Saturday, July 20, 2013

Hopeless geology - 3

Beach visit
On our second day in Barcelona we visited the beach. It is impossible to describe this area with its combination of 17th, 19th and 21st century architecture so I will concentrate on the shoreline.
Barcelona beaches are a little dirty (sand and sea), especially the ones near the downtown. [8]
Catalonia is full of beaches since it is located in the east side of Spain and at the border of the Mediterranean sea. By coming at Barcelona, you are only at 25 minutes of the beach by walking from Plaza Catalunya. I will add that I don’t like much its beaches particularly the Barceloneta area, because it is dirty (sand and sea) – you can swim but it exists a high probability that you cross over a plastic bag in comparison to other ones. [10]

The gravel band
We dipped our feet in the water and I was immediately captivated by the mix of sand and gravel. The gravels were concentrated in a band of 2-3 meters width, parallel to the sea shore. There are two possible explanations:
  • This is caused by the size sorting effect of the waves.
  • This is a band (lens) of gravel that is enclosed between two layers of sand.
I can only speculate about the provenance of the gravel band. It might be local geology, but it might even be artificially transported foreign soil. In the city anything can happen.
The port area was quite amazing – the biggest floating mall in the world, a man made Egyptian sand beach, and lots of other businesses. [9]
Gravel sample
I did not have time to collect a large and representative sample. It was getting dark quickly and we were tired. And collecting stones is an antisocial activity if the others are not interested in geology. On my own I would have stayed longer and would have a more complete collection.
It is notoriously difficult to determine stone types. From this sample I can only recognize:
  • Granite - upper left
  • Limestone - center right - I should do an acid test to be completely sure, but I don't have hydrochloric acid available
  • Flint ? - lower right
  • Quartz ? - top middle right
All the others are a total mystery. Especially this one. 3 cm long with beautiful rectangular Crystals, a porphyritic structure. Very hard, certainly igneous rock.
If the gravel band is natural it is most probably a river sediment. The rounded shapes are consistent with a long transport. It is most probable that the rounded shapes were not produced on the beach by wave action but during the river transport. This might have happened recently but also during one of the last ice ages.
Most likely the gravel comes from the Pyrenees or its promontories. The gravel might be relatively recent (1000-100.000 years).
Barcelona's coastal location and the proximity of the surrounding coastal mountain ranges leads to frequent flash flooding in the region. Many of the seasonal rivers on the outskirts of the city continue to pose flood risks. A normally dry river channel near Sant Pere de Ribes has evidence of a past flood level line in a river cliff. [3]
But the gravel might also be older (5-2 million years).
The lower Quaternary deposits overlie the Pliocene series, which is formed by a regressive sequence composed of marine blue marls followed by sands and marls associated with gravel lenses that pass progressively to the lower Quaternary sediments.
Quaternary alluvial detrital sediments are mainly composed of layers of red clay, yellow silt, and minor sand and gravel. The thickness of the alluvial detrital sediments is variable, attaining 20 m in the creeks. [2]
And the gravel might be even older (12 million years).
Twelve million years ago, the Mediterranean was taking on much of the shape it has today. Sea levels were dropping, and it would not be long, geologically speaking, before the Mediterranean became isolated from the Atlantic and, literally, dried up. Rivers were working tirelessly at the erosion of uplands old and new, pouring gravel, sand, and mud down to the ever-shifting coast. A great delta formed close to where Barcelona is today, its sediments eventually heaved up to form the mass of Montjuic. [11]
 And it might even be that the source of the gravel is an even older, conglomerate stone from the surrounding mountains.
 A spectacular example of conglomerate can be seen at Montserrat, near Barcelona. Here erosion has created vertical channels giving the characteristic jagged shapes for which the mountain is named (Montserrat literally means "jagged mountain"). The rock is strong enough to be used as a building material - see Montserrat abbey front at full resolution for detail of the rock structure. [4,5]
The granite
 The three rounded granites on the right are from the beach. The angular one on the left I picked up in a Barcelona park. They are definitely the same stone type.
For economic reasons the granite I picked up in Barcelona must come from a nearby (max. 100 km) source. It would not be efficient to transport the heavy stone over long distances. This is confirmed by the same type granite I saw in the station at Figueres. (The colours look different in the pictures but in reality they were the same.)
Most probably it has been mined in one of the local stone quarries, like this one. I saw this one from the train between Barcelona and Figueres, but it is more probably a sandstone quarry that a granite quarry (from the shape of the incision).
This is further confirmed by this sample from the Barcelona museum. It is 600 - 300 million years old:

Granite: plutonic igneous rock - Dating:Upper Paleozoic - Provenance: Espanya,  Catalunya, Girona, Alt Empordà, Cantallops - The sample comes from the old quarry at Lamas, located on the highway from La Jonquera to Cantallops. The area is marked by widespread granite outcroppings from the axial zone of the Pyrenees, from the Upper Paleozoic superior. The granite has a granular texture with equigranular crystals of 4-6 mm. [6] 
And by this description of local Barcelona geology:
The Collserola mountain range forms the northern barrier to Barcelona's expansion. It overlooks the city. The Collserola is composed of two types of rock: metamorphic (slates, quartzites, metamorphic limestones) and igneous (granite). [7]

References: [1]
Variscan and Alpine structure of the hills of Barcelona: geology in an urban area,
Estructura herciniana y alpina de las colinas de Barcelona: geología en una zona urbana
P. Santanach, J. M. Casas, O. Gratacós, M. Liesa, J. A. Muñoz, F. Sàbat [2] [3] [4] [5]
Granite sample [6] [7] [8] [9] [10] [11]

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