Faulting and porosity modification in the Sherwood Sandstone at Alderley Edge, northeastern Cheshire: an exhumed example of fault-related diagenesis

Abstract

Abstract The Alderley Horst is an exhumed, titled structure close to the eastern margin of the Cheshire Basin, broadly analogous to the titled fault blocks in the East Irish Sea Basin. Sediments of the Lower and Middle Triassic Sherwood Sandstone Group are exposed at the crest of the horst whilst its flanks are capped by sediments of the Mercia Mudstone Group. Extensive diagenesis and porosity modification have taken place in the Sherwood Sandstone across the Alderley Horst. Diagenetic modification includes reduction of hematite grain coatings and quartz cementation, together with Cu-Pb-Zn-Fe sulphide and barite mineralization. The distribution of this cementation is spatially related to faults that cross the horst and its style and extent varies between individual faults and between fault sets of different orientations. Different spatial distributions are observed for quartz cements, barite, sulphides, ferroan calcite, reduction of hematite grain coatings and secondary ore concentrations. Sandstone units close to the base of the Mercia Mudstone Group are more intensely altered than lower units showing that hematite reduction and mineralization are concentrated at the crest of the horst. Fault-related mineralization and porosity modification post-dates all of the fault movement as brecciation and deformation of barite and sulphide cements are not observed. The intensity of mineralization decreases at structurally lower levels on the WNW-ESE oriented faults, so precluding them as conduits for the vertical migration of mineralizing fluids. However, the horst-bounding Kirkleyditch Fault was probably an important migration pathway that enabled valving of fluids from dowthrown sources in the Mercia Mudstone Group, Manchester Marls and Carboniferous shales, although the possibility of fluid migration up-dip from the south cannot be completely discounted. The mineralizing fluids were ponded at the crest of the horst where they were baffled by pre-existing structural barriers.