Sack trough

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Relief map of the area around Alfeld an der Leine with A : Hilsmulde and B : Sackmulde (with seven mountains , foothills and sack forest )

The trough is a geological structure a good 20 kilometers long and up to six kilometers wide north to southeast of the city of Alfeld in southern Lower Saxony in Germany. From a tectonic point of view, it represents a syncline (trough), the axis of which runs in a northwest-southeast direction roughly parallel to the Leine River .

The sack trough is named after the Alfeld district of Sack, which is roughly in the middle of the trough .

geography

Geographically, the sack hollow in the Hildesheim district is part of the Leinebergland or the northwestern Harz foreland . In terms of landscape, it is divided into the three wooded mountain ranges Seven Mountains (up to 395  m above sea  level ), foothills (up to 353  m above sea level ) and Sackwald (up to 374  m above sea level ), which geologically, however, form a unit. The center of the sack trough around the village of Sack is about 140  m above sea level. NN up to 260  m above sea level NN significantly lower than the surrounding mountains and is mainly used for arable farming.

The sack trough is crossed by the two country roads  469 and 485, which connect Alfeld via Langenholzen and Sack with the towns of Adenstedt and Sibbesse to the east .

geology

The area of ​​the sack trough is part of the subhercyneal basin and consists of a halokinetic colored sandstone vault that extends northwest-southeast , the flanks of which are formed by layers of shell limestone and Keuper layers . The rise of the Zechstein salts in salt domes and the sinking of the areas in between in the Leine area gave rise to the saddle and trough structure that exists today .

morphology

The morphology of the area is essentially determined by the hardness and weathering competence of the rock in the subsoil. The storage conditions of the rock layers resulted in a layered landscape with Hercynian (northwest-southeast) ridges and valleys. The mountain ranges are formed by the rocks of the lower and middle red sandstone , the upper shell limestone and certain rocks of Cretan age, in particular Hilssandstein and flame marl .

The ridge-forming rocks are composed of sandstones or limestones . The valleys between these mountain ranges are asymmetrical due to the collapse of the layers . They are formed by the rocks of the Upper Buntsandstein , the Keuper and the Jura . These are clayey sediments . In addition to valley cuts and ridges, various planed areas can be made out. These can be found in the area of ​​the Middle Muschelkalk and the Minimuston .

Geological and paleogeographical development

Typical handpiece of the flame marl in the sack trough

The geological development history of the area is closely linked to the events in the Germanic Basin . The strata in or near by begin with sediments of the Permian age.

These are deposits from the saltwater cycle of the Zechstein Sea . Depending on the degree of evaporation , claystones , limestones, dolomites , anhydrites , rock salts or potash salts were deposited in a cyclical sequence. During the Mesozoic era , the sack trough was in the northern edge of the Germanic Basin. The deposit area belonged to the Hessian depression between the Hunte threshold and the Eichsfeld threshold .

The Triassic (251.0 ± 0.4 to 199.6 ± 0.6 Ma ) sediment sequences begin with the red sandstone. During the red sandstone there was extensive deposition of siliciclastic sediments in northern Germany . After the final evaporation of the Zechstein Sea, up to 1500 m of red-colored silica plastic were deposited in northern Germany. The sedimentation environment during the red sandstone was arid to semi-arid and the sedimentation occurred cyclically. These cycles can sometimes be correlated with one another over several hundred kilometers . The lower red sandstone consists of a sandstone-mudstone alternation and was deposited in a large, undivided basin. The pouring took place from the surrounding high areas of the Harz Mountains as well as from the Rhenish and Bohemian Massif , mainly from the south and south-east.

The middle red sandstone is divided into 4 cycles, each starting with a coarse sandstone and becoming finer-grained towards the hanging wall. Following WURSTER (1964), these are of limnic and fluvial origin.

During the red an epirogenic lowering of the sedimentation basin took place. Similar to the Zechstein, an epicontinental shallow sea formed within which evaporites ( red saline ) were deposited . The red clays were deposited during the Upper Röt, and the transition to the fully marine shell limestone facies began . The direction of the pouring did not change during the Middle and Upper Buntsandstein.

The sediments of the shell limestone are divided into lower, middle and upper shell limestone. At the beginning of the shell limestone, a transgression through the Upper Silesian Porte created shallow marine conditions. The Lower Muschelkalk is characterized by a cyclical alternation of chalky and marly sediments. The individual cycles begin with a calcareous conglomerate , followed by an alternation of limestone and marl, and dolomitic yellow limestones follow in the hanging wall. These cycles reflect a change in marine, lagoon and hypersaline conditions, which can be traced back to cyclical changes in water depth and thus ventilation, salinity or the connection to the open sea as well as climate changes.

During the Middle Shell Limestone , marl, dolomites and gypsums were deposited. The sedimentation milleu ranged from lagoon to hypersalinar. The Upper Muschelkalk, the lithological further into Trochiten- and Ceratitenkalk may be broken, dumped the soil of the Germanic Basin, making the connection to the Arctic Ocean interrupted and instead of this, the opening of the Belfort Gap was initiated. This led to a Tethyal impact on the area. A transgressive phase improved aeration and salinity of the water column and pure fossil-rich limestone was deposited. At that time , the sea ​​area was divided by thresholds that created reef-like shallow water areas within which the trochitic limestone formed. After the sedimentation basin was lowered, the ceratite limestone was deposited.

The turn from Muschelkalk to Keuper is marked by a regression phase; continental sediments increasingly formed again . The Keuper deposits were poured into the sedimentation area from the north, which WURSTER (1964) associates with tectonic shifts on the edge of the Bohemian Massif. While sand and claystone formations predominate in the Upper and Lower Keuper, the Middle Keuper is characterized by gypsum and marl. This suggests arid conditions in the Middle Keuper. The deposit area is characterized by an extensive delta system , within which ingressions have also led to lagoon and brackish conditions. With the addition of silica plastic from the hinterland, the proportion of carbonate sediments is reduced.

The beginning of the Jura (199.6 ± 0.6 to 145.5 ± 4.0 Ma) is characterized by a transgression that restored fully marine conditions in the deposit area. The North German Basin formed a shallow shelf sea , which on the Hessian road to the southern Jurassic sea was connected. In Lias it came accordingly to deposit glaukonitführenden clays steps to HARMS (1984) Hettangian , Sinemurian and Pliensbachian belong. In areas with a low-oxygen environment, fine-grained siliciclastics with high proportions of fossils and high levels of organic material were deposited , from which the well-known black slate of the Lias would later form with continued diagenesis . During the Malms , the Hessian Basin was closed and the Lower Saxony Basin was formed , which was preserved until the Lower Chalk . Dogger and Malm are not represented in the sack trough.

The chalk deposits (145.5 ± 4.0 to 65.5 ± 0.3 Ma) are discordant on the sediments of the Triassic and Jurassic ages. At the onset of the Lower Cretaceous, clayey formations still predominate, which are replaced by coastal sands ( Hilssandstein ), followed by flaming marls , which are made up of clay to marl stones, which can be chalky, pebbly and glaconite-bearing. During the Albs or Cenomans in particular, the sea transgressed from the north via the Triassic and Jurassic deposits into the western Harz foreland .

The Upper Cretaceous in turn is characterized by fully marine formations. This is where the very pure coccolith limes ( Weißpläner ) appear.

During the Tertiary (65.5 ± 0.3 Ma to 2.588 or, depending on the view, 1.806 Ma) the sea receded from the area, the deposit area was now on the edge of the epicontinental sea, which covered large parts of northern Germany. Tertiary sediments are hardly or not at all to be found in the area of ​​the sack trough, they were removed .

The Quaternary is characterized by the alternation of glacials and interglacials . The various ice advances left extensive deposits of thin loose rock , which can only be found in parts of the area, as well as Aeolian loess deposits , which are primarily to be found in the windless valley cuts of the area.

tectonics

The area is part of the northern Harz foreland. This is characterized by Hercynian stroking hollows and saddles. The tectonic elements originated in the Upper Jura during the Alpid orogeny . This development was influenced by the migration of Zechstein salts from the area of ​​the sack trough into the Leinetal saddle. ABDALLAH (1962) writes that it is not a depression in the tectogenetic sense, but a negative bump. This can be explained as a salt emigration basin from which the salt penetrated into the fracture zone of the Leinetalattels and rose there. The ascent of the Zechstein salts began in the Keuper and arched the overburden . In the Upper Cretaceous, the evaporites broke through the overlying post-Permian sediments.

According to JORDAN (1989), the formation of the Leinetalattel goes back to a regional disturbance of the Mesozoic overburden. The area northeast of the fault shifted to the area to the southwest. The jump height is 800–1000 m. The Zechstein salts rose along this fault zone, i.e. the weak zone created in this way in the overburden.

literature

  • FJ Harms: Explanations for sheet No. 4025 Freden . Ed .: Lower Saxony State Office for Soil Research. Hanover 1984, p. 1-168 .
  • Dierk Henningsen, Gerhard Katzung: Introduction to the geology of Germany . 6th edition. Spektrum Verlag, Heidelberg 2002, ISBN 3-8274-1360-5 .

Individual evidence

  1. ↑ a b c C. Hinze: The upper Buntsandstein (Röt) in the mountainous region of southern Lower Saxony . In: Geological Yearbook . tape 84 . Hanover 1967, p. 637-716 .
  2. ↑ A. Herrmann: Epirogenic movements in the Germanic red sandstone basin and their significance for lithostratigraphic parallelization between northern and southern Germany . In: Geological Yearbook . tape 81 . Hanover 1962, p. 11-72 .
  3. ↑ J. Wolburg: Sedimentation cycles and stratigraphy of the red sandstone in northwest Germany . In: Geotectonic Research . tape 14 . Stuttgart 1961, p. 7-74 .
  4. ↑ P. Wurster: Crustal movements, sea level fluctuations and climate changes of the German Triassic . In: Geologische Rundschau . tape 54 . Stuttgart 1964, p. 224-240 .
  5. ↑ H. Boigk: On the structure and facies of the red sandstone between Harz and Emsland . In: Geological Yearbook . tape 76 . Hanover 1959, p. 597-636 .
  6. ↑ a b M.G. Schulz: Fine stratigraphy and cycle structure of the Lower Muschelkalk in N-Hessen . In: Communications from the Geological-Paleontological Institute of the University of Hamburg . tape 41 . Hamburg 1972, p. 133-170 .
  7. ↑ A. Kumm: Trias and Lias . In: A. Kumm, L. Riedel & W. Schott (eds.): The Mesozoic in Lower Saxony. 1st department . Oldenburg 1941, p. 1-328 .
  8. ↑ JP Groetzner: Stratigraphic-facial investigations of the Upper Muschelkalks in southeastern Lower Saxony between Weser and Oker . Dissertation. Ed .: Technical University of Braunschweig. Braunschweig 1962, p. 1-124 .
  9. ↑ a b F. J. Harms: Explanations on sheet No. 4025 Freden . Ed .: Lower Saxony State Office for Soil Research. Hanover 1984, p. 1-168 .
  10. ↑ S. Keller: The upper chalk of the sack trough near Alfeld . In: Geological Yearbook . Volume A64. Hanover 1982, p. 3-171 .
  11. ↑ AM Abdallah: The Leine valley axis between Seesen and Alfeld . In: Roemeriana . tape 4 . Clausthal-Zellerfeld 1962, p. 1-52 .
  12. ^ H. Jordan: Geological hiking map Leinebergland 1: 100000 . Ed .: Lower Saxony State Office for Soil Research. 2nd Edition. Hanover 1989.

Coordinates: 52 ° 0 ′ 19.8 ″  N , 9 ° 51 ′ 57.8 ″  E