Structural and lithological controls upon fluid migration within the Chalk and Upper Greensand aquifers in the Chilterns and Lambourn Downs
The influence of lithological heterogeneities, structural discontinuities and discontinuity surface mineralisation upon groundwater migration within the Chalk and the Upper Greensand of southern England has been investigated. Lithological heterogeneities in the Chalk succession include marl seams, hardgrounds, tabular flints and nodular flints. Each of these heterogeneities has a lower intrinsic porosity and permeability than in the calcite rich chalk. The influence that each of these heterogeneities exerts upon groundwater flow is assessed in relation to the porosity and permeability of the Chalk. Clay minerals, iron oxide and dendrites of manganese hydroxide are forms of discontinuity surface mineralisation. The paragenesis of these deposits is predominately from superficial deposits resting on the Chalk. The presence of mineralisation reduces the porosity and permeability of discontinuity surfaces. Mineralisation reduces the exchange between water in discontinuities with that in the matrix. In the matrix, parallel to the discontinuity surface, a zone of enhanced porosity occurs, formed by the dissolution of the Chalk matrix. Parallel to this zone is a zone of reduced porosity which has been formed by the precipitation of calcite as overgrowths upon the matrix. Mineralised discontinuity surfaces provide routes for rapid migration of groundwater in both the saturated and unsaturated zones. There is no discernible relationship between the physical properties of discontinuities (i.e. orientation, dip, dip direction) and the occurrence of mineralisation. The distribution of discontinuities within the Chalk is not uniform. Clusters of similarly orientated discontinuities occur separated by less disturbed material, which causes the Chalk act as an anisotropic aquifer. Solution formed cavities have been observed at the point of intersection between discontinuities, or at the intersection of discontinuities with lithological heterogeneities. The cavities extend laterally along the line of intersection. These cavities allow rapid migration of groundwater within the saturated zone. Inclined solution cavities are formed where the discontinuities have differing orientations or where the lithological heterogeneities are dipping. Such cavities provide preferential routes of groundwater recharge through the unsaturated zone. The Chalk is a less effective aquifer at depth due to the decreased occurrence of solution enhanced discontinuities. The Chalk and Upper Greensand aquifers are not in hydraulic continuity in locations where the basal unit of the Lower Chalk, the Glauconitic Marl is present.