A Salt Pillow Structure in the Southern North Sea
P.F. Owen, N.G. Taylor, 1983. "A Salt Pillow Structure in the Southern 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
Download citation file:
This note describes the major features of a seismic section recorded over a Zechstein salt pillow in the southern North Sea. We made an attempt to explain these in terms of the geological history of the area.
The seismic data were acquired and processed in 1979 as part of a regional survey to give improved data at the deeper levels. in this example the migrated version of the line was used for the interpretation because it shows the detailed structure more clearly than the normal stacked section. The features illustrated are: (1) the asymmetric shape of the pillow; (2) listric faulting on the northeast side of the pillow wall; and (3) the displacement of the crest of the Base Tertiary reflector which does not lie immediately over the crest of the Triassic.
Permian — in this part of the salt basin some 500 m (1,640 ft) of halite were deposited with the dolomites and anhydride of the Upper Permian (Zechstein). The Zechstein evaporites overlie continental sediments of Permian age, and Carboniferous Coal Measures. We have termed these earliest units "Basement" on Figure 4, since they do not affect the subsequent structural history.
Triassic — Regional studies show that the Early Triassic deposits, mainly mudstones, were evenly distributed. This section shows that: (1) over 1,000 m (3,281 ft) of Triassic sediments were deposited by the Late Triassic; and (2) the youngest Triassic sediments thin by 150 m (492 ft) over the present day pillow.
This indicates that halokinesis could have begun as early as the Late Triassic. A 1,000 m (3,281 ft) overburden is probably sufficient to make underlying salt unstable (Trusheim, 1960). Alternatively, movement of the basement horst feature may have itself induced salt pillowing above it.
Jurassic — There is no evidence of thinning in the Lower to Middle Jurassic interval against the pillow, and therefore it is unlikely that there was any growth of the pillow at this time.
Between the Middle Jurassic and Lower Cretaceous, two closely related processes occurred: (1) major growth of the salt pillow; and (2) listric faulting on the northeast flank.
Figures & Tables
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.