THE IRON MOUNTAINS GEOPARK Annex no. 2: Geological Heritage including Abstract HISTORY OF THE EARTH IN TWO DAYS November 2015 2 ABSTRACT Iron Mountains are the key to geological history of central Europe. Proterozoic gneisses and volcanosedimentary complexes (subaerial and submarine volcanism) are present. The earliest life forms have been found – stromatolites. The Paleozoic comprises Cambrian siltstones, Ordovician quartzites, Silurian shales, Devonian limestones and Carboniferous sediments. Rare fossils include trilobite and graptolite fauna. The third oldest ichnofossil of Zoophycos type in the world has been reported. The Mesozoic (Upper Cretaceous) sedimentary cover has a unique development, with abundant fauna and flora. Sandstones and spiculitic marlstones provide excellent conditions for groundwater accumulation. Tertiary rocks are represented by basalt enclosing olivine-rich xenoliths. Geological story of the area is completed by Quaternary loess and sandy gravel. Over a hundred geosites have been described from this area. The above facts became grounds for listing the Iron Mountains as a National Geopark. It allows a geotourist to explore the history of the Earth in only two days. 3 GEOLOGICAL HERITAGE B.1 Geological conditions of the area Regional geological characteristics of the Iron Mountains National Geopark The Iron Mountains National area an exceptional phenomenon within Geopark (IMNG) lies around the contact the whole of Europe (see the practical of several regional geological units. sample of the geological excursion ˮ, freely The Bohemian Massif is the most “History of the Earth in two days available also at website extensive relict of the Variscan Orogen http://www.geovedy.cz/cs/files/Geovedy_ in Europe. As such, it often attracts visitors Brozura_ZS_A5_web.pdf). Scientists and for its varied geological composition. renowned geologists of the past were Geological variety of the Iron Mountains aware of this fact and studied the Iron over a relatively small area can be Mountains region. Historical line of their presented within one or two days, which activities is shown in Fig. 9. makes this Fig. 9. Early times of geology and history of research in the Iron Mountains. Source: IMNG Archive. General geological characteristics Massif, the Iron Mountains represent of the IMNG area are based on the regional a component of the Variscan Orogen. geological pertinence of its separate parts Based on current knowledge, the western (Fig. 10). Much like the whole Bohemian part of the IMNG area is ranked within 4 the Teplá-Barrandian Zone – the so-called of the “platform ˮ units. The oldest of these Bohemicum. This includes the so-called rocks are represented by a relict Iron Mountains Proterozoic and the Iron of the Permo-Carboniferous Jihlava Mountains (or Chrudim) Paleozoic. Central Graben near Kraskov. Cretaceous part of the IMNG area is formed sediments are of wide areal distribution, by plutonic rocks of the Iron Mountains located along the western, northern and plutonic complex (Iron Mountains eastern limits of the IMNG. They or Nasavrky pluton) and the Oheb concentrate to the „Long Furrow“ area and Crystalline Complex. This complex the Chrudim area. Tertiary rocks are belongs, in terms of the regional geological dominated by volcanics at the Košumberk subdivision, to the Kutná Hora–Svratka Castle near Luže (Fig. 11). region which is, in turn, ranked Quaternary rocks have a wider to the Moldanubicum s. l . distribution in the area of Rosice and The eastern part of the area is formed Chrtníky. They have the character of loess by rocks of the so-called Hlinsko Zone rich in molluscan fauna. Quaternary (Rhenohercynicum) and rocks processes (mostly weathering and erosion) of the Poli čka Crystalline Complex are prominent at a number of sites, taking (Moldanubicum). The area is also reached their share in the geomorphic character by rocks of the Kutná Hora Crystalline of the whole area. As such, they pose Complex (Chot ěbo ř area) and the Svratka a vital aspect of the Geopark geology. Crystalline Complex (area of Otradov and Below, you can find the descriptions Prose č). of the individual regional geological units These pre-Variscan and Variscan lying in the Geopark territory. units are overlain by post-orogenic rocks Fig. 10. Geological subdivision of the territory of the Iron Mountains National Geopark with designated regional units. 5 Fig. 11. Ruins of the Košumberk Castle built on Tertiary basalt (a painting from the 19th century). Iron Mountains in different periods these days. In spite of this, there are many of geological history sites displaying clear evidence – often very peculiar – of volcanic activity. Proterozoic Another phenomenon is linked with Proterozoic is the oldest geological the presence of the Iron Mountains Fault. unit represented in the Geopark territory. This major structure is now well visible It is generally constrained by ages 2.5 because it was associated with the uplift billion years to 542 million years. of the Iron Mountains by 600 m or more The oldest rocks within the Geopark have (Fig. 12). This uplift did not take place been dated to less than 700 million years. within a single day, of course: it lasted During their long history in the Iron several million years. The presence Mountains, the Proterozoic rocks were of major deep-reaching ruptures allows subjected to several major pressure and the ascent of hot fluids. Such process along temperature changes. As a result, most the Iron Mountains Fault resulted rocks of this age have been markedly in the origin of ores and other minerals. metamorphosed. Large accumulations of minerals represent The Proterozoic era in the Iron deposits which can be – in ideal case – Mountains is connected with two important exploited. The high number of mineral phenomena. One of them is volcanic deposits along the Iron Mountains Fault activity, both subaquatic and subaerial. also gave name to the adjacent mountain Prominent superficial products like range. volcanic cones have not been preserved till Fig. 12. The range of the Iron Mountains from the southeast (12a) and northwest (12b). 6 Moldanubicum of individuals collected during the whole history of paleontological research in this Moldanubicum is represented area (Fig. 13). Much more abundant fossil by several lower-order units in the territory finds in the Geopark territory include of the IMNG. The largest area is covered cephalopods with straight, conical shells by the Oheb Crystalline Complex (genus Orthoceras or similar), brachiopods in the SW and W parts of the area, a unit (a group similar to bivalves), and crinoids ranked to the Kutná Hora–Svratka Region which belong to the group of echinoderms. (Moldanubicum s. l. ). Lithologically most The Mrákotín area yielded finds of yet abundant types are red orthogneisses another interesting group: graptolites. intercalated with fine-grained biotite At a first glance, graptolites resemble shiny paragneisses and amphibolites. Quartzitic lines on dark stone; in fact, these are paragneisses and serpentinites are also colonies of tiny organisms ranked among present. Stratigraphic rank of the meta- the group of hemichordates. sedimentary units and the age of pre- Variscan orthogneisses are unknown. Paleozoic rocks in the Geopark territory posed an important source Rocks pertaining to the Kutná Hora of building material, especially in the past. Crystalline Complex are exposed Very hard Ordovician quartzites were used to the south of the Oheb Crystalline for the construction of buildings. Today, Complex, in the E and NE surroundings one of the largest and most attractive of Chot ěbo ř. They are represented quarries in the Geopark – the Prachovice by reddish orthogneisses, banded Quarry – is opened in the Paleozoic rocks: migmatites and biotitic greywacke Silurian and Devonian limestones (for paragneisses. Two-mica migmatites and Silurian geology in the IMNG. biotitic migmatites are present The importance of this area in the past and in the immediate surroundings present is stressed by the educational of Chot ěbo ř. Rocks in the E part trail “Around the Prachovice Quarry ˮ. of the Geopark area (around Otradov and The trail also provides views of otherwise Prose č) are ranked to the Svratka inaccessible mining area. Occurrences Crystalline Complex. They are dominated of Late Paleozoic (Permo-Carboniferous) by porphyroclastic orthogneisses with rocks in the IMNG area are rare, being frequent intercalated bodies concentrated solely to the area of Kraskov of amphibolites, limestones and skarns. near Se č. Reddish fluvial sandstones to conglomerates locally contain finer- Paleozoic grained tuffitic beds. Paleozoic is the general term for These fluvial sediments were the era constrained by the ages of 542–251 reported to yield rare finds of fossilized million years. This long stage in the Earth wood (araucarites). The relict near history is subdivided into several periods. Kraskov documents the southern end Each of the periods is represented – at least of the so-called Jihlava Graben, which to a limited degree – in the Geopark developed on the deep-seated P řibyslav territory. The Paleozoic era is linked with Fault. Besides this relict, isolated one of the most typical and most famous occurrences of Permo-Carboniferous rocks fossil representatives: a trilobite. This of the Jihlava Graben have been also ancient arthropod can be found at several described from the SW surroundings sites in the IMNG. Its finds are sporadic of Hradec Králové. and rare, with only several dozens 7 Fig. 13. Brloh. A quarry face in quartzite (Paleozoic – Ordovician) and boulders of the surf facies (Mesozoic – Cretaceous). The quartzites contain trilobite fauna. Mesozoic display effects of ancient sea surf – large, abraded boulders (Fig. 13). In contrast to the Paleozoic record, Mesozoic rocks preserved in the Geopark After moving only a few hundred represent only a relatively short time span metres basinward, the visitor finds himself (ca. 100–85 million years). In spite of this, in an “open ˮ sea full of sharks, fish, the Geopark features sites with unique bivalves and cephalopods. Abundant geology, which are worth our attention.