Article 68

Knowledge Of Chalk Seas And The End Of Dinosaurs – The Cretaceous Period.

Peter Toghill’s Chapter 11 reports that increased plate tectonic activity in the Cretaceous led to large amounts of new ocean crust being formed at the World’s ocean ridges. This in turn, led to continuous but erratic increases and decreases in global sea levels none of which, according to Hallam (1997), have been below current levels while those above current levels are measurable in hundred of metres, while according to Vail (1977) some of these have been below current levels for relatively short times in the Cambrian, the Carboniferous, the Triassic, the Jurassic and the Cainozoic. However, relative increases in sea level affected the Yorkshire Basin from the beginning of the Cretaceous while the Wessex Basin, including the Weald, experienced deltaic and fluvial conditions for 30 million years of the period. The first marine transgression produced the Lower Greensand over southern England and at the same time distant volcanoes supplied ash for Fuller’s Earth deposits. The second transgression, following mid-Cretaceous Earth movements, produced the Gault Clay and Upper Greensand which spread as far west as Devon as well as Yorkshire, where the Red Chalk was deposited near to Market Weighton. A major transgression at the base of the Upper Cretaceous produced up to 500 m of chalk over most of Britain and north-west Europe, with sea levels rising to 300m more than present levels. Sea levels fell at the end of the Cretaceous and uplift and doming affected the area.

Chalk is a unique type of limestone that is not found anywhere else in the geological column and cannot be seen forming anywhere in the world today. It was formed as a calcareous ooze of micron-sized plates of dead plankton and marine algae called coccoliths. Sponges grew on the sea bed and provided silica after death from which the well-known flint nodules of the chalk were later formed. The southern part of the North Atlantic started to widen during the Cretaceous and a new sea formed as far north as between Labrador and the Bay of Biscay. Rifting in the Rockall area did not lead to sea floor spreading in the Cretaceous, and the formation of the northern areas of the North Atlantic had to wait for the Tertiary period. Southern England lay around 40 degrees North by the end of the Tertiary. The end of the Cretaceous saw one of the greatest mass extinctions of all time with 75% of all species becoming extinct, most notably the dinosaurs and the ammonites. While it is popularly believed that collision with an asteroid or comet caused this mass extinction, it is much more likely to have been caused by a combination an asteroid collision, plate tectonic/volcanic events, and the long term decline of many animal groups and species.

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