Wessex Basin facts for kids
The Wessex Basin is a special area of land and sea in southern England, stretching into the English Channel. It's important because it holds petroleum, which is a type of oil and gas. The part of the basin on land is about 20,000 square kilometers, and the part under the English Channel is about the same size.
This basin is a type of geological feature called a rift basin. It formed a very long time ago, between the Permian and early Cretaceous periods. This happened because the African Plate and the Eurasian Plate (which are huge pieces of Earth's crust) were moving apart. Later, in the late Cretaceous and again in the Cenozoic era, the basin was pushed upwards, a process called inversion. This was a distant effect of the Alpine orogeny, which was a huge mountain-building event.
The Wessex Basin is usually split into three main parts: the Winterborne-Kingston Trough, the Channel Basin, and the Vale of Pewsey Basin. Because there's a lot of interest in finding oil and gas here, scientists have gathered a lot of information. This has helped them understand how the basin changed over time.
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How the Basin Formed and Changed
The Earth's crust in this area became thinner and stretched out. This caused the land to sink, creating deep, uneven valleys called half-grabens. These changes happened in stages, starting in the Permian period and finishing in the early Cretaceous.
By the middle of the Cretaceous period, the stretching of the crust stopped. The region then continued to sink slowly, but without new faults forming. This was partly due to the ground cooling down after the earlier stretching.
Then, the basin started to push upwards, a process called inversion. This began in the late Cretaceous and continued through the Cenozoic era. This upward movement happened at the same time as the Alpine orogeny, a major mountain-building event where the African and Eurasian plates were pushing together.
Layers of Rock and Earth's History
Deep below the Wessex Basin, there are very old rocks from the Devonian and Carboniferous periods. These rocks were pushed together and folded during an ancient mountain-building event called the Variscan orogeny.
After this mountain-building, the old faults in the rocks became active again. This caused different parts of the land to sink at different rates. Sands from this time sit on top of the older Devonian and Carboniferous rocks. Scientists can tell when the basin first started to form by studying these sands.
Later, a lot of rock (about 10 kilometers thick!) was worn away by erosion. Then, new sediments from the Permian period were laid down. These sediments were from a dry, desert-like environment.
The Triassic period followed, bringing new layers of sand and silt. At first, these were mainly on the western side of the basin. But as the sea level rose, they also appeared in the southwest.
During the Jurassic period, the area was mostly covered by the sea, and marine sediments were deposited. However, by the end of the Jurassic, the sea level dropped, leaving behind shallow marine sediments.
The Cretaceous period brought big changes. The old faults deep in the ground became active again, and the edges of the basin lifted up. About 1000 meters of brackish (slightly salty) and freshwater sediments were deposited. Later in the Cretaceous, most of the deposits were chalk. A clear break in the rock layers shows when the basin started to invert.
How the Basin Flipped Upside Down
The basin's inversion, or "flipping," was caused by the movements of the Alpine orogeny. This created a series of gentle folds in the rock layers called monoclines along the basin's edges.
Inversion happens when old faults, which once caused the ground to pull apart, start to push together. In the Wessex Basin, the faults that caused the land to stretch and sink during the Mesozoic era later reversed their movement. This caused areas that were once high to sink, and areas that were low to rise. The more an area was pushed upwards, the more it changed from its original shape.
Scientists can also see signs of inversion in the types of sediments deposited. For example, erosion during the late Cretaceous and Paleogene periods is one sign. Also, the chalk layers changed from marine (sea) sediments to non-marine (river) sediments. Cracks in the chalk also filled with younger sediments.
Today, the Wessex Basin has many east-west trending folds, like monoclines and anticlines. These sit above the old faults. The Purbeck-Isle of Wight fault is a clear example where you can see how the basin looked before and after the inversion.
Oil and Gas in the Basin
Because there's a lot of oil and gas here, scientists have used special techniques like seismic profiling to map the ground beneath South Dorset, Hampshire, and the Isle of Wight.
Most of the oil and gas is found in rocks from the Mesozoic era. The first wells were drilled in 1937. The Kimmeridge Oil Field, discovered in the 1950s, is the largest offshore field and is still producing oil today. Wytch Farm is the main onshore field, which started producing in the 1970s. The oil and gas here come from three main source rocks: the Blue Lias Formation, the Oxford Clay Formation, and the Kimmeridge Clay Formation.
See also
- East Midlands Oil Province
- Geology of Hampshire
- London Basin
- Petroleum reservoir
