Shoreface pinch-out style on the front of the brent delta in the easterly tampen spur area

Abstract

The architecture of the front of the Brent delta (Rannoch and Etive Formations), near the delta's northerly termination on the easterly Tampen Spur has been examined. The aim of the work has been partly to document and explain the deterioration of sandstone thickness and quality near the pinch-out area, and partly to comment on the likelihood of sand accumulation beyond the pinch-out area. The internal geometry of the delta front has been mapped in two dimensions by tracing the distribution of three shoreface facies within the Rannoch Formation and three mouth bar to foreshore facies within the Etive Formation. The facies units from the two formations form a complex of downlapping sandstone tongues which form aggradational to strongly progradational packages characterised by upward-coarsening trends, or transgressive tracts showing a landward-stepping of units with a dominance of upward-fining (or upward increase in mica content) trends. Strongly progradational sandstone tongues, which bring wedges of the Etive Formation downwards into the Rannoch lithosome, are usually marked by erosional boundaries, interpreted in terms of relative sea-level fall or pauses during the rise of sea level, allowing sediment supply/accommodation ratios to be high. Transgressive tracts expand rapidly basinwards and are the direct cause of the splitting of the sandy Rannoch/Etive shoreface into thinner sandstone units near the pinch-out area. The latter feature, on the youngest part (latest Bajocian) of the prograding delta front, is believed to signal the impending turn-around of the deltaic system from a regressive to an overall transgressive mode. Although the distal sandstones are thin and of poor quality, the abundance of erosion surfaces in both the Rannoch and Etive Formations in the pinch-out area could suggest that significant volumes of sand have been resedimented northwards into the undrilled areas of the basin.