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Some potentially attractive reservoirs, containing anomalously thick (10s to a few 100 m), cross-stratified sandstone, have been locally encountered within both the classic regressive (lower Brent) and the transgressive (upper Brent) segments of the Brent Delta. Three documented cases of these sandstone bodies are re-examined. They are internally dominated by simple or compound dunes, and typified by two types of deepening-upward succession, recording a retrogradational or transgressive shoreline history. Type I is expressed as a single estuarine succession changing upwards from erosive, coarse-grained channelized deposits into outer estuary tidal bar deposits. The estuary is underlain and overlain by deltaic deposits. Type II lacks significant basal river deposits but is composed by stacked mixed-energy and tide-dominated estuarine deposits. It is underlain by deltaic deposits and overlain by open marine sediments. Considering the structural evolution in the northern North Sea basin, we suggest (as did some earlier researchers) that these sandstone bodies were local, but sometimes broad transgressive estuaries, formed at any time during large-scale Brent Delta growth and decay. The estuary generation was likely triggered by fluvial incision coupled with active faulting, producing variable accommodation embayments, where tidal currents became focused and deposition became transgressive. The spatial variations of the interpreted estuary deposits were linked with variable, fault-generated accommodation. The relatively simple, lower Brent estuarine units were created by short-lived, fault activity in places, whereas the complex, stacked upper-Brent estuarine units were likely a result of more long-lived, punctuated fault-induced subsidence leading into the northern North Sea main rifting stage. The thick cross-stratified units potentially accumulated in the hangingwall of large bounding faults.

Detailed observations on the Rannoch Formation in several deep Viking Graben wells indicate that the ‘classical’ wave-dominated Brent delta-front shows coupled storm–tide processes. The tidal signals are of three types: I): alternations of thick cross-laminated sandstone and thin mud-draped sandstone, whereby double mud drapes are prominent but discretely distributed, II): a few tidal bundles within bottomsets and foresets of up to 10 cm-thick sets cross-strata, and III): dm-thick heterolithic lamination showing multiple, well-organized sand–mud couplets. During progradation of the Brent Delta, the Rannoch shoreline system passed upward from 1) a succession dominated by clean-water, storm-event sets and cosets frequently and preferentially interbedded with type I tidal beds, and occasional types II and III tidal deposits, toward 2) very clean storm-event beds less frequently separated by types II and III tidal beds, and then into 3) a thin interval showing muddier storm-event beds mainly alternating with type II tidal beds. It is likely that those variations in preservation bias of storm and tidal beds in each facies succession result from combined effects of 1) the frequency and duration of storms; 2) river discharge; and 3) the absolute and relative strength of tides. Tidal deposits are interpreted as inter-storm, fair-weather deposits, occurred preferentially in longer intermittent fair-weather condition and periods of lower river discharge, and well-pronounced in the distal-reach of delta-front. The formation and preservation of tidal signals between storm beds, indicate that the studied Rannoch Formation was most likely a storm-dominated, tide-influenced delta front 1) near the mouth of a large Brent river, where a significant tidal prism and high tidal range might be expected, and 2) in a setting where there were relatively high sedimentation rates associated with high local subsidence rates, so that the storm waves did not completely rework the inter-storm deposits. The documentation of the unconventional Rannoch Formation contributes to our understanding of mixed-energy coastal systems.

This case study addresses fault reactivation and linkage between distinct extensional episodes with variable stretching direction. Using 2-D and 3-D seismic reflection data we demonstrate how the Vesterdjupet Fault Zone, one of the basin-bounding normal fault zones of the Lofoten margin (north Norway), evolved over c. 150 Myr as part of the North Atlantic rift. This fault zone is composed of NNE-SSW- and NE-SW-striking segments that exhibit a zigzag geometry. The structure formed during Late Jurassic and Early Cretaceous rifting from selective reactivation and linkage of Triassic faults. A rotation of the overall stress field has previously been invoked to have taken place between the Triassic and Jurassic rift episodes along the Lofoten margin. A comparison to recent physical analogue models of non-coaxial extension reveals that this suggested change in least principal stress for the Lofoten margin may best explain the zigzag-style linkage of the Triassic faults, although alternative models cannot be ruled out. This study underlines the prediction from physical models that the location and orientation of early phase normal faults can play a pivotal role in the evolution of subsequent faults systems in multi-rift systems. acceptedVersion

This study investigates the depositional architecture of turbidite lobes from the Southern Laga Basin (Lower Messinian, Central Apennines) and the external controls on its development. Physical stratigraphy and facies analysis of a 1700 m thick stratigraphic interval enabled us to unravel the hierarchical partitioning of lobes and compare the internal organisation of four units of different age and degree of topographic confinement. A four-fold hierarchy of depositional bodies recording short and long term variations in sediment input to lobes was recognized and includes: i) event beds, composed of sandstone-mud cap couplets deposited by single turbidity currents: ii) single lobes, comprised of few m thick bed sets with variable degree of amalgamation; iii) lobe sets, consisting of few ten of m thick stacks of single lobes that are typically depocentre-wide and iv) lobe complexes, representing high rank units of greater internal complexity. Field mapping of channel-lobe transition sectors and onlap terminations of lobes highlighted a significant stratigraphic change in size and morphology of the receiving basin topography which mainly resulted from sediment filling and onlapping of basinal slopes. The lobe depocentre evolved from an initial funnel-shaped trough of an approximate area of ?150 km2 to a wider morphology reaching a maximum area of over 500 km2, which suggest a notably more confined setting for the two older lobe complexes. Comparison of external shape and internal organisation of lobe sandbodies from the studied complexes points out how this different degree of confinement resulted in development of two main depositional architectures: i) sheet-like confined lobes, comprised of laterally contained sandbodies that in the axial part of the depocentre show flat geometry and little internal variability in facies partitioning and degree of amalgamation and ii) semi-confined lobes with a 'jigsaw' architecture, internally comprised of shingled to compensational patterns of building blocks with lobate to more complex planform and highly variable facies partitioning. The architecture of confined lobes is interpreted as having resulted from flow containment of incoming turbidity currents by lateral basinal slopes and deposition under confined accommodation space whereas that of semi-confined lobes would record deposition from freely expanding turbidity currents in a wider depocentre allowing for lateral migration of depositional lobes. The different architecture of confined vs. semi-confine lobes has important implications for the hydrocarbon industry as it results in significantly dissimilar structures of permeability and connectivity.