The Rosso Ammonitico limestone of Sassorosso (Garfagnana, Italy) – Geology is the Way

The Mediterranean Region is home to one of the most beautiful, yet mysterious, sedimentary rocks in the world: the limestone Rosso Ammonitico. The beauty of this rock is pretty obvious from the picture above, but why a mystery? Because nothing like this forms in the current seas. Therefore, it is only through close study of the beautiful structures preserved in red that we can attempt to understand the processes that produced it limestone layers.

The Rosso Ammonitico is almost everywhere from Spain to Italy to the Balkans and Turkey. The ages vary 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 European geological history when Mesozoic carbonate platforms with tropical reefs and Bahamas-like islands began to sink, pressurized by plate tectonics, leaving space for a relatively deep sea with open waters rich in marine creatures . . Indeed, as the name ‘Ammonitico’ suggests, this limestone contains fossils of ammonites, squid-like extinct creatures with spiraling shells and cousins ​​of the distant nautilus – one of my favorite animals.

Many of the spectacular features of these limestones are very noticeable at the scale of outcrops, especially if you get some help from an active quarry. Some time ago I visited this quarry near a village Sassorosso, in Garfagnana, where they exploit the Rosso Ammonitico, which here has a Cinematic – Pliensbachian age (199 – 183 million years)

Overview of the Rosso Ammonitico quarry near Sassorosso (Corfino, Garfagnana, Italy)

The quarry is still active because the Rosso Ammonitico is a popular ornamental stone. This allowed me to have several views of these red limestones on the polished frontages of the quarry.

Red ammonite wall

Ammonical Red

Ammonical Red

Beautiful, isn’t it? The Rosso Ammonitico shows a nodular texture, with nodules of calcite-rich white sediments surrounded by reddish limestone carrying clay, interspersed with red layers of red marble and shales. The red color indicates the presence of oxidized iron, which suggests that this precipitate is formed in an oxygen-rich environment. The vertical walls didn’t allow me to give a coin or something to give you an idea of ​​the scale, but you can tell from the height of the pickle in the first picture that the layers are 5 to 15 cm (2 – 6 inches) dense and single nodules ranging in size from a few millimeters to several centimeters (less than an inch).

Ammonical Red

Ammonical Red

Ammonical Red

The modules 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 corrugated or indented, squeezed together. This creates beautiful patterns. The most beautiful part, though, in my opinion, is where the thin, reddish layers of shale cut across the nodules.

Ammonical Red

Ammonical Red

Ammonical Red

These red layers define the bedding of the Rosso Ammonitico but do not look much straight and pale. They appear to ‘mix’ with the carbonate nodules. They are somewhat irregular, with bights and bifurcations that surround small lenses of carbonated sediment. Extracts of limestone completely surrounded by clay are common in the red layers.

Ammonical Red

Ammonical Red

Ammonical Red

Can you see in the slider above the reddish ‘pipes’ that cut through the light red limestone (dotted lines)? They are holes dug by bentonic creatures that lived on the seabed, digging the sediment to find food or shelter.

Speaking of fossils … have you looked in detail at some of the ‘nodules’?

These spiral objects are the ammonite fossils I mentioned at the beginning of this post. If you look carefully (I haven’t outlined them, and I promise I will in one of the next posts) you can see that the ammonite shell is divided into a series of small chambers by structures called septa. Ammonites were everywhere in the Mesozoic seas. All that remains today of this taxon of animals, which contained thousands of known species, is the nautilus (video below) – which is actually a nautiloid rather than an ammonoid (ie the shell is different). It does give you an idea, though, of what the seas looked like, teeming with crimson creatures pushing themselves around in the ocean, hanging in water.

As this is a quarry cut, sections can be found through the shell (planispiral), and sections oblique or perpendicular to the shell. I’ll let you identify the ammonite in the next photos. Tell me in the comments if you find them!

Ammonite in rosso ammonitico

Ammonite in rosso ammonitico

Large ammonite fossils can pierce the link between the limestone and clay layers, like the large ammonite below.

Ammonite in rosso ammonitico

If you look closely at the ammonite above (and the others), the fossil outline is red because it now contains reddish clays. The original ammonite aragonite shell is gone, dissolved and replaced by a calcite mold that filled the fossil’s inner chambers. Structures witnessing the dissolution of calcite and aragonite and the chemical deposition of a new carbonated material are common in the Rosso Ammonitico. Indeed, various authors suggested that dissolution and deposition of carbonates to be the main driver for the formation of the nodules.

How did the nodules form?
We do not know for sure, because there is nothing like the Rosso Ammonitico in today’s oceans. The main idea is that the presence of calcite and aragonite in the sediment, dissolving in water at different rates, caused the water in the sediment to become locally saturated in carbonated calcium, deposited by the formation of the nodules. The growth of the nodules ‘pushed’ the insoluble material (red clay) apart, producing the red shades we see today in this rock. Such a reaction must have been very, very slow, but could proceed thanks to the ultra-slow sedimentation rates of the Rosso Ammonitico. For example, the Rosso Ammonitico outcrop I showed today is 30 m thick and testifies to 15 million years of geological history. If you do the math, it’s 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 super-slow sedimentation rates, as creatures can feast on the sediment for thousands of years before its official burial. The dissolution and formation of nodules has probably continued for a very long time under the seabed. Then, in detail, the load of the overlying sediments began to deform the nodules and push them together. You can find the study that offers this provenance here.

origin of Rosso Ammonitico

Do you have any other ideas?
Understanding how the Rosso Ammonitico was formed is a geological curiosity, which is why there is not much research about it. It may be just me, but I don’t mind if (probably) research has no practical applications for us. I just want to know if I would like to see more studies on the Rosso Ammonitico and similar rocks that we know very little about. Did you find this blogpost interesting? Do you have another idea? Write in the comments below or go into the field and research the beautiful, red, nodular limestones of the Rosso Ammonitico.

References
Jenkins (1974). Origin of red nodular limestone (Ammonitico Rosso, Knollenkalke) in Mediterranean Jurassic: a diagenetic model.
Rosso Ammonitico (in Italian) – a very well written wikipedia page.

Did you like this post?
I like writing long and detailed geological posts, but it takes a lot of time to get into the field and study the literature about the rocks I show. If you found this read worth a coffee, you can offer me one!

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