Bryozoans in the English Chalk

Dr Paul D Taylor (UK) We are very fortunate in Britain to host one of the most remarkable deposits in the entire geological record, the Chalk. The Late Cretaceous Chalk (with a capital ‘C’) is an extremely pure limestone, famous for the White Cliffs of Dover and responsible for the landscape of rolling hills and dry valleys, forming the ‘downs’ and ‘wolds’ that stretch through England from Devon in the southwest, to Yorkshire in the northeast. The economic importance of the Chalk to the early human inhabitants of Britain was enormous because the flints contained within it could be fashioned into axe heads and hard cutting tools. Why is the Chalk so special geologically? It is a rare example of a pelagic sediment – an open ocean sediment – that was deposited over the continental shelf. This occurred at a time when global sea-level was high and the supply of terrigenous clastic sediment into the sea was minimal. The Chalk is an oceanic ooze composed mainly of the disaggregated plates – coccoliths – of coccolithophores, planktonic microalgae with exquisitely engineered skeletons of calcite. Unfathomable numbers of coccolithophores sank to the seabed over a period of some 35 million years to produce the thick accumulation of Chalk that today extends over northern Europe and into western Asia. The Chalk is a favourite hunting ground for fossil collectors, yielding beautifully preserved specimens, especially of echinoids. But closer inspection of the Chalk shows that the dominant macrofossils are often bryozoans. These colony-forming invertebrates … Read More

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