D. Ham, 1983. "The Central Graben, North Sea", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The Central Graben of the North Sea is an elongate graben about 70 km (43.5 mi) wide extending north-northwest to south-southeast for some 400 km (248.5 mi) midway between the British and Norwegian Coasts (see location map). The main graben is of Late Jurassic age but an earlier Permian basin is recognized as a broader feature extending westward into the Forth Approaches Embayment and eastward into the Norwegian Danish basin.
Line AA which runs east-northeast to west-southwest, perpendicular to the western edge of the main part of the Central Graben, is one of a series of lines shot by British Petroleum (BP) in 1981 to provide high quality regional data in the North Sea. They were shot with a high multiplicity of cover, 60-fold, and a large air gun array to bring out the deep data. The interpretation was done on the migrated section (which is shown at half the horizontal scale of the stacked section). The reduced scale makes angular relations easier to recognize, and the migration has improved the resolution, (for example, minor faulting on Horizon F between SP's 1000 and 1400).
No depth section is presented, but a depth scale has been added to the key section. The single velocity-time curve used for conversion is adequate for the central and eastern part of the section, but it underestimates the depths to the west of the major fault.
There were several stages in the development of the Central Graben:
1. Subsidence during the Permian allowed the formation of a much wider evaporite basin, containing dolomite, anhydride, and halite sequences. Thick sequences of mudstones and thin sandstone were deposited in the Triassic.
2. Thermal uplift, centered about a triple junction at the northern end of the graben, occurred in the Early Jurassic.
3. The graben system was formed in the Middle to Late Jurassic when crustal collapse initiated a period of faulting and subsidence. The large scale erosion which took place in the previous episode, continued on the higher parts of the structure, and considerable thicknesses of mudstones and marginal sandstones were deposited in the rifts.
4. At the end of the Jurassic there was further tilting and faulting before the deposition of a thick sequence of Early Cretaceous calcareous mudstones. There was also minor halokinesis at this time.
5. Again, in the Late Cretaceous, there was widespread deposition of limestones and calcareous mudstones. Movement was concentrated on adjustment across existing major faults.
6. Rapid subsidence of the Central Graben resumed in the Tertiary, the earlier carbonate sedimentation being replaced by terrigenous clastics. There was a recurrence of salt movement which occasionally developed into piercement structures.
The graben edge is defined on the section by the major fault near SP 1900 which is downthrown to the east by over 5 km (3.1 mi) at Base Zechstein level (Horizon H), but by little more than 1 km (0.6 mi) at Base Cretaceous level (Horizon E).
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.