Occurrence of organic-matter-rich beds in Early Cretaceous coastal evaporitic setting (Dorset, UK): a link to long-term palaeoclimate changes?

摘要: In Dorset (southern U.K.), the Durlston Bay and Lulworth Cove sections expose lowermost Cretaceous coastal marine non-marine partly evaporitic sediments (the so-called Purbeckian facies). An interval with organic matter (OM)-rich layers is recognized in both sections. This OM-rich 20 m thick middle of section. Within these beds, a large OM accumulation recorded, total carbon (TOC) up to 8.5 wt%. High hydrogen index (HI) values (up 956 mgHC/gTOC) point Type I OM, generally considered as derived from algal-bacterial biomass. contrasts present underlying overlying intervals, displaying general lower TOC HI values, consisting degraded material higher proportions terrestrial OM. organic-rich can be interpreted period enhanced primary productivity within lagoonal/lacustrine settings at times low sea level. It could also correspond increasingly reducing waters. The distribution found correlates well existing long-term palaeoclimatic patterns, i.e. latest Jurassic (late Tithonian)-Early (early Berriasian) semi-arid climate replaced during middle-late Berriasian by more humid climate. pattern recorded elsewhere on margins Tethys, indicating widespread change. Dorset, deposition beds occurs exactly transition between two regimes, probably first increase humidity, which led freshwater supply into lakes lagoons. suggests direct or indirect control local development and/or anoxia/ dysoxia restricted shallow water masses. proposed environmental model links production preservation changing, climate-driven, hydrological regime.