Depositional controls on diagenetic evolution of the Lower Cretaceous Wealden Sandstones (Wessex Basin, southeast England)

Depositional processes hold crucial controls on the diagenetic processes, events and evolution in sedimentary basins. Using the example of non-marine, Lower Cretaceous Wealden Sandstones from Wessex Basin in southeast England, this study reveals the depositional controls on diagenetic evolution in a sedimentary basin. Seventy-four surface samples, from 6 field locations across the Wessex Basin in Dorset and Isle of Wight were subjected to laboratory analyses such as optical petrography, scanning electron microscopy and Quantitative Evaluation of Minerals by Scanning Electron Microscopy analysis. The degree of compaction in the Wessex Basin sandstones includes moderate packing with most grains in a mix of point and long contact, with rare grains displaying concave-convex or non-grain contact and a loose packing dominated by non-grain contacts, with minor point and long contacts. The authigenic mineral phases identified in the sandstones are quartz, kaolinite, calcite and halite in decreasing abundance. The dissolution of felspars observed in the Wessex Basin sandstones possibly provided the main source of silica for the quartz overgrowths, with silica sources adjacent to the site of cementation. The small amount of quartz overgrowths may have formed during shallow burial. The partial dissolution of quartz observed in the Wessex Basin sandstones is considered likely to be the result of flushing of organic-compound rich waters derived from shale interbeds, under low temperature conditions (c. 25-50 degrees C). The precipitation of authigenic minerals within the Wessex Basin had a limited impact on the porosity and permeability of sandstones. Detrital mineralogy appears to be the main depositional control on diagenesis and may also explain the low abundance of authigenic kaolinite in the Wessex Basin. Similarly, the shallow burial depth of the sandstones is considered to be the main factor inhibiting more pervasive quartz cementation within the basin.