Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence

Abstract

Although the ‘Mid-Cimmerian event’ or unconformity has been recognized over much of Europe, its exact stratigraphic relations and causal mechanism have remained unclear. Application of a genetic sequence stratigraphic approach (using 17 marine condensed sections and maximum flooding surfaces) to Jurassic sequences across NW Europe allows the stratigraphic succession to be subdivided into a series of time-slices (genetic stratigraphic sequences) and allows the true nature of the unconformity to be determined. They indicate that the main event’s correlative conformity falls in the Aalenian near the break between the opalinum and murchisonae ammonite biochronozones. Further study of the associated spatial and temporal variation indicates that systematic truncation of stratigraphy occurred throughout the North Sea domain (the oldest stratigraphies subcrop in areas adjacent to the triple junction) with subsequent progressive onlap towards the same area. When integrated with igneous evidence, these observations are interpreted to confirm regional (Toarcian–Aalenian) domal uplift, resulting from the impingement of a broad-based (> 1250 km diameter), transient plume head or ‘blob’ at the base of the lithosphere. Progressive pre-rift, Aalenian–early Bathonian marine onlap records differential subsidence in response to the initial deflation of the dome while central regions may have continued to rise. Subsequent subsidence post-dated Bathonian–Callovian volcanism but still pre-dated the timing of most significant (Kimmeridgian–Volgian) rifting. Such temporal relations demonstrate that North Sea volcanism is inconsistent with a classic ‘passive’ rift model. Instead, it seems more appropriate to equate Mid–Late Jurassic North Sea development with an ‘active’ rift model following mantle-driven thermal doming. Integration of sedimentation patterns with basin development suggests that the early Toarcian–early Kimmeridgian succession records a long-term, second-order regressive–transgressive episode related to regional tectonism. Comparison with the current chart of coastal onlap and global sea-level change highlights the correlation of the Intra-Aalenian event with one of the most significant regressions (the 177 Ma event separating the Absaroka and Zuni first-order megacycles). The knowledge that this part of the curve appears to be based exclusively on sections from Dorset and Yorkshire, within and adjacent to the region affected by regional doming, suggests that there remains a need to test this part of the chart using sections from outside the uplifted area and emphasizes the impracticality of using just two relatively closely spaced sections in trying to define a truly global signal. Clearly, sections should be taken from several areas and preferably from different plates as tectonically uncoupled as possible. Until then, the worry will remain that regional tectonic events could overprint any global signal and be erroneously interpreted as abrupt changes in global eustasy. The fact that doubts such as these can be cast upon parts of the eustatic sea-level chart suggests that it is still someway off being a valid global standard with true predictive capabilities.