Evolving depositional geometries in the North Sea Jurassic rift

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Abstract
The tectonic history of the North Sea during the Middle and Upper Jurassic consists of initial basin-scale uplift, followed by subsidence. The subsidence is at least partly associated with a major rifting episode culminating in the Early–Mid-Kimmeridgian. Shallow marine depositional cycles of progradation and retrogradation are recognized throughout this time interval, with a periodicity of 1–5 million years. Twelve of these units, selected from data-constrained study areas, are described, and quantified data are presented on their geometry, duration, and the implied variations in accommodation volume. Sediment input rates into the basin decreased through the Middle and Upper Jurassic as hinterlands were drowned, and source/basin relief decreased. A corresponding evolution in the geometry of the units is seen, from wide tabular units in the Bajocian/Bathonian to narrow ribbon-geometry units in the Kimmeridgian. The continued decrease in sediment input resulted in an interval of basin-wide marine condensation. On seismic data this forms a prominent surface of marine onlap known as the Base Cretaceous (or Late Cimmerian) ‘Unconformity’. This pattern of decreasing depositional dimensions of shelf/shoreline systems is to be expected in marine rifts without access to continental drainage systems, where sediment supply decreases with syn-rift and post-rift subsidence of drainage basins. This decrease may happen throughout the late pre-rift to early post-rift, and the change from structure-independent to structure-dependent facies trends may not occur suddenly at the onset of rifting.

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