POSTDEPOSITIONAL EVOLUTION OF MUDSTONES .1. EARLY DAYS AND PARENTAL INFLUENCES

Ancient mudstones almost invariably record significant modification by very early diagenetic processes in the form of distinctive authigenic mineral assemblages. In the past, these have mostly been described fr om concretions-striking segregations of sulphides and carbonates self- organized into sub-spherical bodies. Thanks to relatively recent advan ces in electron microscopy (particularly back-scattered electron image ry) it is now a fairly simple matter to identify and characterize thes e same assemblages where they more commonly occur-dispersed through th e fine-grained matrix of mudstones. Earliest diagenesis is almost excl usively restricted to microbially-mediated redox reactions. Within rec ent unconsolidated sediments, these reactions take place in vertical s equences-the 'biogeochemical zones' of Claypool and Kaplan. Causal lin ks can now be identified between early diagenetic mineral assemblages and microbial process zonation patterns. The patterns themselves vary systematically with depositional setting. One very important aspect of depositional setting is rate of sediment accumulation. This rate cont rol is more influential than previously suggested. It determines which diagenetic minerals will form and their amount as a proportion of the final sediment. This dependency permits early diagenesis and sea-leve l change to be linked through predictable lateral variation of biogeoc hemical zonation with sediment transport path (or bypass). It also pro vides clues as to the development of cementstones and horizons with di screte concretions-both of which reflect fluctuations in sediment accu mulation rate. Finally, these relationships suggest why some mudstones are more likely than others to cause cementation in sandstones as a c onsequence of compactional de-watering and solute export.