The Mediterranean Region is home of one of the most beautiful, yet mysterious, sedimentary rock in the world: the Rosso Ammonitico limestone. The beauty of this rock is pretty obvious from the pic above, but why mysterious? Because there is nothing like this forming in present-day oceans. Hence, we can only try to understand the processes that produced it by studying in detail the beautiful structures preserved in its red limestone layers.
The Rosso Ammonitico is practically everywhere from Spain to Italy to the Balkans and Turkey. The age varies from place to place, from the Middle Triassic (247 million years ago) to the Upper Jurassic (145 million years ago), but the characteristics and meaning of this rock are the same. The Rosso Ammonitico marks a dramatic moment in the geological history of Europe when Mesozoic carbonate platforms with tropical reefs and Bahamas-like islands started to sink, under the pressure of plate tectonics, leaving place to a relatively deep sea with open waters teeming with marine creatures. Indeed, as the name ‘Ammonitico’ suggests, this limestone contains fossils of ammonites, extinct squid-like creatures with spiraling shells and distant cousins of the nautilus – one of my favorite animals.
Many of the spectacular features of these limestones are well visible at outcrop scale, especially if you have some help from an active quarry. Some time ago I visited this quarry close to the village of Sassorosso, in Garfagnana, where they exploit the Rosso Ammonitico, which here has a Sinemurian – Pliensbachian age (199 – 183 million years old)
The quarry is still active because the Rosso Ammonitico is a popular decorative stone. This allowed me to get several views of these red limestones on the polished quarry fronts.
Beautiful, isn’t it? The Rosso Ammonitico shows a nodular texture, with nodules of white calcite-rich sediment surrounded by reddish clay-bearing limestone, interrupted by red layers of red marls and shales. The red color indicates that oxidized iron is present, implying that this sediment formed in an oxygen-rich environment. The vertical walls did not allow me to put a coin or something to give you an idea of the scale, but you can tell from the height of the pickaxe in the first picture that the layers are 5 to 15 cm (2 – 6 inches) thick and individual nodules vary in size between a few millimeters to several centimeters (less than an inch).
The nodules sometimes show clear boundaries. In other cases, the boundaries of the nodules gradually fade into the surrounding reddish sediment. The outline of the nodules is often curvilinear and many appear wriggly or indented, squeezed along one another. This creates beautiful patterns. The most beautiful part is however, in my opinion, where the nodules are interrupted by the thin, reddish layers of shales.
These red layers define the bedding of the Rosso Ammonitico but they do not look much straight and planar. They look as if they ‘mingled’ with the carbonate nodules. They are rather irregular, with bights and bifurcations that surround small lenses of carbonatic sediment. Fragments of limestone entirely surrounded by clay are common in the red layers.
Can you see in the slider above the reddish ‘pipes’ that cut through the pale red limestone (dashed lines)? They are burrows dug by bentonic critters that lived on the seafloor, excavating the sediment to find food or shelter.
Speaking of fossils… have you looked closely at some of the ‘nodules’?
These spiraling objects are the ammonite fossils I mentioned at the beginning of this post. If you look closely (I haven’t outlined them, and I promise I will in one of the next posts) you can see that the shell of the ammonite is divided into a series of small chambers by structures called septa. Ammonites were everywhere in the Mesozoic seas. All that is left today of this taxa of animals, which consisted of thousands of known species, is the nautilus (video below) – which is actually a nautiloid rather than an ammonoid (i.e. the shell is different). It gives you an idea, however, of how the seas must have looked like, teeming with shelled creatures propelling themselves around in the ocean, suspended in water.
Since this is a quarry cut, you can find sections through the spiral of the shell (planispiral), and sections that are oblique or perpendicular to the shell. I will let you identify the ammonite in the next pics. Tell me in the comments if you find them!
Large ammonite fossils can pierce the contact between the limestone and clay layers, like the big ammonite below.
If you look closely at the ammonite above (and the others), the outline of the fossil is red because it now consists of reddish clays. The original aragonite shell of the ammonite is gone, dissolved and replaced by a mold of calcite that filled the internal chambers of the fossil. Structures witnessing the dissolution of calcite and aragonite and the chemical precipitation of new carbonatic material are common in the Rosso Ammonitico. Indeed, various authors suggested that the dissolution and precipitation of carbonates to be the main driver of the formation of the nodules.
How did the nodules form?
We do not know for sure, because there is no equivalent to the Rosso Ammonitico in present-day oceans. The leading idea is that the presence of both calcite and aragonite in the sediment, dissolving in water at different rates, caused the water in the sediment to become locally over-saturated in calcium carbonated, which deposited forming the nodules. The growth of the nodules ‘pushed’ the insoluble material (red clays) apart, producing the red tones we observe today in this rock. Such a reaction must have been very very very slow, but could proceed thanks to the ultra-slow sedimentation rates of the Rosso Ammonitico. For example, the outcrop of Rosso Ammonitico I have shown today is 30 m thick and testifies 15 million years of geologic history. If you do the math, it is about 2 mm (0.7 inches) for 1000 years. Practically nothing! And in other areas of the Apennines this formation is even thinner. The presence of burrows is another indication of ultra-slow sedimentation rates, since critters can banquet on the sediment for thousands of years before its definitive burial. The process of dissolution and formation of nodules likely kept going for a very long time underneath the seafloor. Then, at depth, the load of the overlying sediments started to deform the nodules and push them together. You can find the study proposing this origin here.
Do you have other ideas?
Understanding how the Rosso Ammonitico formed is a geological curiosity, which is why there is no much research about it. Perhaps it is only me, but I don’t care if research (apparently) has no practical applications to us. I just want to know and I would like to see more studies on the Rosso Ammonitico and similar rocks we do not know much about. Did you find this blog post interesting? Do you have another idea? Write in the comments below or go to the field and research the beautiful, red, nodular limestones of the Rosso Ammonitico.
Jenkyns (1974). Origin of red nodular limestone (Ammonitico Rosso, Knollenkalke) in the Mediterranean Jurassic: a diagenetic model.
Rosso Ammonitico (in Italian) – a very well-written wikipedia page.
Did you like this post?
I like to write long and detailed geological posts, but it takes a lot of time to go to the field and study the literature about the rocks I show. If this read was worth a coffee for you, you can offer me one!