Limestone depositional cycles, sea level and the sun

Author: Paul Wood.

3 October 2013 - Professor Maurice Tucker

Professor Tucker was introduced by Steve Livera who observed that he originally invited him when he was still at Durham. However, he is now based at Bristol University and still agreed to come to Scarborough for the lecture, for which Steve thanked him.

He started by explaining that he became interested in fossils as a child because he found some in the garden of his home in London (even though they shouldn't have been there!). This led him to a career as carbonate sedimentologist.

With many excellent photographs, he discussed areas where limestones form today such as reefs and barrier bars. Studying details of lagoons shows us the origins of carbonate sand and also microbial mats such as stromatolites that become specific types of carbonate deposits. There are also cool water carbonates in higher latitudes and deep sea floor oozes that form carbonate muds.

There is much concern today about climate change, rising sea levels and increasing sea water acidity damaging corals. Also a recent study has shown that Saharan dust may be poisoning some corals. Controls on carbonate deposition were temperature, salinity and the different limestone forming organisms that change over geological time. For most carbonate deposition, the water depth should be less than 10m. Accommodation space allowing carbonate deposition is controlled by subsidence rates, and both local and eustatic (global) sea level changes.

Limestones are particularly important as oil reservoirs because they can have high porosity. They are also widely used as building materials. The deposition of carbonates is generally well organised into depositional cycles composed of sequences divided into parasequences. An example in Yemen has more than 100 parasequences - metre scale cycles. There are three possible causes for this cyclic behaviour, tectonic, sedimentary and climatic change resulting from Milankovitch rhythms. Good examples of cycles can be seen in the Triassic of the Dolomites and the Chalk. In the Chalk individual beds can be traced from Flamborough Head across the sedimentary basin to the south of France. Individual beds of Chalk may be correlated with changes in the orientation of the Earth's axis (precession).

LIDAR scanning at Wath quarry near Hovingham shows packages of thickening upward beds which are laterally continuous over a large area. The clay partings defining the beds are most likely caused by climate variations.

Prof Tucker ended by telling us that 'Beds have a message. We may have missed it by not studying the beds in detail'.