Surf cliff on the Steigerberg

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Surf cliff on the Steigerberg with the surface layers of the Alzey formation. View from the northwest.

As surf cliff on Steigerberg is in a gravel pit on Steigerberg in Rheinhessen open-minded part of about 30 million years ago ( mya cliff incurred) of the sub-Oligozänmeeres the Mainz Basin called, which is a unique in Europe in this form Geotop concerns.

Geographical location

The disused gravel pit is located in the Rhineland-Palatinate district of Alzey-Worms about 25 kilometers southwest of Mainz near the K5 between the villages of Eckelsheim and Wendelsheim on the western flank of the Steigerberg. The cliff complex extends over an area of ​​approximately 40 × 70 meters at a height of about 15 meters.

geology

Regional geology and stratigraphy

The gravel pit with the fossil cliff complex is located on the southwestern edge of the Mainz Basin , which is a partial basin at the northwestern end of the Upper Rhine Rift. The Upper Rhine Graben and the Mainz Basin were formed as a result of the long-range effects of the Alpid mountainous formation . The lowering of the Upper Rhine Graben and the Mainz Basin began in the Eocene . In the course of the Oligocene and Miocene there were several sea invasions in the Upper Rhine Rift, including the Mainz Basin. Of these, the sea ingress in the Lower Goligocene, the so-called Rupel transgression, was the only one that created fully marine conditions in the Mainz Basin, i.e. H. the living conditions in the Rupel Sea, such as B. the salinity, corresponded to that of a real sea. In addition, the highest sea level levels in the entire Tertiary of the Mainz basin were reached during the Rupel transgression. The reason for the Rupel transgression was a phase of accelerated sinking of the basin structures in conjunction with a global ( eustatic ) rise in sea level.

The rocks that make up the cliff complex represent the pre-tertiary subsurface of the basin. They originated in Unterperm, 260 million years before the Rupel Transgression, and are part of the basin filling of the Upper Carboniferous-Unterpermischen Saar-Nahe Basin , which sank into the mountain range after the Variscan mountain formation in what is now southwest Germany. Lithostratigraphically , they belong to the Donnersberg Formation , a volcanic-sedimentary sequence in the lower part of the Nahe Group . The latter is in turn part of the Rotliegend series. The cliff, a rhyolite , d. H. a solidified acidic lava, is called Kreuznacher rhyolite (not to be confused with the purely sedimentary Kreuznach formation in the upper part of the Nahe group).

While the Kreuznach rhyolite belongs to the Rotliegend, the cliff complex that was formed from it is geologically much younger. Together with the marine sands and gravels that cover the cliff complex in the pit at Steigerberg, it is placed in the upper Rupelium (around 30 mya) and lithostratigraphically referred to as the Alzey Formation . The Alzey Formation, which was deposited in waters close to the coast, meshes with the clayey-marl sediments of the Bodenheim Formation in the area around the Steigerberg , which represent sedimentation further away from the coast in deeper waters. The fossils in the Rupel sediments of the Mainz Basin show that the climate in Rheinhessen at that time was subtropical. Among other things, there are many heat-loving mussels, various species of sharks and rays, and even manatees.

The cliff complex

Total view of the cliff complex from the southeast. Most of the complex is taken up by the surf platforms.
Left: About four meters high stump of the cliff of the first stage with surf caves. Right: View into the upper surf cave through the gap in the upper center of the left image. Left: About four meters high stump of the cliff of the first stage with surf caves. Right: View into the upper surf cave through the gap in the upper center of the left image.
Left: About four meters high stump of the cliff of the first stage with surf caves.
Right: View into the upper surf cave through the gap in the upper center of the left image.
Small cave in one of the cliff walls.

The rhyolite cliff complex consists of four terrace-like steps, each consisting of an approximately horizontal surface and each bounded by an approximately three to five meter high, NNW-SSE-oriented, almost vertical wall, so that overall a stair-shaped geometry results. The lowest and at the same time oldest terrace area does not consist of rhyolite, but of weakly solidified sand and clay stones from the Nahe group. The topmost and youngest exposed terrace area disappears into the sandy and gravelly sediments of the Alzey formation on the east wall of the gravel pit.

The vertical walls have horizontal notch-like structures that are interpreted as surf coves. They also show larger and smaller cavities that z. T. reach relatively far into the rock.

The horizontal surfaces are traversed by approximately parallel grooves running at a 90 degree angle to the vertical walls, which are about 0.05 to 0.1 meters deep. In addition, cup-shaped depressions with a smooth inner surface often appear on the surfaces. Both grooves and depressions are presumably the result of erosion processes caused by surf or currents in interaction with boulders that were moved by the water. The kettle-like depressions are presumably strudel pots. Striking are some isolated rocks standing on the horizontal surfaces. These are interpreted as "surf pillars" in the broadest sense. On the rocks there are also traces of organisms that lived there, e.g. B. Oysters ( Ostrea callifera ), mussels ( Perna sanderbergi ) and barnacles ( Balanus stellaris ).

All of these features taken together show that this is a fossil rocky coastline that has typical features of today's rocky cliff coasts. The horizontal surfaces are surf platforms created by the receding coastlines, represented by the vertical walls. What is special is that the terraced or step-like structure of the complex documents the rise in sea level during the early phase of the Rupel transgression.

History of origin

In today's Bad Kreuznach area, in the upper Unterperm, faults associated with the tectonic subsidence of the Saar-Nahe basin, acidic magmas rose and penetrated relatively close below the surface of the earth into already deposited sediments, where they formed horizontal rhyolite after cooling -Bearing, today's Kreuznacher rhyolite, formed. In the subsequent geological ages, more sediments were deposited in the region.

These overlying layers had been eroded again up to the upper Rupelium, and on the southwestern edge of the Mainz Basin, where no tertiary sediments had yet been deposited, there were a number of witness mountains made of Kreuznach rhyolite. When the sea fell in the course of the Rupel Transgression, these mountains turned into islands and their slopes became cliffs that the Rupel Sea gnawed at. The steep coast of one of these islands is the one that is handed down today in the sand pit on Steigersberg.

The surf of the Rupel Sea, which hit the cliff, created an ever deeper groove at the foot of the cliff. The resulting overhang eventually broke under its own weight , aided by natural weathering processes . Due to the interplay of erosion and breaking, the coastline shifted inland. After a relatively rapid rise in sea level, this process continued about four meters further up the cliff. The cliff has now been further abraded at this level and moved further inland. Since the abrasion now took place entirely in the relatively erosion-resistant rhyolite, a surf platform formed in front of the cliff, which represented the non-eroded cliff foot at and below the waterline. Boulders that came from the broken cliff areas were rolled back and forth by the waves on the surf platform and cut grooves and whirlpool pots in the rock slab and caves in the cliff face. After another rapid rise in sea level, the whole thing continued again a few meters further up the cliff. A new surf platform was created, whereby the "ceilings" of the caves formed during the previous cliff stage were removed, so that channel-like structures were created whose walls, insofar as they did not continue laterally in the cliff wall, now as a kind of "surf pillar" isolated on the old, now well below sea level surf platform remained standing.

A third rapid rise in sea level has been handed down in the surf cliff on Steigerberg. On the seaward side of the active surf platform, pebbles accumulated that came from the abrasion of the cliff and further turned into fine-grained clastic sediments (fine gravel, sands). These boulders, gravel and sands are now known as the Alzey Formation.

The continued rise in sea level meant that the early stages of the surf cliff were completely covered by the sands of the Alzey Formation and only came to light again after the work in the gravel pit.

Discovery and Conservation

The cliff was discovered in 1997 during mapping work by the State Geological Office of Rhineland-Palatinate on the site of a gravel pit. The geotope was then freed from gravel and sand in 1999 in cooperation with the State Geological Office, the employment office at the time and the Christian Youth Village Association . In the winter of 1999/2000, unexpectedly large damage was caused by frost blasting in some areas of the cliff area. In 2002, the cliff was therefore covered with sand again to protect it from further erosion, after another form of conservation and exploration failed due to funding.

literature

  • Stephane Rousse, Philippe Duringer, Karl RG Stapf: An exceptional rocky shore preserved during Oligocene (Late Rupelian) transgression in the Upper Rhine Graben (Mainz Basin, Germany). Geological Journal. Vol. 47, No. 4, 2012, pp. 388-408, doi : 10.1002 / gj.1349 .
  • Kirsten I. Grimm, Matthias C. Grimm: The Alzey Formation (Rupelium, Mainz Basin) on the Steigerberg near Eckelsheim: Sedimentological, sequence stratigraphic and biostratigraphic investigations of a transgressive coastal system. Geologica et Palaeontologica. Vol. 39, 2005, pp. 79-108, Marburg.

Web links

Commons : Brandungskliff am Steigerberg  - Collection of pictures, videos and audio files

Individual evidence

  1. ^ Lutz Geißler: The surf cliff on the Steigerberg silted up. ( Memento of October 27, 2012 in the Internet Archive ) Geoscientific Public Relations Network, February 9, 2008, accessed on August 7, 2011

Coordinates: 49 ° 47 ′ 12 ″  N , 7 ° 59 ′ 19 ″  E