Monday, 4 April 2016

Chicxulub’s peak ring: exceptional geologist bling

Sometimes there are not many places to look for a specific geological feature. The peak ring of the Chicxulub crater is the perfect example: it is the only unequivocal instance of a peak ring known on Earth. Some others may have existed in the past but have since been eroded through geological time. When geoscientists come to study peak rings, they can look through a telescope, at Venus or The Moon. But when testing hypotheses on Earth, the only known analogue is located beneath the Yucatán Peninsula, Mexico.

Source: ECORD
A peak ring results from rock displacement when a meteorite collides with a rocky body. At Chicxulub, current hypotheses suggest that the impact was so powerful the rocks at the site behaved like a fluid. Like a droplet into water, the collision generates an uplift at the centre of the crater, this uplift then collapses outwardly to form a peak ring. International Ocean Discovery Program (IODP) Expedition 364 will investigate specifically, and for the first time, the peak ring of Chicxulub Impact Crater: here the structure forms a circle 80 km in diameter. In April and May 2016, 30 km north from the coast of Yucatán, an international team on board Liftboat (L/B) Myrtle will attempt to drill and sample a 1.5 km deep borehole to reach the peak ring. The feature is now preserved beneath 17 m of water and more than 650 m of Cenozoic carbonates at the drilling site.

The logo for Expedition 364, ©ECORD
The main expedition goal is to shed light on the physical processes involved in the formation of peak rings. The collision also likely resulted in the sterilisation of the impact site; and so another key objective aims to evaluate if and how microorganisms re-established in the peak ring; and the role of hydrothermal systems at the post-collision event site. Geophysicists, sedimentologists, geochemists, microbiologists, micropalaeontologists and petrologists from the UK, Germany, Belgium, Austria, Netherlands, France, China, Japan, Australia, USA, Canada and Mexico will work together during the project to address these questions.

The European Petrophysics Consortium (EPC) is heavily involved in this expedition, as part of the ECORD (European Consortium for Ocean Research Drilling) Science Operator (ESO). Johanna Lofi (University of Montpellier), Erwan Le Ber and Laurence Phillpot (University of Leicester) will coordinate all of the downhole and core petrophysical measurements. Data collected downhole provide a continuous record of rock properties along the borehole. A multi-sensor core logger (MSCL) is used to measure physical properties of recovered cores, at a resolution of 2 cm. By comparing downhole and core logging the exact depth of the cores in the borehole can be calculated; a very important aspect of this expedition, as key horizons such as the PETM and the K-Pg boundary should be sampled. Finally, the combination of downhole and core petrophysical measurements will be crucial to test current hypotheses of peak ring formation. This expedition represents an exceptional opportunity to investigate a truly exciting geological feature: this unique peak ring, testimony of one of Earth’s most dramatic events.

Equipment being loaded onto the L/B Myrtle, ©Sally Morgan, Leicester

After the offshore phase of the expedition, core samples will be sent to the IODP Bremen Core Repository in Germany, where further analyses will be done this autumn. EPC will continue to coordinate the acquisition of more petrohysical data on the core material recovered.

You can keep up-to-date with project developments through the expedition webpage and by following @EPC_Research.