The geology of the Portsmouth region : a perspective of the Wessex and Hampshire Basins

This paper results from the scene-setting presentation given at the opening of the 16th Extractive Industry Geology Conference at Portsmouth University in September 2010. The geology and structure of the Hampshire region is, at first glance, simple and laid open for inspection in rolling countryside with a subdued topography of scarps and long shallow dip-slopes of the Chalk downlands and broad synclines preserving Palaeogene strata, all of which are cross-cut with languid streams in wide gravelfilled valleys. However, the structure of the underlying and co-extensive Wessex Basin, with its Permian to Cretaceous infill, up to 3.5 km thick in places, provides the grain of the country we see today. This Wessex Basin-infilling tells a story of massive tectonic extension and normal faulting, related to the opening of the proto-Atlantic, and an equally massive phase of tectonic compression and reactivation of some of the faults in a reverse sense, as the result of the later collision (principally of Miocene age) of the African and European tectonic plates (the Alpine Orogeny). This extensive geological history relates to an imprint, preserved in a Palaeozoic basement, of deep-seated thrusting created at the initiation of the supercontinent Pangea (when the continental masses of Gondwana and Laurentia collided during the Variscan Orogeny). It is the break-up of Pangea that provides the depositional accommodation space for the Mesozoic sediments and through time has given the continental plates we are familiar with today.