Integrated stratigraphy and mineralogy of Lower Jurassic mudstone successions, Cardigan Bay and Cheshire basins, UK.

Abstract

The deposits of shallow epicontinental seaways dominate much of the preserved Early Jurassic (~ 201.4-174.1 Myr) sedimentary record. Extensive research has been conducted on these sedimentary successions to understand paleoclimate changes using various methods, but key petrographic parameters linking to provenance and transport mechanisms within these rocks is still incomplete, and distinct gaps for an integrated stratigraphy and timescale for this interval persist.

Quantitative techniques based on mineralogical mapping are now available to fully characterize and interpret sedimentary textures and composition across all size categories and SEM-based quantitative mineral mapping is used here as the main approach to fill knowledge gaps about the nature of fine-grained Early Jurassic sedimentary sequences in the UK. Extended stratigraphic textural and compositional datasets were obtained for two key locations with robust bio- and chemostratigraphic controls, the Mochras and Prees 2C drill cores. This study focuses on clay-sized and silt-sized sedimentary particles, providing further insights into transport mechanisms, sedimentary source change, depositional environment, sea-level change, and major changes occurring in the Earth system at the time. Additionally, these data were complemented by high-resolution core scanning XRF data and determination of natural remnant magnetization to feed into an improved Earth system understanding and integrated stratigraphy for the Early Jurassic.

Principal findings of this thesis are that grain size in the two localities can be used to assess sequence stratigraphy, seawater oxygenation and, partially, diagenetic history of the strata. Mineralogical analysis highlights distinct responses to palaeoenvironmental change and phases of intense climatic perturbations, but also provides evidence for the likely overall influence of monsoonal climates on sedimentation in the UK basins at the time. Primary palaeomagnetic signatures are found to be sufficiently well preserved in the studied rocks to warrant further work towards a step change for the quality of the Early Jurassic geomagnetic polarity time scale.