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Examples of Sedimentary Basins

The Turpan Basin

The Turpan basin is a sedimentary basin located in North West China.It is part of the Northern Xinjiang - Beishan mountains region.This region includes other large sedimentary basins such as the Junggar,Turpan and Yining basins.These large basins consist mostly of sands,shales and sandy conglomerates.The sandstones mostly consist of  Feldspathic litharenite and litharenite.The Turpan basin is a large sedimentary basin that was formed in the late permian.There are two types of subsidence that lead to the formation of the Turpan basin.The first was thermal susidence that took place during the late Permian and through the early triassic.The second is flexural subsidence that took place throughout the Middle triassic to Early Tertiary which produced periods of extremely high subsidence rates.Very high susidence rates are also common in the Late Triassic,Early Jurassic and Early cretaceous.

The composition of the sandstones vary in time especially with its quarts content.In the permian the quartz content was only 11%.In the Triassic the quartz content increased to 27%,In the lower Jurassic it was 35%,In the upper Jurassic it was 36% and even reaching 47% in the Cretaceous and Tertiary.

Fig 5.1.1. Map of the Turpan Basin.

Fig 5.1.2. Subsidence curve of the Turpan Basin.
The are two main subsidence sources .These include thermal subsidence and flexural subsidence.Thermal subsidence occurs as magma is made at the mid ocean ridge it cools and builds a sedimentary basin.This is done through thermal reequillibriation.As thermal susidence occurs the curve is strongly concave down and and then smoother in a downward direction.This indicates that at  the beggining of the subsidence the subsidence rate is very high as the sedimentary basin is rapidly subsiding.After the magma cools that was produced at the mid ocean ridge the subsidence rate is alot slower and is mostly due to the weight of overlying sediment and water load.The highest rates of subsidence occur in the Late Permian  and early Triassic.High rates of subsidence occur where there are large amounts of coarse clastic deposition.This reflects that high rates of subsidence are linked to the rapid erosion of nerby source regions.The greatest amounts of subsidence occur about 50 million years after the onset of extension.This period coincides with the onset of thermal subsidence and the evolution of the basin.

There are four main periods of subsidence.The rapidly subsiding period occurs in the upper Permian and the Early Triassic.The actively subsiding period occurs during the middle triassic to the Jurassic.The reducing subsiding period occurs in the cretaceous and the increased subsiding period occurs in the early tertiary.
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