Mollisol diapirism
The Mollisoldiapirismus (mollisol = active layer; διαπείρειν diapeirein = gr. For "permeate"), partially also Kohlediapirismus mentioned, is an operation close to the ground substrate, the acting through longer freezing and subsequent thawing of water during the ice age was caused. He is best known from several opencast mining outcrops in Saxony , Saxony-Anhalt and Thuringia . The lignite forms diapiral to dome-like bulges, which were created by the coal's water saturation and subsequent ascent. The properties of mollisol diapirism are largely similar to those of halokinesis , the movement of salt underground.
Occurrence and research history
Mollisol diapirism is a phenomenon called cryoturbate , that is , changes in the subsurface caused by freezing and thawing processes under the influence of cold-time conditions. It occurs in areas with extensive coal distribution , which on the one hand lies at a relatively shallow depth below today's surface (around 20 m), but on the other hand is overlaid by repeatedly changing sediment overburden. Especially in Saxony and Saxony-Anhalt, but also in Thuringia , these conditions are partially met. A large part of this region was also covered several times by glaciers in the recent geological past in the Pleistocene . Such subsurface formations were observed for the first time here at the beginning of the 20th century in open-cast lignite mining outcrops in the Leipzig lowlands , especially in the Weißelster Basin and in the Geiseltal . The first more precise descriptions in the form of mostly unrelenting static coal pressings were made in the mid-1950s by Otfried Wagenbreth based on observations in the Profen opencast mine near Zeitz in what is now Saxony-Anhalt. Lothar Eißmann was able to provide a comprehensive explanation of the origin of these pressings in 1978 .
process
Under normal circumstances, lignite , which is mainly found in the deposits in central Germany, has a specific density of 1.15 g / cm³. This is slightly lighter than with clastic sediments such as gravel , sand , silt or clay , where it varies between 1.8 and 2.1 g / cm³. However, the loading pressure of the cover layers on the coal, mostly consisting of clastic sediments from the last glacial periods, is not sufficient to lead to a plastic deformation of the solid coal banks, neither is the high water content of 50 to 60%, which is below is given for both the Geiseltal and the Profen opencast mine.
During the cold periods of the Pleistocene (2.5 million to 11,000 years ago) there were multiple advances from the inland ice from north to south, during which longer-term glacial and periglacial conditions also set in in what is now Central Germany. In periglacial (Eisvorland) passed permafrost , ranging up to 400 meters deep, in direct ice cover still several tens of meters. In the course of this, not only pore ice formed in the originally solid coal seams, but also ice veins and ribbons up to massive ice bodies, which, due to the permeability of the lignite, also resulted in a large amount of water being withdrawn from the environment. The freezing of the water in the coal broke the original grain structure. This led to a comprehensive overprinting of the coal, so that what was once a solid to lumpy coal seam was turned into a rather small-grained substrate made up of only 1 mm particles on average. The reduction of coal from one lump to one granularity exceeded a factor of 10 in relation to each other. In the subsequent thawing process at the end of the glacial period and in connection with the retreat of the glaciers to the north, the permafrost areas also dissolved. This resulted in a high water saturation of the coal in the thawed soil (Mollisol) - caused by the melting of the ice in the coal and by groundwater from the melting of the glaciers - which, due to the previous breaking (freezing) of the grain structure, turns it into a liquid (queasy) state shifted, which caused the complete destruction of the structure of the coal by loss of the internal angle of friction and the shear strength .
The serious changes within the near-surface brown coal during and at the end of the cold periods under the influence of the permafrost and then the liquid water led to a gradual activation of the Mollisol diapirism. Initially, small coal bulges formed, which arose during the glacial period due to the ice formation in the seams and the associated expansion of the volume . These possibly resembled the palsas made of moorland and peatland in today's permanently frozen areas. The actual Mollisol diapirism only started with the thawing process and the breakdown of the permafrost. Due to the now significantly higher density of the overlying sediments, they sagged down in the thawing soil (Mollisol) and displaced the lighter and, due to the water saturation, now pulpy coal, which had to flow off to the side. The lighter coal pulp then rose in weak zones and formed diapiral to dome-like coal bulges, which in some cases also pierced the slopes or were partially exposed on the surface through later erosive processes. Furthermore, the sagging of the heavier deck sediments caused the formation of sometimes very extensive marginal depressions, which are laterally bounded by the coal diapirs. The coal rise was completed with the end of the settlement movement of the overlying sediments ( isostatic compensation ) or with the drying up of the coal supply underground. Since the Mollisol diapirism took place in a larger part of the near-surface coal seam, laterally staggered bulges formed in various regions, such as the Geiseltal, and thus led to the formation of characteristic coal ripples with depressions in between. In some outcrops, such as Profen, however, it was observed that individual coal diapirs had been activated during several successive cold phases. Overall, this autoplastic-gravitationally induced process resembles the salt movements in the subsurface ( halokinesis ). In the development process, however, it differs significantly from similar, diapir-like charcoal deformations in the subsurface, which are caused by ice load and ice advancement, as they have become known from the neighboring area to the north, such as the Muskau folds .
meaning
The coal, which is diagetically changed by the influence of the Ice Ages , is of no economic importance, as the powdery substrate is not suitable for briquetting . Because of its nature, miners also call it “coffee grounds charcoal”. However, the marginal depressions created by Mollisol diapirism are of great scientific importance. Sediments were deposited in these during the warm periods occurring between the cold periods . Since these sediment traps did not completely erode during the subsequent glacial periods, fossil remains of the flora and fauna of the Pleistocene have often been preserved. These basin structures thus represent important geological and palaeontological information stores.
Individual evidence
- ^ Johannes Weigelt: The coal pressings in the Geiseltal mines "Leonhardt", "Pfännerhall" and "Rheinland". Yearbook of the Hallesches Verband NF 7, 1928, pp. 68–97
- ^ Otfried Wagenbreth: Quaternary geological observations in the area of the Profen opencast mine near Zeitz. Freiberger Forschungshefte C, 21, 1955, pp. 40-92
- ↑ a b c d Lothar Eißmann: Mollisoldiapirism. Journal for Applied Geology 24 (3), 1978, pp. 130-138
- ↑ a b c d Matthias Thomae and Carsten Sommerwerk: On the origin of the Neumark-Nord site (Geiseltal). In: Harald Meller (Hrsg.): Elefantenreich - Eine Fossilwelt in Europa. Halle / Saale 2010, pp. 39–44
- ↑ a b c Matthias Thomae: Mollisoldiapirism - cause of the preservation of the Neumark-Nord site (Geiseltal). In: Jan Michail Burdukiewicz, Lutz Fiedler, Wolf-Dieter Heinrich, Antje Justus and Enrico Brühl (eds.): Knowledge hunters . Festschrift for Dietrich Mania. Halle / Saale, 2003, pp. 601–605
- ↑ Manfred Kupetz: Geological structure and genesis of the shrub moraine Muskauer fold arch. Brandenburg Geoscientific Contributions 4 (2), 1997, pp. 1–20
- ↑ Jaqueline Strahl, Matthias R. Krbetschek, Joachim Luckert, Björn Machalett, Stefan Meng, Eric A. Oches, Ivo Rappsilber, Stefan Wansa and Ludwig Zöller: Geology, paleontology and geochronology of the Eem basin Neumark-Nord 2 and comparison with the basin Neumark-Nord 1 (Geiseltal, Saxony-Anhalt). Quaternary Science Journal 59 (1-2), 2010, pp. 120-167