Sediment and groundwater geochemistry of the chalk in southern England

Murphy, Amanda (1998) Sediment and groundwater geochemistry of the chalk in southern England. (PhD thesis), Kingston University, .


Chalk was deposited during a period of high global sea-level some 70 -100 Mya, but subsequent erosion has removed most of the onshore chalk in the UK. The biostratigraphy and lithostratigraphy of the Chalk have been extensively studied, and geochemical studies have concentrated on C, O and Sr stable isotopes. This study involves high-resolution inorganic (Si, Ti, AI, Fe, Mg, Ca, Mn, Sr) geochemical characterisation of a suite of 480 outcrop (uppermost Albian - basal Campanian Chalk from the Culver Cliff /Whitecliff section on the eastern Isle of Wight) and borehole (uppermost Cenomanian - middle Coniacian Chalk from Banterwick Barn, Berkshire) samples of varying lithology and sedimentology. Phosphorus and S were determined in borehole Chalk. Selected outcrop samples were treated with 1M acetic acid in order to obtain trace-element and mineralogical (X-ray diffraction) data from the insoluble (non-carbonate) residues. Porewater compositions were obtained from the borehole, and sediment stable isotope ([delta][sup]13C, [delta][sup]18O) and organic carbon data were determined. Both outcrop and borehole Chalk display consistent lithological and stratigraphic geochemical trends. Manganese and Sr are not lithologically controlled, but reflect varying seawater compositions and diagenetic (hardground formation, mineralisation) processes, and commonly exhibit an inverse correlation. Molar Ti/Al correlates very well with global eustatic sea-level (low Ti/Al occurring during high sea-level) and remanence magnetism, reflecting the drowning of continental areas and reduction of detrital sedimentation. Low Mn and high Si/Al commonly correlate with sequence boundaries. Exceptions are found where hardgrounds coincide with sequence boundaries, resulting in high Mn. Ewellent correlation between the Banterwick Barn, Fair Cross and Winterbourne Chalk boreholes of the western London Basin may be achieved using CaC0[sub]3, Sr and Mg data which together with geophysical data confirm the extent of sedimentary thinning at Banterwick Barn. Excellent correlation between porewater Si and sediment Si/Al is found at Banterwick Barn in the saturated zone of the aquifer. In the unsaturated zone, leaching by meteoric water reduces porewater Si levels.

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