Geological Mapping, Stratigraphy, Palaeontology and Mineralizations of the Palaeozoic Around Valério’s Quarry and the Museum of Trilobites (Arouca Geopark), Canelas, Northern Portugal

Abstract

The studied area, a sector in the reverse limb of the Valongo Anticline adjacent to the Museum of Trilobites and to Valério’s Quarry (Canelas, Arouca), is located in the Central-Iberian Zone of the Iberian Massif. The main objective of this study was to obtain a detailed geological map, through the recognition of the lithologies, stratigraphic units, fossils, structures and mineralizations. The Palaeozoic succession from Cambrian to Carboniferous was studied, with the exception of the Devonian, which does not outcrop in the area. Regarding lithologies, a succession of volcano sedimentary layers has been found in Cambrian (Montalto Formation, Middle Cambrian) and in the Lower Ordovician (Santa Justa Formation), either underlying Floian massive quartzites (Tremadocian?) or overlying Floian massive quartzites (Floian). At the top of the Middle Ordovician (Valongo Formation, Dapingian-Darriwilian), near the contact with the Upper Ordovician (Sobrido Formation, Hirnantian), a ferruginous layer with abundant siliceous nodules occurs. In the base of the Sobrido Formation, massive quartzites underlie diamictites. Concerning the fossil record, abundant icnofossils such as Cruziana and Planolites were identified in the quartzites of the Lower Ordovician. Also, different somatofossils occur in the Middle Ordovician slates, namely trilobites (between the largest trilobites in the world) and other arthropods, graptolites, cephalopods, brachiopods, gastropods, bivalves and echinoderms (cystoids and crinoids). In Silurian, besides quartzites (Rhuddanian), light grey slates bearing graptolites, namely Monograptus (Llandovery), are dominant. In Carboniferous (Gzhelian) breccias, sandstones and slates with plant fossils, namely with articulated plants and ferns, occur. Regarding the mineral resources of the area, beyond the slates of Middle Ordovician (Valongo Formation) exploited in Valério’s Quarry, there is a Roman mining work, the Gralheira d'Água mine (Au-As type mineralizations), associated to the Lower Ordovician (Santa Justa Formation) quartzites and volcano sedimentary layers, exploited for gold. Antimony mineralizations (Sb-Au type mineralizations) associated to the volcano-sedimentary succession of the Cambrian-Ordovician transition, were found and studied by Scanning Electron Microscopy (SEM). The final work resulted in a geological map, covering an area of 1200m by 900m, at a scale of 1: 3,400.