Astronomically-calibrated magnetostratigraphy of the Lower Jurassic marine successions at St. Audrie's Bay and East Quantoxhead (Hettangian-Sinemurian; Somerset, UK)

Abstract

Astronomically-calibrated magnetostratigraphies are becoming the standard in the Geomagnetic Polarity Time Scale (GPTS) because in time these records can be accurately compared for consistency. The Lower Jurassic GPTS is however still poorly resolved with major inconsistencies between different records from the Paris Basin and the continental Newark and Hartford basins, while only the latter have been astronomically-calibrated. Here, an independent cyclostratigraphy, astronomical calibration to the 405-kyr eccentricity cycle and magnetostratigraphy for the lower Jurassic from coastal outcrops in Somerset, United Kingdom, covering the Hettangian and lower Sinemurian Stages is presented. These results fully confirm the magnetostratigraphic patterns derived in the terrestrial Hartford Basin, eastern USA, with a dominant normal polarity interrupted by three short reverse chrons: H24r, H25r, and H26r. The cyclostratigraphic analysis of the marine strata from the St. Audrie's Bay/East Quantoxhead section is further extended and improved yielding an independent astronomical calibration to the 405-kyr, long eccentricity cycle and a floating astrochronology for the new magnetostratigraphy. The astronomical tuning shows consistency with the Hartford Basin at the 405-kyr, long eccentricity time scale and minor differences at the 100-kyr, short eccentricity time scale. These inconsistencies will have to be resolved to further improve the Geologic Time Scale 2012, in which the Hartford Basin magnetostratigraphy forms the basis for the Lower Jurassic. The St. Audrie's Bay/East Quantoxhead composite section could serve as a unit stratotype section for the Hettangian Stage.