Thursday 26 January 2012

A modern turbidite deposit from the Mediterranean Coast of Spain.

Turbidites are sedimentary rock formations formed by submarine landslides. They are very distinctive, as water is very efficient at separating sediments by particle size, since larger, heavier particles will sink rapidly but smaller, lighter particles will remain in suspension for longer, taking more time to settle out. The upshot of this is that turbidite deposits show a clear gradation of particle sizes, with large particles at the base and a steady decrease in particle size moving up the stratigraphic column; smaller, finer debris will also spread further out, giving a horizontal grain size distribution as well as a vertical one. Turbidites are valued by sedimentary geologists, as they are distinctive and tend to cover wide areas, helping to build a clear picture of the geology of a region. They are prized even more highly by palaeontologists, as they often contain excellent fossils, since organisms can be buried rapidly, helping them to enter the fossil record. For example the famous soft bodied Cambrian fossils of the Burgess Shale come from turbidite deposits.

In a paper published in the journal PLoS One on 25 January 2012, a team lead by Anna Sanchez-Vidal of the Grup de Recerca Consilidat (GRC)-Geociències Marines, Departament d'Estratigrafia, Paleontologia i Geociències Marines at the Universitat de Barcelona, describe the results of a storm on the Mediterranean Coast of Spain on 26 December 2008.

The storm was the largest recorded in 25 years of monitoring on the section of coast, though anacdotal evidence suggests that storms on a similar scale occurred on the same coast in the nineteenth and early twentieth centuries, when formal weather records were not being kept. The storm progressed for about 1000 km in a northwest-to-southeast direction along the Catalan Coastal Shelf, causing widespread shore erosion and coastal flooding. On the continental shelf it caused scouring of the seabed, and severe disruption to seagrass, brown algae (seaweed) and gorgonian (soft coral) communities.

Map of the effected area, from Sanchez-Vidal et al. (2012).

In the Blanes Canyon, at the western end of the effected area, where sedimentation was being monitored, there was strong increase in the current speed through the canyon, followed by a marked increase in the grain size of particles being deposited at the shallower end of the canyon. This was accompanied by a marked increase in turbidity in the water throughout the canyon. Finally there was an increase in the deposition rate for fine particles throughout the canyon.