Olaf Michelsen

Structural development of the Fennoscandian Border Zone, offshore Denmark

Abstract The Hans-1, Saeby-1 and Terne-1 wells are located within the Danish part of the Fennoscandian Border Zone and provide significant new data pertaining to the evolution of this important tectonic belt. All three wells encountered Palaeozoic and Mesozoic rocks. The Cambrian to Lower Silurian deposits can be closely correlated to sections known from Bornholm and Scania, and tectonic activity within the Fennoscandian Border Zone cannot be verified during this time interval. The thick Upper Silurian shales were probably deposited in a rapidly subsiding foreland basin, marginal to the speculative Caledonian deformation front which lies south of the Ringkobing-Fyn High. A major hiatus comprises the Devonian and Early Carboniferous. Upper Carboniferous sediments were deposited before a phase of rifting. Upper Carboniferous intrusive and extrusive volcanic rocks suggest tectonic activities, heralding the rifting phase. Extensive block faulting occurred in the border zone during the Rotliegende and more than 650 m of reworked volcanic rocks were deposited in an alluvial fan environment. Syndepositional erosion created a major unconformity before the deposition of post-rift Zechstein sediments. During the Mesozoic the Fennoscandian Border Zone was divided into two zones: the Sorgenfrei-Tornquist Zone to the west and the Skagerrak-Kattegat Platform to the east. The Sorgenfrei-Tornquist Zone continued to experience tectonic activity in Triassic and Middle Jurassic times. The zone has experienced an uplift of 1700–2000 m during Late Cretaceous to Early Tertiary inversion tectonics and the Late Tertiary uplift of Fennoscandia.

High-frequency sequence stratigraphy of Upper Cenozoic deposits in the central and southeastern North Sea areas

Abstract During late Cenozoic times, southwestward prograding deposits filled in the central and southern part of the epicontinental North Sea Basin. The Ringkobing-Fyn High acted as a topographical high partly separating the central and southeastern North Sea into two embayments, which were connected towards the west by a deeper seaway located in the Central Graben area. The stratal geometry deduced from seismic sections indicates different basin physiography, subsidence rates, and sediment influx between the two embayments. Eustatic sea-level cycles of several frequencies interacted with low subsidence rates in the southern embayment to generate composite relative sea-level cycles each composed of relative sea-level cycles of higher frequencies. Forty-five Upper Cenozoic sequences are identified in the southern embayment, whereas only 21 sequences are recognized in the northern embayment. Major hiati and intervals of starved sedimentation may therefore be present in the northern embayment. The sequences stack into nine composite sequences, each reflecting a depositional cycle representing lowstand succeeded by transgressive/highstand conditions. In general, a composite sequence is characterized by a period of increasing sediment accumulation rates.

Cenozoic evolution of the eastern Danish North Sea

This paper provides a review of recent high-resolution and conventional seismic investigations in the eastern Danish North Sea and describes their implications for the development of the eastern North Sea Basin. The results comprise detailed time-structure maps of four major unconformities in the eastern Danish North Sea: the Top Chalk surface (mid-Paleocene), near top Oligocene, the mid-Miocene unconformity, and base Quaternary. The maps show that the eastern Danish North Sea has been affected by faulting and salt diapirism throughout the Cenozoic. Carbonate mounds, erosional valleys and pockmark- or karst-like structures were identified at the top of the Upper Cretaceous–Danian Chalk Group. Strike-parallel erosional features and depositional geometries observed at near top Oligocene and at the mid-Miocene unconformity indicate that these major sequence boundaries can be attributed to large-scale lateral changes in sediment supply directions. Increases in sediment flux to the southeastern North Sea at the Eocene/Oligocene transition and in the post-Middle Miocene appear to correlate with similar events world wide and with long term δ18O increases, indicating forcing by global factors, i.e. eustasy and climate. Stratal geometries observed on the seismic data indicate that the so-called ‘Neogene uplift’ of the eastern Danish North Sea may have been hundreds of metres less than previously suggested. It is argued that late Cenozoic uplift of the basin margin and of mountain peaks in southern Norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late Cenozoic denudation and dissection of topography created in the Paleogene. The late Cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late Cenozoic tectonics, the cause of which is conjectural. A series of shallow thrust structures and an associated system of deep, buried valleys were mapped. Thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet, and it was followed by deep incision due to subglacial melt-water erosion, probably during the Elsterian glaciation.

Factors controlling the Cenozoic sequence development in the eastern parts of the North Sea

The causal relationship between the Cenozoic sequence development in the southeastern North Sea Basin and sea-level changes, climatic fluctuations and tectonic events is unravelled by relating variations in the relative sea level and base level, based on interpretations of seismic surveys, to published δ18O variations and eustatic changes. The latter curve is based on the Earths orbital forcing, and here informally termed as the GSI curve. The analysis shows that the Cenozoic sequence development in the southeastern North Sea was influenced by climatically and tectonically induced sea-level changes. The major Cenozoic sequence stratigraphic boundaries (lower order) are highly influenced by tectonic events, e.g. uplift of Fennoscandia and increased subsidence rates in the basin centre. Reactivation of Mesozoic fault zones controlled the deposition of minor sand bodies transported to the centre of the basin during the Late Palaeocene by mass flows. The location of an Oligocene mound structure, which constitutes part of a sequence, is controlled by the overall palaeotopography of the basin and local fault-related depressions. Correlation between (i) the ages of our sequences and the δ 18O variations in the Oligocene succession, and (ii) the GSI curve and the base-level fluctuations of the late Miocene and younger sequences, show that the generation of the higher order sequence boundaries were driven by glacio-eustatic sea-level changes. A climatic control of the sequence formation due to glacio-eustatic sea level changes is therefore suggested for the Oligocene and Pliocene sequences, and probably also for the Upper Miocene sequences.

Stratigraphy and facies distribution of the Utsira formation and the Pliocene sequences in the northern North Sea

Abstract The Upper Cenozoic deposits of the northern North Sea have been analysed in order to establish a regional and detailed stratigraphy. The Utsira Formation is subdivided into four log-units and mapped, and two main depocentres are outlined. The lower part of the Utsira Formation consists of thick marine, mounded sand bodies, interpreted as overall stacked lowstand fan deposits, while the upper part of the formation consists of more clayey-silty intervals, indicating increased relative sea level. The succeeding progradational Pliocene deposits are subdivided into 13 high-frequency depositional sequences and are mapped. The sequences are grouped into four composite sequences. Each of the four Pliocene composite sequences is composed of one or two rather locally distributed, prograding sequences (lowstand sequence set), succeeded by one or two more widely distributed aggrading-prograding sequences (transgressive-highstand sequence set). Boundaries between the composite sequences are recorded as marked changes in distribution of depocentres and sequence architecture. The regional uplift of Scandinavia is believed to be the main control on sediment input, feeding the succeeding general prograding Pliocene sequences. Oscillations of the eustatic sea level punctuated the tectonically controlled progradation and affected variations in the accommodation space, and thus created the high-frequency sequences.

Occurrence of major sequence stratigraphic boundaries in relation to basin development in Cenozoic deposits of the southeastern North Sea

The post-Danian Cenozoic succession in the southeastern North Sea is subdivided into seven informal “major sequence stratigraphic units”, comprising twenty-one sequences. The boundaries of the units are easily recognized on seismic sections over the entire study area. The North Sea is an epicontinental basin. It underwent considerable changes during the Cenozoic as regards subsidence rates, direction of sediment transport, and position of depocentres. The six unit boundaries are interpreted to reflect some of the major events in the development of the basin. The boundary between units 1 and 2 (earliest Eocene) is associated with a regional hiatus followed by a large relative sea-level rise, whereas the boundary between units 2 and 3 (Middle Eocene) coincides with a shift in the direction of sediment transport from north to west. The boundary between units 3 and 4 (Eocene-Oligocene transition) marks an important change in the depositional environment of the North Sea with a shift in the direction of sediment transport from west to mainly northeast. In the northeastern part of the study area, thick prograding deposits of unit 4 are situated on distal deposits of units 1, 2 and 3. The transition between units 4 and 5 (latest Oligocene) shows a pronounced basinward shift in onlap, and it is interpreted to reflect a major fall in the relative sea level. The boundary between units 6 and 7 (Middle Miocene) is identified on a shift from a prograding reflection pattern to an aggrading pattern. The lower part of unit 7 is interpreted to represent a starved depositional environment, related to a pronounced increase in subsidence rate. The North Sea sequences are correlated with the Danish onshore deposits. The sequence boundaries generally coincide with an unconformity in the Danish deposits. A glauconitic horizon characterises the initial deposits overlying each of the unconformities.

Detailed stratigraphic subdivision and regional correlation of the southern Danish Triassic succession

This paper provides a regional, log-based lithostratigraphic correlation of the Danish Triassic deposits with deposits known from the southern North Sea Basin, including the northwest German and Dutch onshore areas. A new lithostratigraphic nomenclature, mainly adapted from the Dutch nomenclature, is suggested for the Danish area south of the Ringkobing-Fyn High, replacing the previously used nomenclature. The new nomenclature comprises the Lower Triassic Bunter Shale and Bunter Sandstone Formations, the Lower to Middle Triassic Rot and Muschelkalk Formations, and the uppermost Middle Triassic and Upper Triassic Keuper and Sleen Formations. In total, 22 members are also defined from the southern North Sea into the southern Danish area. This main marine southern facies province is separated from a continental northern facies province by the northern flank of the Ringkobing-Fyn High trend. Units of the southern province only crossed the central and western part of the Ringkobing-Fyn High during the Rot and the Muschelkalk transgressions, whereas the basin north of the eastern part of the high was influenced by the southern facies province during the Early to early Late Triassic.

Oligocene sequence stratigraphy and basin development in the Danish North Sea sector based on log interpretations

Abstract Five sequences are defined in the Oligocene succession of the Danish North Sea sector. Two of the sequences, 4.1a and 4.3, have been identified onshore Denmark. Two types of prograding lowstand deposits are recognized. Sand-dominated deposits occur proximally, comprising sharp-based forced regressive deposits covered with prograding low-stand deposits. Clay-dominated prograding lowstand deposits occur distally in the sequences. The highstand deposits are proximally represented by thick prograding sandy deposits and distally by thin and condensed intervals. The main sediment input direction was from the north and the northeast. A succession oif lithofacies, from shallow marine facies dominated by sand to outer shelf facies dominated by clay, is mapped in each of the sequences. An overall southward progradation of the shoreline took place during the Oligocene, interrupted only by minor shoreline retreats.

Geological indications for Palaeogene uplift in the eastern North Sea Basin

Abstract The timing and effect of the Cenozoic uplift of Scandinavia has been investigated using a multi-disciplinary approach involving sedimentological, seismic and biostratigraphic data from the Danish and the adjacent Norwegian parts of the North Sea Basin. It is concluded that significant uplift took place periodically throughout the Palaeogene possibly marking an earlier onset of the so-called “Neogene uplift” of Scandinavia. This conclusion is based on a number of sedimentological observations, including smectite content, grain-size variations, kaolinite thermal stabilities and T max values supported by seismic reflection geometries and biostratigraphic data. These data indicate several phases of re-working of Palaeogene and older sediments situated further to the east and northeast during the middle to late Eocene and during the middle to late Oligocene. The tectonic patterns were similar during the late Paleocene and the Oligocene with some inversion taking place, whereas no inversion has been observed during the Eocene. Main provenance areas were to the north and northeast during the Paleocene and Oligocene, whereas the Eocene sediments originate mainly from the British Isles to the west. It is proposed that Palaeogene uplift of Scandinavia was associated with regional tectonic movements along crustal zones of weakness, which were reactivated as they accommodated strain induced by the Alpine Orogeny and the opening of the North Atlantic.

Detailed log-stratigraphic study of the Lower Cretaceous in the Danish Central Trough, North Sea

Abstract A detailed log-stratigraphic study was conducted on the Lower Cretaceous Cromer Knoll Group in the Sten-1, I-1, Nora-1, Elin-1, Bo-1 and Adda-1 wells in the Danish Central Trough. Six major, 24 intermediate and a number of minor log-units were identified and correlated between the wells. The log-units are defined by consistent log trends and are separated by distinct shifts in log readings. The log-units are isochronous within the resolution of the biostratigraphy and are calibrated to the standard chronostratigraphic scale. A good correlation between the log-units and published seismic stratigraphy was established. In the homogeneous Lower Cretaceous clay, marl and chalk deposits the log-stratigraphic method used is seen to be an important correlation tool, supplementing the biostratigraphy and seismic stratigraphy. A detailed log-stratigraphic subdivision of the Lower Cretaceous has been established. Depositional hiatuses were found and subsidence and uplift patterns of different subbasins are described. The overall successive changes in the lithological character of the major log-units coincide with the long-term fluctuations of the Haq et al. eustatic sea-level curve.