SEDIMENTATION, SEA‐LEVEL RISE AND TECTONICS AT THE TRIASSIC‐JURASSIC BOUNDARY (STATFJORD FORMATION), TAMPEN SPUR, NORTHERN NORTH SEA

Abstract

Statfiord Formation reservoir rocks (Rhaetian‐Sinemurian) of the Tampen Spur area, Northern North Sea consist of coarse‐ and fine‐grained units in alternation, in a succession up to 300‐m thick in places. The formation is interpreted in terms of coastal fans and fan deltas on a low‐energy coastline.Four broad facies associations are distinguished: subaerial fan and distal fan (partly subaqueous) deposits make up the coarse‐grained units, while the fine‐grained units consist of lagoon‐bay and floodbasin‐bay sediments. The deposits interpreted as subaerial fan‐plain and braided‐stream channels are dominantly cross‐stratified and plane‐bedded, coarse‐grained sandstones and fine‐conglomerates. The distal fan deposits, mainly massive or bioturbated finel/medium‐grained sandstones tend to occur at the base or top of the coarse units.The deposits interpreted in terms of a lagoon‐bay setting consist of wave‐rippled, flaser/lenticular bedded siltstone and fine‐grained sandstone in a “background”of dark grey mudstone with occasional coal beds. This association is thought to have originated largely in front of the fans. The floodbasin‐bay association, consisting mainly of red‐and green‐coloured, massive and bioturbated mudstone and silty sandstone is believed to have accumulated in ephemeral floodbasin lakes or in the inner regions of coastal embayments, between fans. In addition to the main associations, there are also marine reworked sandstones at the top of the formation in some areas.Sedimentation time trends within the coarse‐grained units reflect fan shifting and progradation. On the formation scale, the coarse units become coarser and more laterally‐continuous upward, whereas the fine units become dominated by the lagoon‐bay facies at younger stratigraphic levels.It is argued that the former was a response mainly to tectonics, while eustatic sea level rise accounts for the increase in marine deposits through time. The marine sandstone at the top of the formation suggests a transition from a low energy, protected shoreline to a higher energy, more open one; a change that is related to basin enlargement and increased rate of relative sea level rise at the Statfjord‐Dunlin Formation transition.