Astrochronology of the Sinemurian Stage from the Llanbedr (Mochras Farm) core, NW Wales: Implications for the Early Jurassic timescale
Keywords:
Jurassic, Sinemurian, astrochronology, calcium contentAbstract
Early Jurassic palaeoenvironments were perturbed during episodes of major global climate and biogeochemical change at the Triassic–Jurassic boundary and Early Toarcian Oceanic Anoxic Event (T-OAE). Other poorly understood palaeoclimate fluctuations and carbon-cycle perturbations occurred between these episodes, including the mid-Sinemurian Liasidium Event and the Sinemurian–Pliensbachian Boundary Event. Understanding of the causes and significance of these phenomena is limited by uncertainties in the geological timescale, particularly for the Sinemurian Stage. In this paper we present new multiproxy geochemical data through the Sinemurian and earliest Pliensbachian stages from the Llanbedr (Mochras Farm) borehole (NW Wales). The Mochras record is apparently stratigraphically complete except in the Upper Sinemurian which is affected by a normal fault that removes the lower part of the Oxynotum Zone. A floating astrochronology is developed for the Sinemurian Stage based on identification of the 405-kyr orbital eccentricity “metronome” in weight percent calcium and stable organic carbon isotopes (δ13Corg) from Mochras, together with data from the Robin Hood’s Bay base-Pliensbachian GSSP section in Yorkshire, UK. The results indicate a minimum duration for the Sinemurian Stage of 7.3 Myr (7.1 Myr from Mochras and an additional 0.2 Myr from Robin Hood’s Bay for the lower Oxynotum Zone). This interpretation of a long Sinemurian requires a short duration (~2 Myr) for the Hettangian stage.Downloads
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