Geochemical Study of the Sinemurian Arietites Bucklandi Zone in the Blue Lias of Lyme Regis (Dorset, UK)

Abstract

Abstract

This is a study of the origin on the cyclicity of the different horizons in the Blue Lias of Dorset UK. The origin of these rhythmic deposits has been attributed to climatic changes promoted by orbital cyclicity. In this study, the aliphatic and aromatic fractions from the organic extracted rock samples of different layers in the Arietites bucklandi zone were analyzed with GC and GC-MS to identify changes in the thermal maturity depositional environment, input of organic matter and redox conditions. There is no significant change in thermal maturity as a function of burial depth as recorded by C31 17α(H), 21β(H) 22S/(22S + 22R) and C27 Ts/(Ts+Tm). Differences in the average chain length (ACL), carbon preference index (CPI), biomarkers ratios (Pr/Ph, DBT/P, and C27, C28, C29 steranes), as well as the presence and abundance of isorenieratane in ten samples reveal significant variations in depositional conditions and type of organic carbon in the sequence. A high TOC content and HI values in the undisturbed laminated shale is related to water stratification and sulphidic conditions in the photic zone with estuarine circulation during humid climatic stages with high runoff of terrestrial organic matter from the continent. Moreover, low TOC and HI in the highly bioturbated light marls correspond to periods of dry climate conditions that promoted anti-estuarine circulation with bottom water oxygenated and preferential deposition of marine organic matter. Climatic conditions seem to influence the preservation and primary productivity of organic matter in the Blue Lias.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016