Christine Fichler

Patterns of basement structure and reactivation along the NE Atlantic margin

A lineament pattern on the NE Atlantic margin is discussed, illustrated by gravity and magnetic images in the Norwegian Sea, and reviewed in the context of onshore field evidence. While most possible fault trends exist, three major sets predominate. A NE-SW left-stepping lineament set defines the gross geometry of the margin, while interposing northerly trends impose a rhomboidal geometry at a variety of scales. The margin is segmented by NW-SE transfer zones, sometimes involving significant offsets. The principal trends are primarily a function of Mesozoic-Cenozoic plate-wide extensional stress fields. Certain Proterozoic and Caledonian lineaments were, however, opportunistically reactivated according to the extension direction. Caledonian NE-SW orogen-oblique shears, typified by the Møre-Trøndelag Fault Zone, were reactivated via (?Jurassic) strike-slip or oblique-slip, and were further exploited during Cretaceous-early Cenozoic extensional episodes leading to continental break-up. Jurassic E-W extension may also have reactivated N-S faults existing in the basement or generated in duplex systems between the NE-SW shears. Precambrian and Caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones.

REGIONAL TECTONIC INTERPRETATION OF IMAGE ENHANCED GRAVITY AND MAGNETIC DATA COVERING THE MID-NORWEGIAN SHELF AND ADJACENT MAINLAND

Abstract Gravity and magnetic field data covering mid-Norway and the Norwegian Sea were processed in order to enhance tectonic features on various scales. The local features were subjected to an unconventional processing technique involving a non-linear, adaptive Wallis filter designed to enhance the smallest wave lengths. When compared with recent structural information derived from seismic data, the processed gravity and magnetic maps show the main structural trends, major fault zones and basin boundaries, thus proving their worth for regional tectonic mapping. Previously undetected NW–SE-trending offshore crustal lineaments are revealed. A landward prolongation of the Bivrost Lineament appears to continue subparallel towards Proterozoic shear zones below the Caledonian nappes in the Rana area, either along the western margin of the Transscandinavian Granite–Porphyry Belt or the NW–SE-trending Mala–Skelleftea Tectonic Zone. A large lineament is also observed as a landward prolongation of the Surt Lineament indicating a relationship with the Storsjon–Edsbyn Deformation Zone, a major, deep, crustal shear zone in the Precambrian of Sweden. A slightly increased seismic activity, which is possibly related to the present ridge push force, is observed along parts of the previously unknown transfer zones. Combined gravity and magnetic modelling indicates a low crustal thickness in the northwesternmost part of the Voring Basin, between the Surt and the Jan Mayen Lineaments. The lack of correlation between the gravity and the magnetic patterns observed on the residual field maps suggests the presence of a shallow Curie isotherm situated above or within the uppermost basement.

Continuation of the Caledonides north of Norway: seismic reflectors within the basement beneath the southern Barents Sea

Abstract The offshore continuation of the Caledonides to the north of Norway is poorly understood, and the seismic signature of these rocks is unknown. Indeed, the seismic signature of basement rocks is in general poorly documented from conventional seismic data. This paper discusses an area in the southern Barents Sea — the ‘Gjesvaer low’ — where inferred Caledonian basement rocks have been studied from conventional seismic data. The Gjesvaer low, which has not been described previously as a separate structural element, is defined on image-processed gravity data. The main processing steps were directional filtering and principal component analysis. The interpretation of the low as a feature within the inferred basement rocks is based on well data, the calculated depth to magnetic basement, seismic signature and velocities. In addition, two-dimensional gravity modelling shows that density variations within the basement rocks may explain the observed gravity anomaly. Although the following model may be simplistic, distinct seismic reflectors within the low may be interpreted as originating from tectonic boundaries within a thrust system. In the most likely evolutionary scenario, the Gjesvaer low and the south-western part of the Nordkapp Basin are interpreted as having been formed as continuous Caledonian structures whose continuity has survived until the present. Subsequent Late Palaeozoic erosion may have removed more than 10 km of Caledonian rocks in the area of the low. Carboniferous rifting reactivated the Caledonian structures and the south-western Nordkapp Basin was formed. The Nordkapp Basin, which was decoupled from the Gjesvaer low in Carboniferous times, subsided while the low was again eroded.

North Sea Quaternary morphology from seismic and magnetic data: indications for gas hydrates during glaciation?

Buried circular depressions and channels in Quaternary strata have been investigated by 3D seismic data and, less commonly, by high-resolution aeromagnetic data. Sub-glacial melt-water drainage channels of various dimensions are the most distinct morphological features. In the same strata, numerous crater-shaped depressions were found, some coinciding with the initiation of channels. The diameter ranges from 500 m to 3000 m and the depth from 20 to 300 m – appreciably larger than common pockmarks. It is argued here that the craters were generated by gas expulsion from melted gas hydrates, combined with melt-water expulsion and erosion. This is supported by: (1) a similarity with published seafloor craters that originate from melted gas hydrates; (2) appropriate physical conditions for the formation and melting of gas hydrates during glacial and interglacial periods; (3) correlation with shallow gas occurrences and seismic gas indications. An increased number of craters occurs above a major fault and some Tertiary hydrocarbon discoveries, which may indicate thermogenic gas. At such locations, an increase in high-frequency magnetic anomalies can be explained by deposition of sediments with contrasting magnetic susceptibilities. This is an alternative explanation for the occasionally increased shallow magnetic anomalies above hydrocarbon fields, otherwise attributed to secondary changes in the magnetic mineralogy.

The Norwegian Sea during the Cenozoic

Based on 2D seismic surveys covering the entire Norwegian Sea (250 000 km 2 ), selected 3D surveys and an extensive well database, the Cenozoic depositional history for the area has been reconstructed. Significant amounts of sediments were fed to the Norwegian Sea during the Cenozoic, while, apart from a thin Quaternary cover, no Cenozoic sediments are preserved onshore. This is interpreted to be the result of several phases of uplift and erosion of the mainland during this period. The sedimentary filling of the basins is, interpreted in a sequence stratigraphic context, aiming towards a dynamic understanding of the depositional history. In the Palaeocene, extensional tectonics prevailed and the Norwegian Sea received sediments from uplifted land areas, both to the east and the west. The input of sediments to the deeper parts of the basin were to some degree determined by the intersection of NW-SE trending lineaments intersecting with older structural features on the shelf. With the onset of sea floor spreading in the Eocene, the tectonic regime changed from extensional to compressional. Extrusion of basaltic lavas dominated the western land areas, while a major transgressive event resulted in the deposition of shaly sediments on the eastern continental shelf. Large parts of Scandinavia were probably flooded during this time period. A deltaic system constituting the ‘Molo Formation’ was deosited all along the castern Norwegian Sea margin, as a response to regional uplift of the Norwegian mainland. Difficulties in seismic ties and the sparse well control have made the actual age of the Molo Formation a subject for discussion. Both Oligocene and Early Pliocene ages have been suggested. New seismic correlations presented in this chapter suggest that the Molo Formation is Early Pliocene in age. Erosional channcls with possible fluvial drainage patterns suggest subaerial exposure over large parts of the continental shelf during the Miocene. Prograding shelf geometries within Middle to Late Miocene sediments support this theory. An unconformity in the Miocene is associated with a strong compressional event leading to flexural doming and inversion of older depocentres on the shelf. Basin scale tectonic movements are the possible causes for both, the unconformity and the compressive movements. An Early Pliocene flooding event shifted the locus of sedimentation in an eastward direction, and the Molo Formation was the first sedimentary unit deposited onto this surface. A marked shift in the prograding style occurred in mid Pliocene, and Late Pliocene/Pleistocene glacial sediments prograded westward as continental ice sheets expanded onto the shelf. Once glacial conditions were established on the shelf, the glacial drainage pattern followed bedrock bondaries and older structural features in the subsurface.

Magnetic expression of salt diapir-related structures in the Nordkapp Basin, western Barents Sea

High-resolution aeromagnetic surveys in the Nordkapp Basin, western Barents Sea, demonstrate the capability of modern, high-resolution aeromagnetic surveys to provide an efficient and promising tool for mapping features related to salt diapirism. Salt diapirs are clearly visible by a small, low-amplitude negative round to ellipsoidal magnetic pattern. This pattern coincides with shallow sedimentary layers deformed by the rising salt during active and passive diapirism. The dimensions of these features coincide in shape and size with those interpreted on gravity and seismic surveys.

Improved salt imaging in a basin context by high resolution potential field data: Nordkapp Basin, Barents Sea

ABSTRACT The seismic imaging of salt diapirs in the Nordkapp Basin gave rise to considerable problems in defining their shape and volume. Independent information was added by integrating the interpretation with high resolution gravity and magnetic data. We developed a novel, iterative workflow, separated into sub‐categories: sediments, salt structures, basement and Moho. Distinctions between the sources of the anomalies from different depths was achieved by utilizing the different decay characteristics of gravity, gravity gradiometry and high resolution magnetic anomalies. The workflow was applied to the southern part of the Nordkapp Basin. It started with the sedimentary model derived from seismics, populated with measured densities and magnetic susceptibilities and a starting model for the base salt. The residual after the removal of this model was interpreted in terms of a crustal model, including flexural isostatic calculations for the Moho with the sedimentary load. The residual after the removal of crustal and early sedimentary model was used to tune the salt model. As these major and minor modelling steps depend on each other, an iterative process was applied to stepwise improve the density and magnetic susceptibility model. The first vertical gradient of gravity and the magnetic field were found to give most information about the cap rock of the diapirs. The improvement in salt imaging, integrated with results from controlled‐source electromagnetic and magneto‐telluric modelling is shown for the salt diapir Uranus, where a well, terminated in the salt, constrains the minimum of the depth to base salt.

Quaternary lithology and shallow gas from high resolution gravity and seismic data in the central North Sea

High resolution marine gravity data and 3D post-stack seismic data from the central North Sea have been jointly interpreted. The accuracy of the gravity data allowed the detection of density contrasts related to a Quaternary sub-glacial melt-water channel, a shallow gas accumulation and a Tertiary gas chimney. The combination of gravity and seismic data is shown to particularly improve the detection of the shallow gas accumulation. The interpretation included visual correlation of gravity images and seismic data performed in a seismic workstation environment, as well as 2.5D gravity modelling along selected seismic profiles. The successful application of this method on shallow targets requires a limited complexity of the shallow strata as well as targets defined by a distinct density contrast and a reasonable size. Data requirements include high-resolution bathymetric and free air gravity data. Bouguer gravity data, which are commonly used in exploration, cannot be used here as densities may vary within the uppermost layer below the sea bottom.

Combined seismic inversion and gravity modeling of a shallow anomaly in the southern Barents Sea

A combination of prestack elastic inversion with gravity modeling has been applied to a shallow seismic anomaly in the southern Barents Sea. The anomaly is crosscutting dipping layers of late Paleocene age. Earlier seismic interpretations indicate a possible origin in gas‐hydrated sediments trapping underlying free gas. Our interpretation includes seismic inversion of a seismic model which consists of a stack of isotropic, homogeneous, and anelastic layers. The unknown parameters are the P‐ and S‐wave velocities, density, and thickness for each layer. As densities are poorly determined by this method, we included gravity modeling, which improved the density estimates. The resulting parameters have been integrated by rock physics calculations and the knowledge of typical attributes of gas hydrate occurrences. The top of the seismic anomaly is interpreted as a reflection from the base of a gas‐hydrated sediment trapping underlying free gas. The base of the anomaly is the gas‐water contact, which deviates fr...

Remote sensing methods in offshore exploration

Abstract The aim of modern remote sensing methods is to enhance variations in digital data sets by applying various numerical filtering techniques and statistical calculations in order to enhance features of interest in the available data. However, a necessary condition for this approach is the availability of an advanced and user-friendly remote sensing system with the possibility to perform interactive processing and “on-the-screen” interpretation. In the present work, a remote sensing system from International Imaging Systems (I2S) was used to interpret geophysical data sets from the Barents Sea region (both offshore and onshore). Features interpreted from the data sets are integrated with other data sets in order to obtain information about the co-variation of features having geological significance. These gravimetric data were used to identify Palaeozoic structural elements in the Barents Sea. Gravity data based on satellite altimetry is also presented and compared with conventional gravity data. The present results indicate that main structural elements can be defined more precisely from the processed data than from ordinary gravity contour maps. An additional benefit of such processing is the detection of subtle trends in the gravimetric field.