The structural and sedimentological evolution of early synrift successions: the Middle Jurassic Tarbert Formation, North Sea

This study addresses the complex relationship between an evolving fault pop ulation and patterns of synrift sedimentation during the earliest stages of extension. We have used 3D seismic and well data to examine the early synr ift Tarbert Formation from the Middle-Late Jurassic northern North Sea rift basin. The Tarbert Formation is of variable thickness across the study are a, and thickness variations define a number of 1- to 5-km-wide depocentres bounded by normal faults. Seismic reflections diverge towards the bounding faults indicating that the faults were active cont contemporaneous with the deposition of the formation. Many of these faults became inactive during l ater Heather Formation times. The preservation of the Tarbert Formation in both footwall and hangingwall locations demonstrates that, during the earli est synrift, the rate of deposition balanced the rate of tectonic subsidenc e. Local space generated by hangingwall subsidence was superimposed upon ac commodation generated due to a regional rise in relative sea-level. In basa l Tarbert Formation times, transgression across the prerift coastal plain p roduced lagoons and bays, which became increasingly marine. During continue d transgression, barrier islands moved landward across the drowned bays. In the southern part of our study area, shallow marine sediments are erosiona lly truncated by fluvial deposition. These fluvial systems were constrained by fault growth monoclines, and flowed parallel to the main faults. We ill ustrate that stratal architecture and facies distribution of early sediment ation is strongly influenced by the active short-lived faults. Local depoce ntres adjacent to fault displacement maxima focused channel stacking and al lowed the aggradation of thick shoreface successions. These depocentres for med early in the rift phase are not necessarily related to Late Jurassic - Early Cretaceous depocentres developed along the major linked normal fault systems.