Controls on Late Jurassic seismic sequences, Inner Moray Firth, UK North Sea: a critical test of a key segment of Exxon's original global cycle chart

Abstract

ABSTRACTA new interpretation of a comprehensive seismic‐ and well‐database has resulted in the subdivision of the Mesozoic into four, basin‐wide, seismo‐stratigraphic depositional megasequences in the Inner Moray Firth (IMF) basin. Regional mapping of the megasequences has led to the construction of a new model for Mesozoic‐Recent basin development in the IMF. It now appears that extensional tectonics was the main control on the basin's evolution during the Mesozoic. Structural geometries suggest that both the Triassic (Tr) and Rhaetian‐mid Oxfordian (Jl) megasequences were controlled by regional broad‐based subsidence associated with local extensional fault activity prior to the onset of renewed rifting in the IMF. In contrast, the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence developed in response to active extension characterised by half‐graben development. Subsequent Early Cretaceous (Kl) deposition appears to have occurred during a further period of broad regional (thermal) subsidence. It is evident that strike‐slip movement on the Great Glen Fault played a negligible role in Mesozoic basin development and it appears only to have had a local control on structural styles during its reactivation in the Tertiary as it accommodated regional uplift and basin inversion.Further subdivision of the J2 megasequence was possible using biostratigraphically‐controlled seismic reflector terminations and led to the definition of five regional seismo‐stratigraphic sequences (J2.1–2.5). Their geometric, thickness and sedimentary facies variations imply that the onlap‐defined sequence boundaries within the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence were caused by syn‐sedimentary extensional tectonism in a fully marine domain, rather than by fluctuations in global sea‐level in a basin that was relatively quiescent tectonically. The new interpretation has particular significance in view of the fact that the Late Jurassic of the IMF was used by Exxon workers to construct part of their chart demonstrating relative changes of coastal onlap and global eustatic sea levels. As they considered that data from the area showed ‘no evidence that tectonics caused the unconformities’, the new interpretation casts doubt on the global applicability of the Late Jurassic section of Exxon's original sea‐level chart. Furthermore, the study demonstrates that reflector terminations within both tectonically active and/or fully marine sequences should be treated with extreme caution and not be used to define either periods of apparent low‐stand or coastal onlap. Indeed, their appearance may sometimes only represent relatively local, auto‐ and allo‐cyclic sedimentary processes such as submarine fan avulsion or channel switching, unrelated to changes in sea‐level. Finally, the work shows that care must be taken in the selection of seismic lines used to establish and illustrate the nature of depositional sequences and their geometries if pitfalls are to be avoided.