olelog

Magnetostratigraphy is a field within stratigraphy that studies the magnetic characteristics of rock bodies. If the magnetic properties of rocks have measurable differences stratigraphically, that is, from one strata to the next, those differences can be used to identify their relationships and identify varying stratigraphic units. Stratigraphic units are known collectively as magnetostratigraphic units (magnetozones). The most useful magnetic property for magnetostratigraphy results from a change in the direction of the magnetization of the rocks. Crystals in rocks are magnetically aligned with the Earth’s magnetic field. The Earth’s magnetic field has changed over the eons and those magnetic alignments are ‘recorded’ in the crystals of rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The change in the earth’s magnetic fields is caused by reversals in the polarity of the Earth's magnetic field, the Earth’s magnetic poles literally change locations. These reversals of Earth’s polarity have taken place many times during geologic history. (http://en.citizendium.org/wiki/Magnetostratigraphy)

The long procedure of sampling and analysis required to obtain a reversal stratigraphy for a succession of sedimentary rocks means that this technique is normally only used when other (biostratigraphic) methods cannot be used or a high-resolution stratigraphy is required. (Gary Nichols, Sedimentology & Stratigraphy, Blackwell, 1999)

Permian-Triassic gap in the fossil record?

Magnetostratigraphy has now been used to determine whether there was a Permian-Triassic gap in the fossil record of the Russian Ural Mountains.

The world’s single most severe mass extinction event which took place at the end of the Permian and start of the Triassic ages, some 250 million years ago. The extinction event, thought to be the result of runaway global warming, wiped out between 80-95 per cent of the planet’s species. Was this extinction event a real biological catastrophe or was it merely the result of gaps in the fossil record? So far it was thought that ten million years worth of rock from around that time was missing in Russia (we are talking about the continental uppermost Permian Russian stages, the Kazanian and Tatarian).

The scientists matched the magnetic record fossilised within the disputed Russian rocks with those from the rest of the World, demonstrating that the Russian rocks do indeed record the run-up to the event and the Permian/Triassic boundary and therefore the fossil losses in these rocks are part of the mass extinction.The sampled sections span the upper Guadalupian to Induan stages without any obvious break, so confirming the traditional view that the Tatarian is Late Permian in age.

Yet another piece matching in a larger jigsaw puzzle.

Reference:
Taylor et al.
Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus
Earth and Planetary Science Letters
Volume 281, Issues 1-2, 30 April 2009, Pages 36-47
doi:10.1016/j.epsl.2009.02.002
• http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4VS3P43-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=26333ea71795423cab79750f8e75508d
• http://www.bris.ac.uk/news/2009/6320.html

(a.o in the Subhercynian Basin north of the Harz Mountains, Germany)

In Late Permian (something like the last five to seven million years of the Permian Period, which ended around 251 million years ago) a substantial area of the North Sea and north west Europe was covered by sea, the so-called Zechstein Sea.

The sea was situated around the equator. It was fed by rivers and ocean flooding once in a while, which flushed salty water into the sea. Since the climate was warm and arid, evaporation equalled or exceeded the inflow, concentrating the salts in the sea. In this way a large thick deposit of salt was accumulated. A sediment layer that it largely present in today’s underground (of the area in question). When sand and other sediments are compacted they become denser than pure salt. Under the pressure from the sediments above the salt becomes rather fluid and thereby mobile, and because the salt is less dense than the overlying rocks (density inversion) the salt will try to rise upwards (positive buoyancy). The best known of such salt movements (halokinisis with a fine word) are salt domes (also known as diapirs). Here is a cross section of an area with salt domes in north western Germany. The zechstein is shown in blue.


The zechstein has significant economic importance in the North Sea Oil province. The salt is impermeable and many oil or gas fields include structural traps associated with a salt dome. (For the information of my North American readers I may add, that similar salt structures are also of economic importance (for oil and gas) in the Gulf of Mexico - A belt of salt domes lies beneath the surface of the Gulf of Mexico. Over 500 mushroom-shaped geological structures formed as the Gulf separated from the Atlantic Ocean).

And now to the Harz Mountains (in Germany) and the area to the north of it. The Harz was formed by an uplift, which affected the whole area of the Harz. The uplift started during the lower Cretaceous (140-97 million years ago) and stopped in the upper Cretaceous (97-67 million years ago), but most of the uplift happened during the Subhercyne Phase (83 million years ago). At that time Africa had started moving northwards, and thereby shortening (compressing) the crust between Africa and Scandinavia. Numerous predominantly NW-SE (so-called Hercynian) faults were created, including at the northern and southern rim of the Harz Mountains. Looking at a map of Northern Germany you may notice that many rivers follow the same NW-SE trend.

The large hercynian fault at the northern rim of the Harz Mounatins is called the Harznordrand Thrust (fault). In an area affected by thrust tectonics (like the hercynian thrust faults), buckling of the overburden layer will allow the salt to rise into the cores of anticlines (the opposite of synclines). Such anticlines formed north of the Harznordrand Thrust, and on both sides of the anticlines you can now see tilted strata dip away from the anticline. This is also the case in my photo below from the Hoppenstedt Quarry (4 km west of the town of Osterwieck). The strata are here dipping 30-40° to the SSW away from the Fallstein Anticline.


The quarry exposes limestones of the Cenomanian and Turonian (that is in the Upper Cretacious period - Turonian 93.5 ± 0.8 – 89.3 ± 1.0 miilion years ago and Cenomanian 99.6 ± 0.9 – 93.5 ± 0.8 million years ago). These limestones were deposited (100-90 million years ago) on the bottom of the Chalk Sea that I wrote about here and here. The 80 m thick succession in the quarry was deposited without major gaps within 10 million years, which means an average of 8 m per million year.

Reference concerning the Hoppenstedt Quarry:

• EDGG, Heft 237 – Excursion Guidebook – 26th IAS Regional Meeting / SEPM-CES SEDIMENT 2008 – Bochum
  Field trip POST2 – Syntectonic sedimentation in front of a late Cretaceous growth fault – the Harz Mountains and the adjacent Subhercynian Basin (Germany) by T. Voigt and H. von Eynatten.

Note: Once again I have made an extremely long and complex story extremely short in the hope of highlighting some principal relationships.