The Weald Basin was a major topographic feature of 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 basins formation commenced during 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.[1] 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 [2], the remnants of the mountain belt can be seen today in Devon and Cornwall in what is known as the Cornubian Massif. The deformation caused, unlike in Devon and Cornwall, little or no metamorphism.[3]
The mountain belt collapsed soon after the orogeny leading to the former northward thrusts to be reactivated as normal faults and lead to the formation of the Weald basin which developed as an extension of the considerably larger Wessex Basin [4]. 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.[5] 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 Jurrasic a north and east shallowing mud-dominated shelf developed [6].
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.[7] 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.[8] 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.[9] 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.[10]. 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
The inversion of the Weald Basin throughout the late Cretaceous and early Tertiary resulted in the formation of the Wealden Anticline and a number of smaller anticlines within the larger structure, The discovery of natural gas while drilling for water at Heathfield railway station, which was utilised to provide the first gas lighting in the UK.[11] and the existence of the same rocks within the Weald basin which are the source rocks for the Wytch Farm oilfield lead to an interest in the petroleum potential of the Wealden anticline, with exploration taking place on Ashdown forest examining the Ashdown Anticline, a large structure over 30 km long x 7 km wide, located in the centre of the Weald Basin in north Sussex; significant quantities of natural gas where found but oil was absent.[12] The Weald Basin, however has yielded significant quantities of gypsum from Jurassic Purbeck beds and a number of brickworks exploit the lower Cretaceous clays.