K-Ar age dates of authigenic illite from sandstones in the UK south Central Graben have a bimodal distribution. In contrast to established hypotheses of thermal triggers, this illite growth is explained by changes in hydrogeological history. Fluid motion during burial can hence be dated. Illite growth at 84-58 Ma on the graben margins was coincident with rapid subsidence of the Graben axis and consequent expulsion of pore fluids onto the margins. This event pre-dated both overpressure, and the secondary migration and accumulation of hydrocarbon in the region. Illite growth was probably caused by increased solute transport rates during pore fluid motion. Illite growth on an axial high (33-30 Ma) occurred during overpressured conditions. This was triggered either by: (1) downward fluid migration as a consequence of overpressure release. These fluids carried carboxylic acids or hydrocarbons from the highly overpressured Kimmeridge Clay Formation into the less pressured Fulmar Sandstone Formation; or (2) a decrease in water-rock ratios as fluid flow declined within the sandstones, as a consequence of overpressure build-up. Published experiments show that illite growth is favoured by low water-rock ratios.