Construction of a Palaeogene tide-dominated shelf : influence of Top Chalk topography and sediment supply (Wessex Basin, UK)

Palaeogene siliciclastic sediments (Reading and London Clay Formations) in the Wessex Basin record an upward change through four tidally influenced deposition systems. The lowest (Reading Formation) was deposited in estuarine and coastal plain environments. The overlying London Clay records discrete upward changes from deltaic, to mixed tidal flat, to tidal sandsheet (‘Bagshot Sands’). Erosional topography on the Top Chalk surface had an important control on deposition: (1) topographic lows controlled the initial sites of marine flooding and sedimentation; (2) lows would have acted as conduits for sediment delivery; (3) lows created additional accommodation by the compaction of thick mud fills; (4) lows may have formed preferential sites for delta growth. As topography was reduced by sediment infill, depositional units tended toward more sheet-like geometries. Heavy minerals reveal that an upward progradational/aggradational trend and the development of more sand-rich facies are related to a change in provenance toward sand-rich Cornubian sources.