KARST MAPPING DENSITY IN THE PORTSDOWN CHALK FORMATION

Cranborne Chase in south central England contains extensive archaeological evidence supporting a large Neolithic population from approximately 3600-3440 BC. Little to no data exists recording the environment that the Neolithic people were living in and how they influenced the landscape through cultivation and related impacts. Typical data archives that would be used for paleoenvironmental reconstruction, such as lakes or peat fens, do not exist in Cranborne Chase because of the well-drained karst landscape. However, during the summer of 2018 a significant drought enhanced the identification of karst features. These features were mapped with Geographic Information Systems (GIS) and aerial photography to assist in the identification of potential paleoenvironmental and archeological archives.

A total of 1,700+ karst features were mapped in the Cranborne Chase. There is significant correlation between karst feature density, or occurrence, and a defined chalk formation: 1,311 located in the Portsdown Chalk Formation; Culver Chalk Formation, 234 features; Reading Formation, 146 features; and other units with minor assemblages included the Poole, London Clay, and Newhaven Formations. It appears the primary factor effecting karst formation is pyrite weathering, providing a major influence on the acidity of the groundwater and fracture dissolution as it produces sulfuric acid. When combined with calcium carbonate it produces gypsum, which has been observed in the Paleogene beds. Solution pipes in the beds are allowing the sulfuric acid to reach the unsaturated zone above and below the water table. The acidic recharge in the Portsdown induces dissolution across the fracture systems connected to the aquifer, producing the potential dolines and preferred flow pathways that are being observed in the study area. A database is now available holding the mapped locations of karst features that could be further assessed as potential paleoenvironmental recorders. This database was created with the intent of providing geographic assistance to further archaeological and paleoenvironmental reconstruction of south central England.