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

Abstract

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