Plattenkalk - Solnhofen
Friday, 5. December 2008, 10:46:00
I ought to write about sediments or sedimentology more often, but my pictures from outcrops are seldom spectacular. When I show a picture like this one:
who cares about the limestone. Everybody stares at the fossil, which is understandable. Nevertheless I stand there with questions like: Why did it die just there? No oxygen available? How did it get fossilised? Why wasn’t it torn apart by scavengers or waves? What was the environment? How come that you can see so many tiny details? and so on... In short questions that might be answered by studying the sediments in which the fossil is found.
My picture is of one of the most famous fossils ever found - the Archaeopteryx lithographica, a link between dinosaurs and birds. I am happy to see that Munnecke et al. have paid the attention to the rock itself that it deserves in a paper published in the December 2008 issue of Sedimentology.
The limestone in which so far ten Archaeopteryx fossils were found are limestone successions of Upper Jurassic (Tithonian) age in the Solnhofen/Eichstätt area. They consist of alternate layers of thin-bedded, laminated, fine-grained, very pure (hard) limestone and softer (inter)layers with slightly lower carbonate contents that are also laminated and show a foliaceous weathering appearance. The extremely fine grained structure explains why so many fine details are visible in fossils from the area. It has also made some of these rocks perfect for lithography, that is one of the early methods of printing images using a flat stone with a completely smooth surface. In one of the geological museums at Solnhofen you can learn more about this printing process. This use of the rock lead to the naming of Archaeopteryx lithographica.
The author was concerned with the proper name for these rocks. Lithographic limestones may sound appropriate, but only a small portion of plattenkalk limestones, namely less than 1% from the classic Solnhofen occurrences, is usable for lithographic techniques. The German word plattenkalk means something like platy limestone, but this term (platy limestone) appears too wide. In 2005 Röper defined plattenkalk as all carbonate marine sediments, in which bioturbation – for whatever reason – partially or completely stopped, so that the primary lamination and fine stratification of the sediments is preserved. (Bioturbation means stirring or mixing of sediment or soil by organisms, especially by burrowing, boring, or ingestion). Now this lack of bioturbation is another reason that the fossils are so well preserved.
The monotonous appearance of these fine-grained mudstones, in particular the softer layers, made it difficult to study them in detail, even with the use of normal microscopes. Todays techniques with electron microscopical examination has made better studies possible.
There is a general agreement that the plattenkalk was deposited in individual basins at the northern margin of the Tethys Sea, where 30 to 90 m thick successions accumulated. The fine lamination of the plattenkalk indicates that life was absent in the bottom of the basins. Possible causes for hostile conditions at the bottom include oxygen depletion, possibly in conjunction with a toxic hydrogen sulphide regime, or hypersaline conditions. Hydrogen sulphide is a smelly nuisance known from stink bombs or rotten eggs. It is a highly toxic gas which hinders respiration just like carbon monoxide. The posture of the dead Archeopteryx indicates damage to the brain, maybe due to suffocation or poisoning. The poor creature died a long, slow death, unable to breath properly.
Plattenkalk successions like these are rare, but often famous, and include examples from the Ordovician of Scandinavia, the Devonian of Germany, the Carboniferous of Montana, the Permian of eastern Greenland, the Triassic of Spain and the Cretaceous of Italy. The most famous example of plattenkalk is probably, however, this Upper Jurassic occurrence of the Solnhofen (or Eichstätt) plattenkalk series. The Altmühl valley is often visited by fossil hunters, but my experience tells me that you have to hunt for several hours to find anything worth collecting (if any at all). Instead enjoy the landscape and the limestone and (coral) reef geology, and not least the splendid geological museums with beautiful (local fossil) collections spread over the area.
Reference:
Diagenesis of plattenkalk: examples from the Solnhofen area (Upper Jurassic, southern Germany) by Munnecke et al. in the December 2008 issue of Sedimentology (doi: 10.1111/j.1365-3091.2008.00975.x).
• http://my.opera.com/nielsol/blog/2007/06/14/interdisciplinarity
• http://my.opera.com/nielsol/blog/2008/03/18/most-famous-fossil-i-was-in-berlin-all-last-week-the-timing-was-not-optimal
who cares about the limestone. Everybody stares at the fossil, which is understandable. Nevertheless I stand there with questions like: Why did it die just there? No oxygen available? How did it get fossilised? Why wasn’t it torn apart by scavengers or waves? What was the environment? How come that you can see so many tiny details? and so on... In short questions that might be answered by studying the sediments in which the fossil is found.My picture is of one of the most famous fossils ever found - the Archaeopteryx lithographica, a link between dinosaurs and birds. I am happy to see that Munnecke et al. have paid the attention to the rock itself that it deserves in a paper published in the December 2008 issue of Sedimentology.
The limestone in which so far ten Archaeopteryx fossils were found are limestone successions of Upper Jurassic (Tithonian) age in the Solnhofen/Eichstätt area. They consist of alternate layers of thin-bedded, laminated, fine-grained, very pure (hard) limestone and softer (inter)layers with slightly lower carbonate contents that are also laminated and show a foliaceous weathering appearance. The extremely fine grained structure explains why so many fine details are visible in fossils from the area. It has also made some of these rocks perfect for lithography, that is one of the early methods of printing images using a flat stone with a completely smooth surface. In one of the geological museums at Solnhofen you can learn more about this printing process. This use of the rock lead to the naming of Archaeopteryx lithographica.
The author was concerned with the proper name for these rocks. Lithographic limestones may sound appropriate, but only a small portion of plattenkalk limestones, namely less than 1% from the classic Solnhofen occurrences, is usable for lithographic techniques. The German word plattenkalk means something like platy limestone, but this term (platy limestone) appears too wide. In 2005 Röper defined plattenkalk as all carbonate marine sediments, in which bioturbation – for whatever reason – partially or completely stopped, so that the primary lamination and fine stratification of the sediments is preserved. (Bioturbation means stirring or mixing of sediment or soil by organisms, especially by burrowing, boring, or ingestion). Now this lack of bioturbation is another reason that the fossils are so well preserved.
The monotonous appearance of these fine-grained mudstones, in particular the softer layers, made it difficult to study them in detail, even with the use of normal microscopes. Todays techniques with electron microscopical examination has made better studies possible.
There is a general agreement that the plattenkalk was deposited in individual basins at the northern margin of the Tethys Sea, where 30 to 90 m thick successions accumulated. The fine lamination of the plattenkalk indicates that life was absent in the bottom of the basins. Possible causes for hostile conditions at the bottom include oxygen depletion, possibly in conjunction with a toxic hydrogen sulphide regime, or hypersaline conditions. Hydrogen sulphide is a smelly nuisance known from stink bombs or rotten eggs. It is a highly toxic gas which hinders respiration just like carbon monoxide. The posture of the dead Archeopteryx indicates damage to the brain, maybe due to suffocation or poisoning. The poor creature died a long, slow death, unable to breath properly.
Plattenkalk successions like these are rare, but often famous, and include examples from the Ordovician of Scandinavia, the Devonian of Germany, the Carboniferous of Montana, the Permian of eastern Greenland, the Triassic of Spain and the Cretaceous of Italy. The most famous example of plattenkalk is probably, however, this Upper Jurassic occurrence of the Solnhofen (or Eichstätt) plattenkalk series. The Altmühl valley is often visited by fossil hunters, but my experience tells me that you have to hunt for several hours to find anything worth collecting (if any at all). Instead enjoy the landscape and the limestone and (coral) reef geology, and not least the splendid geological museums with beautiful (local fossil) collections spread over the area.
Reference:
Diagenesis of plattenkalk: examples from the Solnhofen area (Upper Jurassic, southern Germany) by Munnecke et al. in the December 2008 issue of Sedimentology (doi: 10.1111/j.1365-3091.2008.00975.x).
• http://my.opera.com/nielsol/blog/2007/06/14/interdisciplinarity
• http://my.opera.com/nielsol/blog/2008/03/18/most-famous-fossil-i-was-in-berlin-all-last-week-the-timing-was-not-optimal
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