Geossítio 26 Ordovician glaciation
… and more than 80% of the marine species of the time became extinct!
Around 445 million years ago (Ordovician) a great glaciation took place. It was the most intense glaciation on record since multicellular organisms first appeared and was responsible for the extinction of more than 80% of the marine species that populated the seas at that time.
After the glaciation, 444 million years ago, near the South Pole, sands began to be deposited that gave rise to the quartzite rocks that form reliefs that stand out in the landscape here (quartzite ridges): the Crista de Galinheiros.
The place name “Galinheiros” is probably related to the hypothetical large number of woodcocks that once lived here. In medieval times, it was a point of reference and a landmark for limiting territorial influences, as documents dating from 1153 attest: “in uilla quos uocitant Canelas subtus mons Galliero discurrente ribulo Pauia territorio [de Arauka (?)]”.
The quartzite rocks were covered by silt and clay that form light-toned shales containing materials (dropstones) released by the melting of icebergs adrift in the ancient Rheic Ocean.
Este geossítio faz parte do itinerário C: Paiva, the surprising valley.
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Informação Geológica
This geosite shows evidence of the glaciation that took place at the end of the Ordovician Period (Hirnantian), around 445 million years ago, the most intense glaciation on record.
This glaciation peaked in northwest Africa, where the South Pole was located at the time, and was responsible for a significant drop in sea level, contributing to the 15 million year old depositional hiatus in the rocks found in this region. In this sense, and in the post-glacial period, around 444 million years ago, at Paleoantarctic latitudes, sands began to be deposited, which gave rise to massive quartzites at the base of the Sobrido Formation, which locally form a residual relief known as Galinheiros.
These quartzite levels were later covered by siltstones and clays that gave rise to shales, where dropstones (rock fragments – clasts released by an iceberg deposited on the seabed) were interspersed in the middle and upper part of the Sobrido Formation. Here it is possible to find clasts embedded in the shale-gravel matrix, which bear witness to the occurrence of a veritable “rain of clasts”, resulting from the fusion of icebergs that were then drifting in the Rheic Ocean.
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