Late Jurassic Sedimentation in the Boreal-Tethyan Seaway: climate modelling, geochemistry, and petrography of the Kimmeridge Clay Formation

Abstract

Climate exerts a strong influence on sedimentation. Understanding the processes behind the spatial and temporal heterogeneities in sedimentary successions can, therefore, be used to reconstruct climate processes in the geological past. Deposited across >1000 km in northwest Europe in the shallow (< 200 m), epicontinental Laurasian Seaway, the Kimmeridge Clay Formation provides an exceptional opportunity to study climate processes and their effect on sedimentation at different latitudes through the Late Jurassic (Kimmeridgian–Tithonian). This thesis presents independent climate modelling, sedimentological, and geochemical datasets from three time-equivalent sections, spanning 1 million years, in the northern and southern extents of the Laurasian Seaway (30–60°N palaeo-latitude) in order to investigate climate dynamics and controls on sedimentation at different northern hemisphere latitudes in the Late Jurassic.
The climate modelling results yield two different hypotheses: 1) HadCM3L indicates that an expanded Hadley Cell and migrated intertropical convergence zone resulted in tropical conditions over the Laurasian Seaway, whereby organic carbon-enrichment in sediments was promoted through enhanced nutrient supply resulting from continental weathering and erosion, and 2) FOAM suggests subtropical-temperate conditions prevailed and that organic carbon enrichment was driven by wind-driven upwelling of nutrient-rich water.
Sedimentological and geochemical analyses for the Ebberston 87 Core, drilled in the Cleveland Basin (Yorkshire, UK), indicates depositional conditions fluctuated between three distinct states and that alternations of organic carbon-rich, carbonate-rich, and clay-rich mudstone and redox conditions were driven by the expansion/migration of the intertropical convergence zone. Analysis of the Swanworth Quarry 1 Core, drilled in the Wessex Basin (Dorset, UK), reveals that although depositional energy differed between the Cleveland and Wessex Basins, sedimentation in both basins was driven by the same, over-arching tropical climate control. Analysis of a third core, drilled in Adventdalen (Svalbard), demonstrates that organic carbon-rich sedimentation occurred in a deltaic setting, which had a markedly higher depositional energy. While the depositional environments in the northern and southern sectors of the Laurasian Seaway differed substantially, similarities between the three studied sections, namely cyclical deposition of terrestrial organic carbon-rich and detrital-rich sedimentation, integrated with published data from throughout the seaway, suggest a low-latitude, tropical influence on sedimentation and organic carbon enrichment across the entire Laurasian Seaway. Furthermore, the palaeogeographic setting of the Laurasian Seaway made the sedimentary system sensitive to subtle changes in weathering and water depths, resulting in distinct modes of sedimentation and biogeochemical cycling.