Stromatolithic limestones are originated by stromatolites, these are made of microorganisms mucilaginous mats growing in shallow flooded, illuminated areas, usually in intertidal areas from flat and calm coasts. Stromatolites still exists, though they use to be much more abundant and extensive at the beginning of life, during the Archean eon. The biggest stromatolites are sinusoidal or globose mats made of several layers of microorganisms that are arranged by oxygen, light, humidity or temperature gradients. Cyanobacteria are the main component of stromatolites; they are responsible for photosynthesis of the organic matter, for fixing atmospheric nitrogen and for secreting the mucilage that provides consistency. Furthermore, archaebacteria, bacteria and unicellular green algae may be present as well. Stromatolites trap sediment particles and accelerate the deposit of calcium carbonate, so over time they become taller and turn to a limestone.
Stromatolithic limestones are identified by their sinuous lines, their fine grain, their microfossils, and the macrofossils according to the period and place of origin. Stromatolithic limestones are a particular type of biomicritic limestones, those that are made of small particles of skeletons and calcium carbonate deposits on living beings.
Stromatolithic limestones are the oldest fossil evidence of life on Earth. Amongst them, the oldest ones are in Australia and date back about 1,250 million years, during the eon Proterozoic, in the Precambrian, although stromatolithic limestones from any ages are found in the world. Also, the most extensive and developed living stromatolites are in Australia, in Sharp Bay.
In the district of Bages, in the road from El Pont de Cabrianes to Artés, one stratum of stromatolithic limestone crops (see pictures in gallery). It is placed on top of the package of marine rocks. It is 2-4 meters thick. It dates from the Bartonian, in the Middle Eocene, about 37 million years. The reddish colour rocks from continental origin lay above this stromatolithic limestone layer. Fossils of shells and sea urchins can be seen into the stromatolithic limestone. The pattern of this limestone rock at El Pont de Cabrianes, its stratigraphic position and its fossil content aim at the origin in a stromatolite in an intertidal area.
The stromatolithic limestones are the proof of the resilience of life on Earth rooting deeper into the origins.
[photos Jordi Badia]