Advance and retreat of the Brent delta: recent contributions to the depositional model

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Abstract
The behaviour and the development of the Brent Group in the Northern North Sea is documented by cross-sections and paleogeographic maps. Although this work is based on a previously published Norsk Hydro study, we present recent progress in the understanding of the Brent Group which has been achieved by a better timing of depositional events, by integrating new well information, and by generating new models based on the new information. The early (Aalenian) lateral infill of the Oseberg Formation was deposited as a response to a relative fall of sea-level, probably generated by a tectonic uplift. Fan deltaic sediments built out towards the west and northwest and backfilled the previously emergent areas during the subsequent relative sea-level rise. The progradation of the Brent Delta (Rannoch, Etive and lower Ness Formations) took place in Late Aalenian to Early Bajocian. The thickness of the Etive relative to the Rannoch Formation indicates thickening due to bathymetric deepening in the area south of the maximum regression whereas subsidence controlled the thickening in the area of the maximum extent of the delta. During the retrogradational part of the delta development (the Tarbert and upper Ness Formations, Early Bajocian to Early Bathonian), increasing tectonic activity can be documented, both in terms of enhanced differential subsidence across faults and by early rotational uplift on some fault blocks.