In Tuscany, south of the city of Livorno, the panorama changes from boring (well, for us geologists for sure) plains with houses and towns to a magnificent coastal scenery with cliffs, small coves, and wood-covered hills next to crystalline waters. The coast around Cala del Leone, between Quercianella and Calafuria, is particularly suggestive. These coastal outcrops – if you want to taste some geology beyond the view – offer beautiful exposures of ophiolites and their sedimentary cover (as I have shown, for instance, in the post about Castiglioncello) and on some of the most important foredeep deposits of the Northern Apennines, the Oligocene – early Miocene Macigno Sandstone, whom I have briefly talked about in the previous post about Manarola.
These rocks are mostly sandstones that deposited 30 million years ago at the front of the (then) submarine Apennine orogen. At the time, tall chains like the Alps were standing up above the sea and being eroded by rivers. The huge sediment supply brought back to the Mediterranean sea produced km-thick sequences of deep water turbidity currents.
Turbidity currents are low density mixtures of sediment and water that, once triggered, may travel for hundreds of km on the seafloor at speed ranging from 10 to 30 km/h. Once they start to slow down, the sediments they carry start to settle on the seafloor, starting from the coarser and heavier down to the finer grain sizes. In the fine-grained sandy/muddy layers at the top cross and parallel laminations may form as the sediments are still drag by a turbiditic current. Mud slowly decantates when the turbidity current is over and continues to deposit from the water column between a turbidity event and the next one. The resulting deposit is a graded fining upward sandstone layer that contains coarse-grained sand or even gravel at the base and fine-grained silt to mud at the top (learn more here).
The turbidites exposed at Cala del Leone are very coarse-grained and there are many layers that were produced by concentrated and hyperconcentrated density flows – dense turbidites indicating that we are in the highest energy part of the submarine fan, likely close to the source area.
Indeed, if you move around the site, you can find many features like this one (use the slider):
This is an erosional contact. The layer at the top, a very coarse-grained sandstone with a basal conglomerate truncates the underlying layer, consisting of medium-grained sandstone. These are, hence, amalgamated beds produced by erosion and subsequent depositions of layers of similar grain size. Amalgamation is a common process associated with turbidity currents and occurs when a turbidity current erodes the seafloor at the moment of deposition. In this case, the presence of amalgamated layers is highlighted by the curved, erosional contact and the sharp increase in grain size but there are examples, here at Cala del Leone, where these phenomena are less obvious.
Erosion and deposition associated with multiple turbidity currents can produce complex depositional sequences, where each new layer of sandstone may amalgamate with the former one, making it difficult to reconstruct the sequence of events. Check the site below, for instance. Before using the slider (which I have set to the right to not tempt you…), how many distinct layers, separated by erosional surfaces, can you recognize?
That wasn’t easy. Layer 1 and 2 are separated only by a thin erosional surface marked by a slight increase in grain size. Layer 2 closes as a wedge between layer 1 and 3. Layer 3 is truncated in half by a big channel formed when layer 4 deposited. Finally layer 4 shows the typical fining upward sequence that starts from basal conglomerate and goes to medium-grained laminated sandstone.
There is another big evidence in plain sight that erosional processes accompany the deposition of these turbidity currents. See the fine-grained, whitish clasts that fill the channel at the base of layer 4? Those are rip up clasts or clay chips. They form when a turbidity current erodes the seafloor, tearing off fragments of mud. Mud is very cohesive compared to sand and can resist as soft clasts even when it’s transported by a turbidity current.
Concentrated density flows
As I have anticipated at the beginning of the posts, these are not Bouma sequence-like turbidites. These flows are very dense and sediment separation is not complete. You can see here details of the base of these deposits that contain poorly sorted sand and gravels. There is a trend of decrease in grain size towards the top, but, generally everything is still heavily mixed. This is a feature of concentrated (and hyperconcentrated) density flows.
Towards the top!
It is not very common to see the top of turbidite beds here. This is, indeed, due to the fact that many layers are truncated, eroded, and amalgamated, but there is another reason I will tell you next time (NO SPOILERS!).
In the few cases, you can see the top preserved, you can see fine-grained sandstone and siltstone layers with nicely developed parallel and cross lamination. These structures form because, in the latest stages of deposition of a turbidite, sandy sediments is dragged by the turbulent flow on the seafloor, producing bedforms. Parallel lamination forms at higher velocity, while ripple marks, responsible for the generation of cross lamination, appear at lower speed.
I couldn’t end this post without showing you some examples of this transition from the field. Again, I have had some fun sketching them for you, but I don’t think it is really necessary here:
As you can see, within this layer, we pass, from bottom to top, to parallel lamination to cross lamination with ripples. Actually the layers at the top start to show another feature of turbidity currents: a convolute lamination where lamination planes delineates convex and concave forms. Why? This is a topic for another post!
Riferimenti bibliografici
Bracci, G., Dalena, D., & Bracaccia, V. (1984). Caratteristiche sedimentologiche dell’arenaria di Calafuria (Toscana). Atti Soc Tosc Sci Nat Mem Serie A, 91, 189-202.
Cornamusini, G. (2004). Sand-rich turbidite system of the Late Oligocene Northern Apennines foredeep: physical stratigraphy and architecture of the ‘Macigno costiero’(coastal Tuscany, Italy). Geological Society, London, Special Publications, 222(1), 261-283.
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