October 20. The Solnhofen Limestone

In several of this month’s episodes I’ve mentioned the Solnhofen Limestone, the rocks in Bavaria where spectacular fossils are found – from the lobster-like Eryon to the flying pterosaur Rhamphorhynchus to Archaeopteryx that we talked about yesterday. Why is this rock so special?

Crinoid from Jurassic Solnhofen limestone (source)

During Late Jurassic time, about 150 million years ago, central Europe was a string of islands, the high-standing areas along seaways defined by rifts that formed as Pangaea began to break apart. The area was close to the Jurassic equator. The array of islands made for some restricted lagoons between them, and in some locations, arid conditions together with restricted circulation made for anoxic conditions where life could not survive. No scavengers, no oxygen to decompose bodies. We’ve heard this story before.

But wait, you say – when we’ve heard this in previous episodes, those stagnant lagoons accumulated organic-rich mud that became black shale. You said this was limestone. What’s the deal? We have to infer that the islands were low-lying, and not shedding much in the way of clastic sediment into the lagoons. Clastics – sand, silt, and mud – are deposited typically in settings where the topographic relief is at least moderately high, so erosion can remove those materials from outcopping rocks on land and dump them into adjacent marine settings. Here in Bavaria during the Jurassic, I think we have to see the setting as something like the modern Bahamas – low islands, and even there the rocks on the surface were probably limestone, not granite or other rocks that would yield quartz sand, silt, and mud. The lagoons were carbonate-rich, so it was fine grained calcite, calcium carbonate, that precipitated out.

The resulting extremely fine-grained rock was also remarkably uniform, so much so that the Solnhofen is called a lithographic limestone – ideal for making lithographic plates – stones carved in fine detail to use in printing illustrations, including multi-color lithographs. It was the quarrying operation in the 19th century for lithographic uses that revealed the spectacular fossils of the Solnhofen.

Fossils are actually not all that common in the Solnhofen limestone, but when they are found, they are preserved in exquisite detail, some of the finest fossils ever found anywhere. In addition to the animals we have talked about, the fauna includes jellyfish with soft parts preserved, free-floating crinoids, beetles, cephalopods, horseshoe crabs, turtles, fish, dragonflies, crocodiles, and more. It is truly a world-class lagerstätte, one of those rare natural collections of spectacularly preserved fossils.

—Richard I. Gibson

Link:
UC Berkeley on Solnhofen 

Photo by Ushakaron, used under Creative Commons license.