Salt Plug Siegelsum in the Northwest-German Basin
P. Lukic, W. Maschek, G.H. Bachmann, 1983. "Salt Plug Siegelsum in the Northwest-German Basin", 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 Northwest-German basin is the classic area of salt plugs (Figure 1). So far more than 200 of them have been discovered (Jaritz, 1973). Thus the "halokinesis," (the processes connected with the isostatic movements of salt; Trusheim 1957, 1960) determines to a large extent the distribution of sediments and the structural geology of Northwest Germany.
Round salt plugs (German: Salzstock) in Germany are 2 to 8 km (1.2 to 5 mi) in diameter. Other salt plugs average 4 to 5 km (2.5 to 3 mi) in width and range from 10 to 20 km (6 to 12 mi) to more than 100 km (62 mi) long. They are sometimes also referred to as salt walls (German: Salzwall). As pointed out by Trusheim (1957, 1960), salt walls are especially common in the deepest part of the basin, where the salt is the thickest. Most German salt plugs consist of the Upper Permian Zechstein salt, although some in the center of the basin also contain Lower Permian Rotliegend salt.
The economic value of the salt plugs is considerable. They are mined for both halite and potassium salt, and they are suitable as artificial solution caverns for the storage of crude oil and natural gas, and are intended for the dumping of hazardous waste. Rising salt plugs have also formed traps for oil and gas.
The salt plug is located near to the Dutch-German border, northeast of the city of Emden. It is 1.5 to 4 km (1 to 2.5 mi) wide and 50 km (31 mi) long stretching from north to south. The seismic line crosses the salt plug in its southern part from northwest to southeast.
The deepest strong reflection in the section at about 2.5 ms corresponds to the base of the Zechstein (i.e. the boundary Upper/Lower Permian). This very distinct seismic marker can readily be correlated and mapped in Northwest Germany. It is dislocated by faults, but not affected by halokinesis. The reflection is of utmost importance for the exploration of the pre-Zechstein, as normally no distinct and correlative reflections occur below it.
The mapping of the base Zechstein reflection encountered a low-relief anticline of 150 to 250 m (492 to 820 ft) closure under the salt plug. in the time section it appears exaggerated by the velocity pullup of the salt. A wildcat discovered a gas field in the Rotliegend red beds. Several other Rotliegend gas fields are known nearby; the biggest is the Dutch Groningen field some 25 km (15 mi) to the southwest.
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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.