The Weald /ˈwiːld/ Basin is a major topographic feature of the area that is now southern England and northern France from the Triassic to the Late Cretaceous. Its uplift in the Late Cretaceous marked the formation of the Wealden Anticline. The rock strata contain hydrocarbon deposits which have yielded coal, oil and gas.
Contents
Formation
The Weald Basin's formation commenced during the Carboniferous, with the rocks which are today basement deposited within a low swamp providing coals which were exploited to the north and east in Kent, but boreholes drilled in the 19th century failed to find this deposit in the area of the Weald. The Carboniferous coals are overlain by Silurian and early Triassic sediments. The sediments where uplifted and faulted within the Variscan Orogeny, with the land now occupied by the Weald Basin being a low external fold belt to the main orogeny, which was located within the present day English Channel. The remnants of the mountain belt can be seen today in Devon and Cornwall in what is known as the Cornubian Massif. Unlike in Devon and Cornwall the deformation caused little or no metamorphism.
The mountain belt collapsed soon after the orogeny, leading to the former northward thrusts to be reactivated as normal faults, and led to the formation of the Weald basin, which developed as an extension of the considerably larger Wessex Basin . Reconstructions of the geometry of the early fault systems in the Weald Basin reveal that for the early history of the basin a series of steep normal faults to the north were active against the London-Brabant Massif, but it is not clear whether this reflects a syn-rift origin for these rocks. The Weald basin gently subsided throughout the Jurassic, Cretaceous and Early Tertiary leading to a thick succession of sedimentary rocks being deposited. During the Early Jurassic a north and east shallowing mud-dominated shelf developed.
Basin reactivation
As a result of the Alpine orogeny the basin was squeezed between the basement to the north and the south, this resulted in the reactivation of the formerly normal faults into minor thrusts (as they had been during the Variscan Orogeny) and the formation of the Wealden Anticline. The region's two surface structural highs (areas of crust and rocks which are uplifted), the Wealden Anticline and the Channel High, are superimposed upon earlier, Mesozoic basins, (the Weald and Channel basins). This nearly exact superimposition of compressional features upon underlying formerly extensional features exemplifies perfectly the principles of structural inversion across a large and well defined geologic feature. The overall uplift produced by the Tertiary inversion in the eastern Wealden basin has been estimated to be as much as 1525 m; which as a result of the large amount of Mesozoic sediments does not reveal the underlying Paleozoic basement. Utilising estimations of the original thickness of Chalk (400–460 m) and other Mesozoic strata indicate a complex fold structure which, in the event it was not eroded during uplift, attained a crestal elevation of 1400 m over what is today Ashdown Forest. However it is likely that erosion kept pace with uplift, resulting in large quantities of sediment supply to the North Sea and the English Channel