Gua Geolgica
Total Page：16
File Type：pdf, Size：1020Kb
B. Scientific description of the European Geopark territory 1- Definition of the geographic region in which the territory is located The Natural Park Sierra Norte is located in the north of the province of Seville (Andalusia, Spain), within a distance of 40 to 80 kilometres from Seville town. The Natural Park covers a region of Sierra Morena, where the dominant landscape is made up of large pastures of oak and cork trees. The vegetation of Sierra Norte is adapted to the Mediterranean climate of hot dry summers and mild winters. The existence of mineral deposits of residents favoured the settlement from prehistory. Roman and Arab peoples have also left many traces in the villages of the region. 2- General geological description The Iberian Peninsula (Spain and Portugal) is made up of three large geological domains: a) Variscan domain. It represents the remains of the Hesperian Massif, a large mountain range materialized during the Variscan Orogeny (or Hercynian Orogeny), in the Upper Devonian and in the Lower Carboniferous. b) Alpine domain. It is made up of the main recent orographical features of the Iberian Peninsula: Betic Mountain Ranges, the Pyrenees and the Iberian Mountain Range. They originated during the Alpine Orogeny in the Palaeogenic. c) Mesozoic, Cenozoic and Quaternarian platform materials. They are sedimentary rocks, diverse in composition, unaffected by the Alpine cycle, which, in general, deposited by filling the topographic depressions originated in previous periods: the Ebro, Douro and Tagus basins; the Guadalquivir basin and its extension into the Portuguese Algarve, as well as the so-called Lusitanian basin, which stretches between the south of Lisbon and Oporto. Variscan Domain Cantabrian Zone W. Asturian-Leonese Zone Central Iberian Zone Ossa-Morena Zone Southern Portuguese Zone Variscan in Alpine domain Alpine Domain Mesozoic, Cenozoic and Quaternarian platform materials The largest part of the “Sierra Norte de Sevilla” Natural Park is located in terrain belonging to the Hesperian (or Variscan) Massif, although there are also a few 7 outcroppings of filling materials from the Guadalquivir Depression. The Hesperian Massif has been divided into several areas on the basis of its stratigraphical, structural, metamorphic and magmatic characteristics. The five areas under consideration, as arranged in a North-South direction are: Cantabrian Zone, Western Asturian-Leonese Zone, Central Iberian Zone, Ossa-Morena Zone and Southern Portuguese Zone. Almost 90 % of the Natural Park’s area is located in the Ossa-Morena Zone; 10% lies in the en la Southern Portuguese Zone, and less than 1 % is made up of Guadalquivir Depression rocks. Central Iberian Zone Ossa-Morena Zone Southern Portuguese Zone Post-Palaeozoic Rocks Suture units Sierra Norte de Sevilla Natural Park Ossa-Morena Zone The Ossa-Morena Zone is a continental block made up of rocks formed between the terminal Proterozoic and the Carboniferous, which was suturated to the Central Iberian Zone and to the Southern Portuguese Zone during Variscan Orogeny; in both boundaries basic oceanic-affinity rocks have been recognized. The zone is characterized by the widespread presence of tectonic ametamorphic foliations and of abundant magmatism, concentrated in three time periods: the late Precambrian, the Cambrian-Orodvician and the Carboniferous. The stratigraphic sequence consists of two Precambrian formations, whose tecto-sedimentary interpretation is difficult, plus one complete Palaeozoic series. The vast majority of rocks in this Zone show an intense degree of distortion. The oldest materials, preceding the Palaeozoic, are specially distorted, having sustained the effects of two orogenic processes: the Cadomian, which took place at the end of the Precambrian period, and the Variscan, at the end of the Palaeozoic. Locally, on top of the Variscan base made up of distorted rocks, there appear the remains of what used to be small continental basins which were filled with 8 sediments and volcanic materials during the Upper Carboniferous, the Permian and the Lower Triassic. Some of them contain cole layers, as in the case of the former Permian-Triassic basins of San Nicolás del Puerto and El Viar. The bigger structures are large laid-down folds verging towards the SW, and ductile superimpositions whose ceiling block moves toward the SW, bearing witness to a first collision event, with an important reduction in Devonian age in the mid-crustal levels. In the Lower Carboniferous there occurred an important change in the tectonic regime, moving into distensive/transtensive conditions with abundant magmatism and formation of sedimentary basins. In the Upper Carboniferous, the collision among all three continental blocks involved (Southern Portuguese Zone, Ossa-Morena Zone and Central Iberian Zone) went on, giving rise to straight folds and leftward tear faults, affecting the structures generated in the Devonian collision period. The southern boundary of the Ossa-Morena Zone is a orogeny suture zone (between tectonic plates), consisting in a strip of complex rocks which are construed as remains of an old ocean floor, distorted and metamorphosed in the early stages of the Variscan Orogeny. At a later stage, the Alpine Orogeny did affect the Ossa-Morena Zone by reactivating old faults which, upon moving, brought about a rejuvenation of the orography. Southern Portuguese Zone The Southern Portuguese Zone occupies the South-western part of the Iberian Massif (in the Natural Park there appears only to the South and the South-east of Almadén de la Plata). In the Southern Portuguese Zone there appear on the surface rocks of an age ranging from the Middle-Upper Devonian to the Autunian (Lower Permian). Between the Late Devonian and the Upper Carboniferous the development is recorded of an extension period which brought about: a) the fragmentation of a Devonian detrital platform, b) abundant magmatism and c) a high temperature gradient in the upper crust. Such is the context in which the large deposits of massive sulphurs being typical of the so-called Iberian Pyritic Strip were formed. From the Upper Viseain onwards, a contractive distortion wave spread towards the SW from the Ossa-Morena Zone boundary, being preceded by flysch deposits. The distortion style is characterized by folds and large superimpositions with which a low-to very low metamorphism degree is associated. The distortion becomes intense around the place of contact with the Ossa-Morena Zone, and it is here that Plutonic rocks are abundant, whose intrusion, in the majority of cases, is posterior to the main stage of tectonic distortion. The main structures verge towards the South and the development of foliation is widespread throughout the area. Age of the park’s rocks Precambrian rocks are represented by the following outcroppings: an anticlinal core to the West of the El Pintado reservoir; a strip of detrital rocks stretching to the North of El Real de la Jara; the marbles of Loma del Viento to the North of Guadalcanal; and a few small-extent outcroppings located close to the South-eastern end. To the Cambrian system belong the largest outcroppings in the region. Even though this type of rocks can be found anywhere in the Park, is it predominant in its Northern half. 9 The Silurian and the Ordovician is represented, in the main, by the so-called Valley Unit, stretching like a narrow strip to the East of the El Pintado reservoir. Materials from the Devonian and the Lower Carboniferous are to be found, above all, in the Park’s section belonging to the Southern Portuguese Zone, to the South of Almadén de la Plata, although there are also some Devonian outcroppings in the Valley’s synclinal as well as in some other places. The rocks from the Upper Carboniferous, the Permian and the Triassic make the fill of what once were the post-Variscan basins of San Nicolás del Puerto, Viar and the surroundings of the El Retortillo reservoir. The Neogene and Quaternarian materials make, in general, deposits in the Guadalquivir depression (Alpine cover) and recent alluvia. There are geological processes currently underway which are still generating new materials and which, within the Park’s environment, become apparent in the formation of soils and alluvia, and in the deposit of travertines, the latter being especially active in the riverbanks of the Huéznar. 3- Listing and description of the geological sites present on the territory of the Geopark Geosites The Geological Society of Spain (SGE), the Spanish Society for the Defence of the Geological and Mining Heritage (SEDPGYM), the Spanish Society of Environmental Geology and Territorial Regulation (SEGAOT) and experts in the field of Geological Heritage, entrusted the Spanish Mining and Geological Institute (IGME) with the carrying out of the work for the Global Geosites project, a task ended in 2007. The methodology used has been the one agreed upon by IUGS and UNESCO. The results of this work as regards Spain, have been the selection and characterization of Twenty Spanish Geological Contexts of international significance, as well as the location and description of One Hundred and Forty-four Sites of Geological Interest (Geosites), forty-five of which are located in Andalusia, which will become candidates for integration into the list of global-significance sites of geological interest (Global Geosites); these geosites have not been approved as yet. Out of the Twenty Spanish Geological Contexts of world significance, twelve are in Andalusia, all being well represented, and out of these, two are very well represented in the Natural Park. Iberian Variscan Orogen. It Constitutes the Iberian Peninsula’s basement, and its rocks, with ages ranging from the Upper Proterozoic to the Carboniferous periods, crop out in the Iberian Massif. This context is unique due to the uninterrupted nature of its outcroppings, and because it shows excellent records that make it possible to analyse the characteristics of the continental crust, as well as the tectonic, metamorphic and magmatic evolution during the late Precambrian and the Palaeozoic.