Geoparque Unesco


Nummulites Bars

Rocks formed by the shells of unicellular marine organisms.
Municipality: Aínsa – Sobrarbe

Between Santa María de Buil and Arcusa, abundant Nummulites can be found on the outcrops.

These fossils, characteristic of the Eocene period and resembling little coins, are usually known as "Dineretes de Sevil".

Geological Context

Nummulites bars are one of the sedimentary materials representative of the Sobrarbe Delta. They are found on the frontal area of the Delta and they are formed by different levels of tabular or slightly lenticular structure. They match the final cycles of the deltaic succession (shallow marine environment).

The Nummulites are unicellular (foraminiferan) marine organisms with a skeleton surrounded by cytoplasm both inside and out. Their level of organization is similar to the amoeba's. Their present-day counterparts have a calcite shell with a high content of magnesium. The shell is formed by several layers of fibrous densely porous calcite. They are lens-shaped, like little coins (=nummus). They are known as "Dineretes de Sevil" in some areas in Sobrarbe. However, they have morphological variations, from flat to globular shapes.

Geological Significance

From the study of the fossil populations of Nummulites: number of species, variations in shape and size, transport degree, associations with other fossils, types of rocks where they can be found, etc., we have been able to reconstruct the sedimentary environment, the depth, the water pressure, the climate, etc. In comparison to other fossil associations in the Eocene basin (north and south faces of the Pyrenees and other alpine mountain ranges), these conclusions allow us to study productive crude oil systems in depth.

Since the sedimentation in the Aínsa Eocene basin was determined by tectonics, the end of the Nummulites colonization is related to the thin detritic sediments invasion during the sea withdrawal from the region, followed by a structural subsidence which dramatically modified the sedimentary environments in this area of the basin.

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