Episodes of phosphogenesis and phosphorite concretion formation in the North Sea Tertiary

Abstract: It is well established that certain periods through geological time have been more favourable to the formation of phosphorite deposits than others. These 'phosphogcnic episodes ' have been correlated with periods of eustatic high sea-levels, global warm temperatures and increased marine productivity, amongst other factors. The development of phosphorite concretions in marine sediments of the southern part of the North Sea Tertiary Basin reflects episodicity corresponding to times of sea-level high-stands. In particular, concretions in Early Eocene marine mudstones make their appearance just above levels which indicate initiation of a more open connection between the Atlantic Ocean and the relatively nclosed North Sea Basin. Similarly, during the Middle Miocene, phosphoritc oncretions occur associated with indicators of oceanic influence. Elsewhere in the world, remote from the North Sea, phosphate-rich sediments also formed during these stratigraphically brief episodes. Occurrences of relatively sparse diagenetic phosphorite concretions inthe North Sea Basin therefore appear to be linked to large scale, possibly global, cycles and not solely to the local diagenctic environment. Most sedimentary rocks contain some phos-phate but some rocks contain anomalously high concentrations. Phosphate-rich sediments have probably been deposited uring all major sea-level transgressions during the Tertiary. How-ever, some periods have been more important than others and resulted in the formation of large phosphorite deposits (Riggs 1987). Many authors have discussed the occurrence and causes of episodicity in the formation of phos-phorites (e.g. Cook & McElhinny 1979; Bento