ASTROCHRONOLOGY FOR THE SINEMURIAN STAGE AND THE LENGTH OF THE EARLY JURASSIC

Studies carried out by multiple team members as part of the International Continental Scientific Drilling Program (ICDP) Early Jurassic Earth System and Timescale project, which aims to produce a stable astronomical timescale for these ~27 million years of geological time, have focussed on the UK Mochras and Prees 2 cores as well as the Global Stratotype Sections and Points (GSSP’s) and adjacent strata in Somerset and Yorkshire coastal exposures (see https://www.icdp-online.org/projects/by-continent/europe/jet-uk/ for project details). These studies have taken a number of approaches, ranging from spectral analysis of a single time series dataset, through to presence/absence of trace fossils and sedimentary structures. Although these methods result in broadly similar outcomes, there are significant differences in detail, highlighting that we have not (quite yet) arrived at a stable composite reference. In this presentation we summarise the progress made particularly with respect to the Sinemurian Stage, and illustrate some areas where agreement has not yet been reached, and outline the steps needed to complete the task. Key findings that do appear robust are i) a short Hettangian stage with a minimum length of 2.25 Myr, ii) a Sinemurian stage with a minimum duration of 6.8 Myr, iii) a Pliensbachian stage with a minimum duration of 8.4 Myr, iv) a Toarcian stage with a minimum duration of 9.4 Myr, and v) prominent long-term cycles with a period of 2.4 Myr. Radioisotopic constraints mean that total Hettangian–Pliensbachian time can be no longer than 17.63 ± 0.67 Myr, whilst from astrochonology the minimum duration of the three stages combined is 17.45 Myr. This arithmetic leaves little-to-no room for missing strata in the JET core and outcrop composites.