Triassic–Jurassic megasequence stratigraphy in the Northern North Sea: rift to post-rift evolution

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Analysis of any sedimentary basin infill should involve an initial separation of the syn-rift and post-rift segments of the succession, particularly where a main aim is to improve prediction of lithology in space and time. The drainage and sediment dispersal patterns and, therefore, the resulting lithology distribution, is usually much more complex in the syn-rift than in the post-rift interval. A varied analytical approach to sequence stratigraphy may therefore be required, especially to honour the local tectonic effects in rifted terranes. Although tectonics are likely to make a fundamental contribution to stratigraphic sequentiality in both settings, the intensity, frequency and area-specific nature of faulting, subsidence and uplift during crustal extension should weigh heavily in the analysis of syn-rift successions. The syn-rift early Triassic basins of the Northern North Sea are compared with the same basins’ mid-Triassic to late-Jurassic post-rift evolution. Further analysis of the post-rift interval shows that the basin had by no means a quiet or uniform development in this phase, but saw the periodic out-building of some 9 major clastic wedges or megasequences, from the Norwegian and Scottish hinterlands. Megasequence boundaries have been chosen to coincide with peak transgression (marine basin) or peak retrogradation (non-marine basin), such that the megasequences have a regressive-to-transgressive structure. This is justified on the grounds of objectivity and simplicity, and the wish to identify individual megasequences with the descriptive and easily understood concept of a clastic wedge. Megasequences have thicknesses between 100 and 1200 m, are time-stratigraphic units of 6–18 Ma duration and should not be confused with higher-order, transgressive-to-regressive sequences. The lower boundary of any post-rift megasequence is a time-line, and will not follow, except locally, a lithostratigraphic horizon. Nevertheless, the lower boundaries can be related loosely to established lithostratigraphic units in the Horda Platform and East Shetland Basin as follows: Scythian (lower Teist Formation) Late Carnian (within middle Lunde Formation) Late Rhaetian (uppermost Lunde Formation) Mid-Sinemurian (within Amundsen Formation) Pliensbachian (within Burton Formation) Early Toarcian (base Drake Formation) Latest Aalenian (lowermost Rannoch Formation) Early Bathonian (lowermost Heather Formation) Late Callovian (middle Heather Formation) The occurrence and broadly similar geometry of the megasequences in both continental Triassic and marine Jurassic successions strongly suggests that post-rift sequentiality was not merely caused by eustatic sea-level changes, but probably included a major contribution from subsidence-rate variation (increased subsidence rates inducing a new megasequence boundary). The main value of mapping the geometry and distribution of the megasequences lies in achieving a better prediction of lithology distribution in the main reservoir fairways, and in creating a basis or framework for analysis of higher-order sequences.