Rune Kyrkjebø

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.

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.

Cretaceous-tertiary palaeo-bathymetry in the northern north sea; integration of palaeo-water depth estimates obtained by structural restoration and micropalaeontological analysis

Temporal and spatial variations in palaeo-water depth are crucial parameters in basin analysis since changes in palaeo-bathymetry detail the amount of sediment underfill during basin evolution. By carefully integrating seismic-stratigraphic observations with palaeo-water depth estimates from structural restoration and micropalaeontological data, changes in accommodation space throughout the Cretaceous-Tertiary post-rift interval are documented on a regional scale in the northern North Sea. Since it is not possible to determine the palaeo-water depth exactly, we have focused on determining most likely water depth figures, and identifying the principal shallowing and deepening trends. The inferred trends from the investigated wells are generally in good agreement with each other on a regional scale, especially when the tectonic position within the basin is taken into account. The inferred general trends are: (1) general shallowing superimposed on several transgressive/regressive events during the Early Cretaceous; (2) deepening from the early Cenomanian to mid-Campanian; (3) shallowing from the mid-Campanian to latest Maastrichtian; (4) deepening in the Early to Late Paleocene; (5) shallowing from the Late Eocene to Late Miocene; (6) deepening from the Late Miocene to Early Pliocene; (7) shallowing during Pliocene time. Early Cenomanian to latest Maastrichtian and the late Eocene to Pliocene events correspond with changes in eustatic sea level, but the deepening/shallowing trends were probably amplified by tectono-thermal effects. The events in the Early Cretaceous, Early to Late Paleocene, and Late Miocene to Early Pliocene cannot be explained by the eustatic sea-level curve, and therefore need to be explained by purely tectono-thermal events.

Vertical movements in south-western Fennoscandia: a discussion of regions and processes from the Present to the Devonian

This review discusses regions of vertical movements of southern Norway and its continental shelf from the Present to the Devonian. The processes examined are distinguished on the basis of their effect into long-wavelength and short-wavelength. On the mega-scale, two main configurations are identified. The older configuration relates to the late- to post-Caledonian stage, which was dominated by orogenic denudation processes that followed the Caledonian Orogeny. This probably included thermal as well as isostatic effects, which contributed to the development of an asymmetrical long-wavelength uplift area with a low-relief eastern flank towards the Baltic countries, and a western hinterland region of high relief, especially above exhumed gneissic regions. The hinterland probably had a rugged topography, similar to that of the present Himalaya. The younger configuration is mirrored by smoother highs and lows of events that were probably, to a large extent, thermally controlled. They include Carbo-Permian, Permian and Jurassic rifts, of which the latter particularly affected the flanks of a south Norwegian high, or dome. From the earliest Tertiary, this feature seems to have been stabilised and supported by a horizontal transfer of hot material associated with the Icelandic plume. Pluming may be superimposed by more recent glacial rebound, which presently interferes with the long-term effects of the North Atlantic asthenospheric plume.

The reconstruction and analysis of palaeowater depths: a new approach and test of micropalaeontological approaches in the post-rift (cretaceous to quaternary) interval of the northern North Sea

This paper explores and tests a new ecelectic approach to micropalaeontological data which are used to reconstruct palaeowater depths in the Norwegian North Sea in the period from the Cretaceous to Quaternary. These new ideas and evidence presented here promote a greater understanding of palaeobathymetry and basin evolution. This account focuses in particular upon Cretaceous to Cenozoic sediments in this region although the new approach has general applicability. Such information is important because palaeobathymetric variations and palaeobasin shape are essential for the recognition of tectonic phases, and understanding the distribution of sediments and source rocks. Palaeobathymetric curves with upper and lower depth limits derived in part from statistical analyses of micropalaeontological data were constructed to present the most likely depth variations through time in the region. The palaeobathymetric curves obtained from micropalaeontological investigations are compared with the palaeobathyemtric variations that were suggested by independent structural restorationms for the region. The outcomes of these two approaches are shown to be essentially similar which implies the soundness of this new approach to micropalaeontological data. Both the structural restoration and micropalaeontological results point to the following: a shallowing in the palaeowater depths in the Early Cretaceous; a deepening in the middle Cretaceous; a shallowing in the Late Cretaceous; a deepening in the Palaeogene; a shallowing in the early Neogene, and a deppening in the late Neogene (Pliocene). The Quaternary interval in wells in this study illustrates the complexity of the palaeoenvironmental record over this period of time, with some wells clearlt showing subsidence while others suggesting relative uplift. This complexity is the result of the impact of ice movement and multiple glaciations and subsequent isostatic re-adjustments.

Cenozoic tectonic subsidence from 2D depositional simulations of a regional transect in the northern North Sea basin

Abstract The Cenozoic depositional history along a regional E–W profile across the northern North Sea has been simulated using a forward process-based simulation program of dynamic-slope type. It involves a depth-dependent, dual-lithology diffusion equation that handles transport, erosion and deposition of sediments. The data used in the simulation were derived from a seismic line calibrated against wells, and from the regional literature concerning the northern North Sea. The most important of the factors used are: the initial basin form (Paleocene bathymetry), tectonic subsidence, isostatic variables, sediment supply (sand-shale), sediment compaction (porosity-depth relationships for sand-shale) and eustatic sea-level changes. The interaction between the data values extracted from the literature could not reproduce a cross-section similar to the observed cross-section from seismic data. Therefore, the subsidence pattern and the initial basin form were reconsidered. The resulting model gave an anomalous Cenozoic subsidence pattern, different from the expected post-rift thermal subsidence, with deviations corresponding to Paleocene and Late Miocene-Pliocene times. The model-derived Paleocene subsidence might have been overestimated by using an over-shallow palaeobathymetric value, although a deepening of the basin is also indicated by biostratigraphic data. The pronounced Neogene subsidence created accommodation space for a thick Pliocene sequence, derived from the uplifted eastern source area.