Jurassic Oceanic gateways of the North Atlantic

Walker, Olivia 2023. Jurassic Oceanic gateways of the North Atlantic. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

This thesis analyses two distinct data sets from West Iberia and the Norwegian North Sea, and aims to improve the understanding of ocean seaways, and associated strata, in the context of Mesozoic continental rifting of the North Atlantic Ocean. For the West Iberian margin, reprocessed 2D seismic profiles, integrated with borehole data, unveil the emergence of a Jurassic seaway approximately 200 km wide. This period coincides with Triassic–Early Jurassic continental rifting, marked by substantial tectonic subsidence. For the first time, the research quantifies the thickness of Mesozoic syn-rift strata and West Iberian salt layers within deep-offshore basins, revealing a thickness variation from 1.7 km to 2.5 km. These findings, coupled with a substantial succession of Lower-Middle Jurassic strata, confirm the extension of the seaway from the Lusitanian Basin into present-day continental-slope basins. Backstripping analysis of Early Jurassic depocentres in West Iberia documents critical tectonic subsidence, leading to the segregation of the seaway into distinct sectors. The study estimates a total subsidence of approximately 5 km in the deep-offshore Peniche Basin during the Late Triassic-Early Jurassic, a notable contrast to the approximately 1.1 km recorded in the proximal Lusitanian Basin. The seismic-stratigraphic record from this time suggests a unified seaway between the Lusitanian and Peniche basins, with significant rift-shoulder exhumation occurring from the Late Jurassic onwards. Moreover, borehole stratigraphy across early Mesozoic basins in West Iberia, Newfoundland, and the North Sea reveals a tripartite depositional sequence of continental, evaporitic, and marine layers, indicative of a co-genetic evolution across the North Atlantic margin. The analysis of the Norwegian Central Graben, conducted with data provided by IGI Ltd., evaluates some geochemical characteristics of source rocks during the Late Jurassic. This dataset, including kerogen types I, II, and III, along with supplemental biomarker analyses, serves as proxies for discerning terrestrial and marine organic matter during deposition phases. The findings indicate that the central North Sea's middle-upper Jurassic units possessed a heterogeneous geochemical signature, shaped by the fluctuating depositional environments typical of rifting. Notably, terrestrial organic matter is identified in various proportions within syn-rift strata, not wholly succeeded by marine organic matter as rifting advanced. Biomarker evidence corroborates the presence of mixed organic matter sources throughout the rifting. In summary, comparisons of the seaways studied in this thesis make it clear that they both witnessed, during some part of their rifting history, salt deposition, with marine incursions present during and after the deposition of the latter. Basin isolation and limited connectivity between oceans gradually evolves into wider, and more connected ocean seaways as the early stages of continental rifting develop. Yet, biomarkers for the upper Jurassic of the Northern North Sea still reveal an important mixing of terrestrial and marine sources of kerogen, some 100 Ma after continental rifting was initiated in both West Iberia, the North Sea, and over the North Atlantic as a whole. As a corollary, this work shows that the presence of these mixed kerogen sources has constructive implications for resource exploration, particularly if the geochemical characteristics of the Northern North Sea are representative of the deep-offshore frontiers of the North Atlantic domain.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Date of First Compliant Deposit:	16 October 2024
Last Modified:	17 Oct 2024 13:00
URI:	https://orca.cardiff.ac.uk/id/eprint/172967

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