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Book Chapter

The Central Graben, North Sea

By
D. Ham
D. Ham
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Published:
January 01, 1983

Abstract

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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Contents

AAPG Studies in Geology

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
A. W. Bally
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American Association of Petroleum Geologists
Volume
15
ISBN electronic:
9781629810188
Publication date:
January 01, 1983

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