|
British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
It is well established that certain periods through geological time have been more favourable to the formation of phosphorite deposits than others. These ‘phosphogenic 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 enclosed North Sea Basin. Similarly, during the Middle Miocene, phosphorite concretions 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 in the North Sea Basin therefore appear to be linked to large scale, possibly global, cycles and not solely to the local diagenetic environment.
