Mainz Basin
The Mainz Basin is a fossil sedimentary basin in Rheinhessen , the filling of which mostly dates from the later Tertiary , around 31 to 19 million years ago ( mya ). In paleogeographical terms , the Mainz Basin was the bay of an arm of the sea during the Oligocene, which briefly connected the North Sea, which then stretched significantly further south, with the western Paratethys , a side sea of the already largely closed western Tethys .
location
|
|
|---|---|
| Geological map of Germany with identification of the Mainz Basin (red circle). |
The Mainz Basin is mostly located in the east of the state of Rhineland-Palatinate , to the left of the Rhine , southwest of the wide arch that the river describes near Mainz , the city that is also the namesake of the basin. Only the extreme north and northwest lie to the right of the Rhine. The basin is roughly a triangle with the corners of Bingen , Hofheim am Taunus and Bad Dürkheim . Geographically and in terms of natural space , the part of the Mainz basin on the left bank of the Rhine largely corresponds to the Rhine- Hessian table and hill country .
Geological framework
The earth's crust of the west and south of the Eastern European craton ( Baltica ) is traversed by a system of Cenozoic rift fractures , preferably north-south , which extends from the North Sea via western Germany and eastern France to the south to the mouth of the Rhone and further to the west Mediterranean stretches. In its south-west German section (between Mainz and Basel) it finds expression in the Upper Rhine Rift , the collapse of which began in the early Tertiary, about 50 million years ago ( Eocene ).
One possible cause of the collapse is an expansion of the earth's crust, which took place as compensation across the compression that the Central European crust experienced due to the collision between Africa and Europe (see formation of the Alps ). The thinning of the crust in the course of the expansion manifested itself in the sinking of an elongated clod, i.e. the formation of a trench structure north of and across the orogenic front of the Alps. The amount of the lowering, which continues to this day, amounts to a total of a few kilometers. Streams and rivers cut simultaneously into the clods (trench shoulders) rising more or less simultaneously to the east ( Black Forest , Odenwald ) and west ( Vosges , Palatinate Forest ) of the Upper Rhine Graben, and the erosion carried away rocks with a thickness of about 2000 m, so there today relatively large areas of the Variscan, folded basement come to light. The eroded masses of rubble were meanwhile poured into the Upper Rhine Trench and buried the Mesozoic layers of the South German table located there. Along the central axis of the trench, these sediments are over 3000 m thick . In some of the faults deep into the crust along which the tectonic movements occurred, magma was able to rise to the surface of the earth, which led to the formation of volcanoes . B. about 18 million years ago ( Miocene ) in the area of the Kaiserstuhl . The continuing movements of the earth's crust in the Upper Rhine Rift are still expressed today in numerous smaller earthquakes . The subsidence rate of the Upper Rhine Rift is still 1 mm per year, the rate of uplift of the trench shoulders is 0.5 mm per year.
The Mainz Basin is a western side basin of the northern section of the Upper Rhine Rift, which is no longer subsided. Strictly speaking, it lies on the western shoulder of the trench in the north of the Upper Rhine Rift, at the intersection of the Upper Rhine Rift with the Upper Paleozoic Saar-Nahe Depression . The Mainz Basin experienced a significantly smaller subsidence than the actual rift region, so that sediment thicknesses of hardly more than 500 m are reached there.
Sequence of layers and paleogeographical development
Basement
The pre-tertiary subsoil of the Mainz Basin consists predominantly of Permian ( Rotliegend ) rhyolites and siliciclastics from the Saar-Nahe Depression , which emerge to the west of the Mainz Basin and continue eastwards in the subsurface of the northern Upper Rhine Rift. The tectonic structure of its subsoil also determines the internal division of the Mainz Basin into a northern part ( Rüsselsheim sub-basin ) and a southern part ( Eisenberg sub-basin or Marnheim Bay ). The two basin parts are separated by the so-called Alzey-Niersteiner-Horst , which is the eastern continuation of the Palatinate saddle of the Saar-Nahe valley, which in turn represents the easternmost section of a threshold running along the central basin axis (the eastern one, also in the northern Upper Rhine Graben and The part of this threshold detected in the Hanau basin is also referred to as the Palatinate-Stockstädter threshold together with the Alzey-Niersteiner Horst ). Correspondingly, the sediment thickness in the area of the Alzey-Niersteiner Horst is very low, and at Nierstein layers of Rotliegend emerge on the slopes above the Rhine ( Nierstein formation ). To the north, the Tertiary of the Mainz Basin extends slightly over the Hunsrücksüdrand Fault to the folded older Palaeozoic of the Rhenish Slate Mountains (including colorful slate, Taunus quartzite ). In the far south it is also underlain by layers of red sandstone and shell limestone . The lack of layers from the younger Mesozoic and often the older Tertiary indicates that the Mainz Basin must have been subject to intensive erosion, at least in the late Cretaceous and in the earliest Tertiary.
Tertiary
Pre-Oligocene
The oldest Cenozoic sediments in the Mainz Basin come from the older Eocene or possibly even from the later Paleocene (> 56 million years). It is local gravel , sand and fine-grained material as well as thin lignite that was deposited by flowing and standing waters. In the south of the basin these are the impure sand and clay layers as well as the layers of the Eisenberg group ( older Eisenberger clay sequence , Eisenberger green clay / green marl / Lymnaeen marl , sandy Eisenberger clay sequence / sandy Eisenberger cover clays , Eisenberger adhesive sand , Ebertsheim formation ). The older Eisenberger tone sequence and the Lymnaeenmargel are also combined with the same old and similarly formed deposits in the actual Upper Rhine Graben to form the Haguenau group . In the north, only one unit, the base clay and base sand (also Eocene base clay ), is separated out for this interval, which includes sediments of different colors , predominantly silty and clayey, partly also gypsum-bearing. All of these rocks do not bite out superficially, but are only known from drilling. The mainly clayey northern counterparts of the part already the early Oligozän ( Rupelian provided) younger layers of Eisenberg group (i. E. Above the Lymnaeenmergel) together with similarly formed deposits in the actual Oberrheingraben to Pechelbronn group summarized. Here the middle Pechelbronn strata are the most powerful and most widespread.
Oligocene
|
|
|---|---|
| Paleogeographic map of Central Europe for the Rupelium with identification of the Mainz Basin (red circle). |
After the Lymnaeen layers in the higher Eocene and the middle Pechelbronn layers or the Ebertsheim formation in the earliest Oligocene already had a weak marine influence in the Mainz Basin (First Rupel transgression) at the time of deposition of the foraminiferous and glauconite layers, one occurred during the Rupelium Significantly stronger global ( eustatic ) rise in sea level (second Rupel transgression, Rupel transgression i. e. S.). The sea penetrated, probably from the north via the Hessian Depression , directly into the Mainz Basin, where it broke against the quartzite rocks on the southern edge of the Hunsrück and further south against rocks made of Kreuznach rhyolite on the coasts of that time. The surf cliff on the Steigerberg at the western end of the Alzey-Niersteiner Horst testifies to this early phase of the Oligocene Sea. At this time, the sandy layers of the Alzey Formation (formerly sea sand) were deposited near the coast , while the clayey-marly to fine sandy layers of the Bodenheim Formation (formerly Rupelton divided into Foraminiferous Marl , Fish Slate and Upper Rupelton ) and the Stadecken Formation ( formerly creeping sand ). The higher part of the Stadecken formation, the creeping sand marl, represents the onset of the Third Rupel Transgression.
Via the Hessian Basin and the Upper Rhine Rift, the marine waters of the Mainz Basin were alternately connected to the then North Sea or the Molasse Sea or the Mediterranean area in the south, at the height of the Rupel Transgression even for a short time with both seas simultaneously. The mean annual temperature at that time was 18 ° C, compared to 10.5 ° C today. The sea and the shoreline would have been comparable to today's Caribbean : subtropical temperatures, sandy bays with palm and laurel trees and crocodiles, sea turtles, manatees and, among many other fish species, numerous rays and sharks in the water. Flamingos, pelicans, rhinos, tapirs, monkeys and elephants lived on land.
Already at the time of deposit of the Stadecken formation, the body of water in the Mainz Basin was increasingly sweetening and brackish conditions replaced the fully marine environment. This development culminates in the middle layers of the Sulzheim Formation (early Chattium ), which are also called freshwater layers (correspond to the Jakobsberg Formation of the southern Mainz Basin, where the name "Sulzheim Formation" is sometimes only used for the brackish deposits below the Freshwater layers are needed which are identical to the Cyrene marl of the older literature). In the upper part of the Sulzheim Formation (sensu lato, corresponds to the lower ceritihi layers of the traditional nomenclature, in the southern Mainz Basin also called Weisenau Formation ), again, brackish conditions arise. In addition, on the northern edge of the basin, starting from the Rhenish Mass, fluvial deposits progress into the basin, some of which are very coarse-clastic and trade under the name Budenheim Formation (formerly milk quartz gravel ). All Oligocene units from the Alzey and Bodenheim Formations up to and including the Budenheim and Sulzheim Formations are also grouped under the name Selztal Group . With the exception of the Alzey and Budenheim Formations, these are predominantly very fine-grained sediments. The extensive absence of coarse-grained deposits in the higher Rupelian and deeper Chattian suggests that the coastlines at that time ran relatively far outside the boundaries of today's Mainz Basin and / or that the surrounding area was only slightly above sea level. Also special is the occurrence of Cretaceous and early Tertiary calcareous foraminifera in the Stadecken and Sulzheim Formation, which may have reached the Mainz Basin from the Alpine region by transporting north along the Upper Rhine Rift.
Miocene
Unlike the sandy to clayey-marl deposits of the Oligocene sea came in lower Miocene and even in younger Chattian predominantly calcareous and marl deposits to sales, the superficial pending in much of today's Mainz Basin and the plateaus and hills of Rheinhessen hill country with give their typical lime flora the character. The corresponding sequence of layers is also referred to as the Limestone Tertiary and lithostratigraphically assigned the name Mainz Group . The calcareous Tertiary facies also occur in the Hanau Basin, but not in the Upper Rhine Graben.
The Mainz Group is subdivided into the Hochheim Formation , which still falls into the upper Chatt (formerly Middle Cerithian Strata ), the Oppenheim Formation , which at least partially falls into the Upper Oligocene (lower part of the sequence formerly known as the Upper Cerithian Strata ), the Oberrad- Formation (upper part of the sequence formerly known as Upper Cerithien layers), the Rüssingen Formation (formerly Inflata layers / Corbicula layers ), the Wiesbaden Formation (formerly Lower Hydrobian layers ) and the Frankfurt Formation (formerly Obere Hydrobien Layers ). The upper layers of the Wiesbaden formation and the Frankfurt formation are the most recent formations only contained in the form of a few erosion relics and do not occur in the southern Mainz basin.
A special characteristic of the so-called Kalktertiärs are coralline algae reefs, in particular at termination of the Mainz basin for Oberrheingraben and especially in the high home formation and in the lower part of the Oppenheim-formation as a barrier reefs occur. In the areas to the west of the barrier and generally in the younger formations of the Limestone Tertiary, stratified limestone and marl stones dominate. Oolithic limestones were formed in very shallow water, and quartz-sand-bearing limestone and marl or even relatively pure sands were deposited near the Paleo coast. Many of the layers are rich in fossils and the delimitation of the formations from one another is partly based on the occurrence of certain fossils, mostly certain species of snails and mussels. Changes in the composition of the aquatic fauna show that the salinity in the deposit area changed relatively strongly over time, from brackish conditions to moderate over-salting. These salinity fluctuations were favored by a temporary constriction of the Upper Rhine Rift, including the Mainz Basin, from both the North Sea and the southern seas.
In the younger Lower Miocene ( Burdigalium ) the sea withdrew from the Mainz Basin and the region has been permanently dry since then. In addition, the west and north of the basin have experienced a moderate uplift since the Middle Miocene. As a result of these events and processes, the youngest layers of the Limestone Tertiary were deeply weathered, karstified and partially eroded. So-called Bohnerztone , clayey weathering residues of the limestone, in which small, bean- or kidney-shaped concretions of “ limonite ” formed. However, from the Middle Miocene onwards, sedimentation took place at least in the river valleys. The most widespread deposits from this period are the gravelly to clayey layers of the Eppelsheim Formation (formerly Dinotheriensand (e) ). They span the period from the Langhium to the Tortonium and represent deposits of the Ur-Rhine . In parts of the southern Mainz Basin, they are represented by limnic-fluviatil layers with an average of finer grains, which are known as the Lautersheim Formation . These contain, among other things, the white Lautersheim clay , which was mined in opencast mines in the 19th and 20th centuries and used for porcelain production. It is possible that the Lautersheim Formation is a little younger than the Eppelsheim Formation and thus equivalent in time to the Dorn-Dürkheim Formation (upper Tortonium). Facially, the latter is largely similar to the Eppelsheim and Lautersheim Formations and is considered to be the youngest Miocene sedimentary unit of the Mainz Basin. However, it is limited to the region around the eponymous municipality of Dorn-Dürkheim .
Post-Miocene
In the subsequent Pliocene (5.3–2.6 mya) weathering and erosion as well as local, predominantly fluvial sedimentation continued. In the north of the Mainz Basin, relatively coarse-grained sediments from the Piacenzium have been handed down, which are called Arvernensis gravel after their type fossil Anancus arvernensis - like Deinotherium an "original elephant". They are considered to be deposits of the Ur-Main. In the south of the Mainz Basin, the Pliocene is represented by white fine sands rich in kaolin, with lenticular inclusions of gravel, coarse sands and clays. These are called Kriegsheimer Sands . Because of the uncertain stratigraphic assignment, they were referred to in older literature as either the White Mio-Pliocene or the White Upper Pliocene .
In the Pleistocene, too, siliciclastic deposits dominate, of which the oldest are only locally distributed. These include the Plio-Pleistocene Weisenau sands and the Cromer period Mosbach sands in the Mainz / Wiesbaden area and the sandy Freinsheim strata in the southern Mainz basin. They are all interpreted as fluvial deposits of the Rhine. In the late Pleistocene, the river sediments became increasingly coarse as the Mainz Basin rises. This also creates river terraces . The Saale (Riss) and in particular the Vistula (Würm) glacial periods are represented by extensive loess deposits. These cover a large area of the tertiary layers, but do not reach a thickness of more than 10 meters. The Lössabfolge is by ground educational horizons and ash deposits of Eifel volcanoes internally divided. The youngest and most powerful ash horizon is that of the Laacher See eruption.
Fossil fauna
The sediments of the Ur-Rhine contain the remains of animals and plants in places, so that today the flora and fauna 10 mya ago in Rheinhessen can be reconstructed very well. These layers are called Dinotheria sands. The name was given after the trunk Deinotherium giganteum ("giant terrifying beast", also known as "tusk elephant", because of its lower jaw incisors formed as downwardly curved tusks), which reached up to 5 m shoulder height. The first fossil of this kind was found near Eppelsheim in the first half of the 19th century. The original is now in the British Museum of Natural History in London. Original casts, like numerous other fossils from the Rhine-Hessian Tertiary, can be seen in the Natural History Museum in Mainz and the Museum Wiesbaden .
A list of mammalian species compiled in 1983 names almost 50 species (e.g. bear , hyena , tapir , ancient horse Hipparion , rhinoceros , saber-toothed cat , antelope , giant sloth ). The 28 cm long thigh bone of the Old World monkey Paidopithex rhenanus, discovered in 1820 near Eppelsheim in the Dinotherien sands, is particularly well-known . He is considered the world's first historical find of extinct representative from the closer relationship of apes .
swell
General
- Kirsten I. Grimm: The Tertiary of the Mainz Basin in the Stratigraphic Table of Germany 2002. Newsletters on Stratigraphy. Vol. 41, No. 1-3, 2005, pp. 347-350, doi: 10.1127 / 0078-0421 / 2005 / 0041-0347
- Peter Schäfer: Sedimentation history and stratigraphy of the tertiary deposits in the southern Mainz Basin (excursion E on April 24, 2014). Annual reports and communications from the Upper Rhine Geological Association, new series. Vol. 96 (Geological excursions in the Palatinate and other scientific articles), 2014, pp. 73-104, doi: 10.1127 / jmogv / 96/0005 (alternative full-text access: ResearchGate )
- Peter Schäfer: Mainz Basin - stratigraphy, paleontology, excursions. Collection of geological guides, volume 79. 2nd completely revised edition, Schweizerbart, Stuttgart 2012, ISBN 978-3-443-15092-1
- Peter Schäfer: Tertiary of the Mainz Basin. P. 200–219 in: State Office for Geology and Mining Rhineland-Palatinate (Ed.): Geology of Rhineland-Palatinate. Swiss beard, Stuttgart 2005, ISBN 3-510-65215-0
- Michael Weidenfeller: Pleistocene of the Mainz Basin. P. 257–263 in: State Office for Geology and Mining Rhineland-Palatinate (Ed.): Geology of Rhineland-Palatinate. Swiss beard, Stuttgart 2005, ISBN 3-510-65215-0
- D. Vogellehner: Paleontology. Freiburg 1987.
Individual evidence
- ^ A b Peter A. Ziegler: European Cenozoic rift system. Tectonophysics. Vol. 208, No. 1-3, 1992, pp. 91-111, doi: 10.1016 / 0040-1951 (92) 90338-7
- ↑ including a part of this system, the Permian Oslo trench , originally known as the Mediterranean Mjösen zone , see Dieter Ortlam: New Aspects of the Cenozoic development in the northern part of the Mediterranean Mjösen zone. Geological survey. Vol. 70, No. 1, 1981, pp. 344-353, doi: 10.1007 / BF01764333
- ↑ P. Schäfer: Sedimentation history and stratigraphy of the tertiary deposits in the southern Mainz basin. 2014, p. 82 f.
Web links
- Of seagrass meadows, coal pigs and Rhine elephants - a journey through time through the Mainz basin - steinkern.de