

The three major mudstones are rich in organic material and represent potential sources of oil and gas where they have been deeply buried and heated. The intervening limestones are more porous than the clays and therefore can potentially act as reservoir rocks if the oil and gas migrates away from the clays. These limestone units also form important building stone resources where they occur at surface as for example in the Portland and Purbeck districts of south Dorset.

The cliffs east of Sidmouth, south Devon showing the Sherwood Sandstone Group in the lower half of the cliff and the overlying Mercia Mudstone Group. P579296.

The oldest rocks of the infill of the Wessex Basin are between 290 and 205 million years old (Triassic sediments) and were deposited on an extensive desert landscape at or close to the equator, they are predominantly red in colour (Plate P579296). These rocks do not occur at surface in the region but they are known from deep boreholes throughout. A maximum of about 1600 m of this sequence is encountered in the vicinity of Lyme Regis, in Dorset. To the north and east these layers are much thinner and occur at depths in excess of 1500 m beneath north and east Hampshire.

These red rocks can be divided into three units:

  • The most widespread and youngest is a series of red mudstones, called the Mercia Mudstone, which was deposited in shallow seasonal lagoons and on broad river floodplains in a desert landscape. They contain significant beds of rock salt. The maximum thickness of these mudstones of about 450 m is proven in boreholes in south Dorset but they are known to thin to about 100 m over much of the remainder of the region.
  • Beneath the mudstones lies a layer of sandstones, the Sherwood Sandstone, up to 250 m thick, that was deposited by large seasonal rivers crossing an arid desert plain. They are encountered at depth south of a line from Wincanton to Portsmouth and are thickest beneath the south-west of the region. They form the major oil and gas reservoir which is exploited at Wytch Farm south of Poole. Towards the west they are also a major aquifer.
  • The lowest and oldest unit (Aylesbeare Group) is only known from boreholes in west and south Dorset and was deposited mainly as large sand river channels and flood deposits and rock screes in a desert. The rocks comprise thin red sandstones and thick mudstones with significant layers rich in pebbles and boulders. In total this unit is up to 800 m thick in south Dorset around Lyme Regis and it is also found in the north-west of the region beneath the area round Devizes and Frome.