S IL IC IF ICAT IONS AND ASSOCIATED CLAY ASSEMBLAGES IN THE CRETACEOUS MARINE SEDIMENTS OF SOUTHERN ENGLAND

ABSTRACT: Silicifications and smectite-dominated clay assemblages are associated in the marine Cretaceous sediments ofEngland. Both are neoformed, and have developed at approximately the same time in the host sediment. Two silicification types, opal-CT and or-quartz, have a distribution pattern unrelated to the stratigraphy orlithology of the Lower Greensand, Upper Greensand and Lower Chalk. The petrography offive examples suggest that the two silicification types were both precipitated from the porewaters of their host sediment, and their silica mineralogy and their associated silicate minerals were controlled generally by the silica concentration of the porewaters and more locally by the pH gradients. Millot (1964) was first to note the association of silicifications developed uring the intrinsic diagenesis of marine sediments and neoformed clay assemblages dominated by smectite, palygorskite and sepiolite. He suggested that the association developed because the parent material for both the silicifications and clay assemblages had a chemical origin in the ions leached by the weathering of the land. In the Cretaceous marine sediments of England silicifications are abundant, rangin