Roy H. Gabrielsen

Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea

Abstract A tectonostratigraphic model for the evolution of rift basins has been built, involving three distinct stages of basin development separated by key unconformities or unconformity complexes. The architecture and signature of the sediment infill for each stage are discussed, with reference to the northern North Sea palaeorift system. The proto-rift stage describes the rift onset with either doming or flexural subsidence. In the case of early doming, a proto-rift unconformity separates this stage from the subsequent main rift stage. Active stretching and rotation of fault blocks during the rift stage is terminated by the development of the syn-rift unconformity. Where crustal separation is accomplished, a break-up unconformity commonly marks the boundary to the overlying thermal relaxation or post-rift stage. Tabular architectures, thickening across relatively steep faults, characterize the proto-rift stage. Syn-rift architectures are much more variable. Depending on the ability of the sediment supply to fill the waxing and waning accommodation created during rotation and subsidence, one-, two- or three-fold lithosome architectures are likely to develop. During the post-rift stage, an early phase with coarse clastic infilling of remnant rift topography often precedes late stage widening of the basin and filling with fine-grained sediments.

Structuring of the Northern Viking Graben and the Møre Basin; the influence of basement structural grain, and the particular role of the Møre-Trøndelag Fault Complex

Abstract A comparison between structural histories of the More Basin of the Mid Norwegian shelf and the northernmost Viking Graben of the northern North Sea, suggests that the basement structural grains of the two areas are not entirely similar. However, considerable differences in timing and stretching magnitudes occur. These differences are clearly seen by Permian times, as contrasting stretching estimates are obtained for this period. The contrasts were even more pronounced in the Triassic–Cretaceous when extension was initiated earlier and terminated later in the More Basin area than in the northern Viking Graben. Furthermore, relatively pronounced late early Cretaceous and (?)Oligocene-Miocene inversion, which affected the More Basin, cannot be identified in the northern Viking Graben. Two reasons for these differences are proposed: firstly, the two basins are separated by segments of the More-Trondelag Fault Complex, and secondly, they had different positions in relation to the opening of the North Atlantic. The More-Trondelag Fault Complex has been active during changing stress conditions since the Palaeozoic, and a correlation between dated events in the onshore part of the fault complex correlates well with the observations made in the offshore basins. This emphasises the regional significance of the More-Trondelag Fault Complex, and seismic activity in the area suggests that the fault complex is still active.

Tectonic impact on sedimentary processes during Cenozoic evolution of the northern North Sea and surrounding areas

Abstract This paper focuses on the Cenozoic evolution of the northern North Sea and surrounding areas, with emphasis on sediment distribution, composition and provenance, as well as on timing, amplitude and wavelength of differential vertical movements. Quantitative information about palaeo-water depth and tectonic vertical movements has been integrated with a seismic stratigraphic framework to better constrain the Cenozoic evolution. The data and modelling results support a probable tectonic control on sediment supply and on the formation of regional unconformities. The sedimentary architecture and breaks are related to tectonic uplift of surrounding clastic source areas, thus the offshore sedimentary record provides the best age constraints on Cenozoic exhumation of the adjacent onshore areas. Tectonic subsidence accelerated in Paleocene time throughout the basin, with uplifted areas to the east and west sourcing prograding wedges, which resulted in large depocentres close to the basin margins. Subsidence rates outpaced sedimentation rates along the basin axis, and water depths in excess of 600 m are indicated. In Eocene times progradation from the East Shetland Platform was dominant and major depocentres were constructed in the Viking Graben area, with deep water along the basin axis. At the Eocene-Oligocene transition, southern Norway and the eastern basin flank became uplifted. The uplift, in combination with prograding units from both the east and west, gave rise to a shallow threshold in the northern North Sea, separating deeper waters to the south and north. The uplift and shallowing continued into Miocene time when a widespread hiatus formed in the northern North Sea, as indicated by biostratigraphic data. The Pliocene basin configuration was dominated by outbuilding of thick clastic wedges from the east and south. Considerable late Cenozoic uplift of the eastern basin flank is documented by the strong angular relationship and tilting of the complete Tertiary package below the Pleistocene unconformity. Cenozoic exhumation is documented on both sides of the North Sea, but the timing is not well constrained. Two major uplift phases in early Paleogene and late Neogene times are related to rifting, magmatism and break-up in the NE Atlantic and isostatic response to glacial erosion, respectively. Additional uplift events may be related to mantle processes and the episodic behaviour of the Iceland plume.

Unconformities related to the Jurassic–Cretaceous synrift–post-rift transition of the northern North Sea

In the Jurassic–Cretaceous North Sea basin, the synrift sequence is separated from the post-rift sequence by the ‘base Cretaceous’ or ‘late Cimmerian’ unconformity. The unconformity covers almost the entire basin, has a distinct character in seismic reflection data and wireline logs, and hence, is easily identified and correlated, making it the most important marker horizon in the area. The unconformity displays great local complexity (in many localities) and great variability on a regional scale (from one locality to another). Thus the unconformity is classified as a nonconformity, a disconformity and an angular unconformity. We suggest that these variations basically reflect different structural position within the basin, so that the short-wavelength variation is dominated by local structural development (e.g. the rotational history of a fault block), whereas the long-wavelength variation reflects basin-scale tectonic, thermal and isostatic processes. The merging of this unconformity with younger erosional surfaces, its complex configuration and polychronous character makes the general term ‘base Cretaceous unconformity’ inadequate. Thus, the term ‘northern North Sea Unconformity Complex’ is used here.

Experiments on clay smear formation along faults

A ring shear apparatus was used to investigate the development of clay smear along faults in sand–clay sequences. Experiments were performed, using six different clay types, different stress conditions (σn=6–500 kPa) and different amounts of clay (4 and 12.5%). The development of clay smear seems to depend on the competence contrast between the clay and the surrounding sand. Clay, when it is less competent than sand, behaves in a ductile manner, resulting in the development of clay smears along the fault. Clay which is more competent than sand behaves in a brittle manner, resulting in the formation of angular fragments. Whether the clay is more or less competent than the sand is dependent on the stress conditions, the initial porosity of the sand and the mechanical properties of the clay. The results suggest that stress conditions allowing the sample to contract will result in the formation of fluid flow barriers, whereas dilation results in the formation of conduits.

The Cretaceous post-rift basin configuration of the northern North Sea

The present analysis suggests that three stages can be identified in the post-rift Cretaceous development of the northern North Sea, namely the incipient (Ryazanian–latest Albian), the middle (Cenomanian–late Turonian) and the mature (early Coniacian–early Palaeocene). The transition from syn-to post-rift configuration was strongly diachronous, suggesting that the thermal state of the system was not homogeneous at the onset of the post-rift stage. This is supported by observed differences between the early post-rift subsidence histories of the southern Viking Graben, the Stord Basin and the Sogn Graben. The incipient post-rift stage was characterized by diverse subsidence. The major structural features inherited from the syn-rift basin (e.g. crests of rotated fault blocks, relay ramps and sub-platforms) had a strong influence on the basin configuration and, therefore, the sediment distribution. In the middle stage the internal basin relief became gradually drowned by sediments. This is typical for basins where sediment supply outpaces or balances subsidence, as was the case in the northern North Sea. Thus, the influence of the syn-rift basin topography become subordinate to the subsidence pattern which was determined by the crustal thinning profile which, in turn, relies on thermal contraction and isostatic/elastic response to sediment loading. The mature post-rift stage was characterized by the evolution into a wide, saucer-shaped basin where the syn-rift features finally became erased. Since thermal equilibrium was reached at this stage, subsidence ceased, and the pattern of basin filling became, to a larger degree, dependent on extra-basinal processes. This simple pattern was influenced by the structural inhomogeneity of the basin. This inhomogeneity may have included the graben units, in turn related to contrasting geometries of the lithospheric structure. The incipient stage of post-rift development was halted by relative uplift/deceleration of subsidence, locally corresponding to 200 m. This is ascribed to a hitherto undescribed thermo-tectonic event. The mechanism of this event is not yet known.

Structure of the Møre basin, mid-Norway continental margin

Abstract In the More basin offshore mid-Norway, the basis of the Cretaceous sequence reaches depths of more than 5000 m. The basin is limited on its eastern margin by a system of extensional faults which probably were initiated in the Triassic and which were active periodically throughout the Aptian. The internal basin floor is divided into subbasins by structural highs which are rotated fault blocks, presumably rooted in the basement. The main structuring of the basin started in Late Triassic and was followed by accelerating subsidence in the Early Cretaceous. Subsidence was interrupted by a mild phase of inversion in the latest Cretaceous, and a more pronounced inversional phase in the Eocene-Miocene. The latter developed folds with wavelengths in the order of kilometers, and local low-angle reverse faults and the inversion of depocenters.

Numerical modeling of Cenozoic stress patterns in the mid-Norwegian margin and the northern North Sea

Numerical modeling of Cenozoic stress patterns in the northern North Sea and the mid-Norwegian margin is presented, and the sense of potential slip along major fault planes belonging to the two areas is restored. We assume that the main regional source of stresses is the Atlantic ridge push as demonstrated by previous studies. Furthermore, we also assume a nearly consistent NW-SE strike for the far-field stress from continental breakup between Greenland and Norway (earliest Eocene) to present day. First, we applied the commercial two-dimensional distinct element method (UDEC) to simulate Cenozoic stress and displacement patterns in the study area. Variations in rheology and major fault zones were introduced into the model. The More-Trondelag Fault Complex and its inferred continuation into the Shetland Platform forms the major mechanical discontinuity in the model. Second, we used the SORTAN method, developed at the University of Paris VI, to predict the sense of potential slip along major fault planes. The input for the SORTAN model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the UDEC modeling. Our results show that the More-Trondelag Fault Complex and its inferred continuation into the Shetland Platform act as a weak fault zone. This fault zone divides the study area into two different stress provinces: the continental margin and the northern North Sea. This result agrees well with the observed differences in Cenozoic structural evolution of the two areas. Compressive structures are observed along the continental margin, whereas relative tectonic quiescence characterizes the northern North Sea during the Tertiary. The restored stress patterns in the northern North Sea and the mid-Norwegian margin also agree well with the observed present-day stress configuration. Our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools (UDEC and SORTAN). As a result, it is demonstrated that oblique-slip motions are mainly expected, in particular, strike-slip and reverse dip-slip faulting are simulated.

Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling

Abstract We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from c. 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was c. 120–125 km, with calculated βmean between 1.38 and 1.40. Permo-Triassic βmean estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic βmean estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic βmean varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic βmean, which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic βmean estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.

Geology of the Norwegian Continental Shelf

In the preceding chapters we have included only a few regional examples and case studies because of space limitations. The present chapter will, however, provide some examples. The North Sea and other parts of the Norwegian continental shelf contain several different petroleum provinces which can illustrate some of the general principles of petroleum geology and geophysics. The geological evolution of these sedimentary basins provides a necessary background to understand the distribution of source rocks and the timing of petroleum migration. The structural history of rifted basins, passive margins and also uplifted basins such as the Barents Sea is critical to the trapping of oil and gas. These basins are very well documented by seismic and well data.