site 25

Where is the site located?

GPS: 49° 52′ 10.76″ N, 15° 37′ 12.32″ E

The area lies in the western part of the Iron Mountains National Geopark, in a prominent depression in relief, hidden in the summit part of the crest of the Iron Mountains. Reddish colour of fields in the near vicinity of Kraskov is a mysterious indication of the presence of rocks coming from Late Paleozoic times, governed by hostile desert climate.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the area of grabens (also called “furrowsˮ) where Upper Paleozoic sediments have been preserved. The grabens have the shape of narrow, elongated depressions, mostly controlled by faults. Such areas deeply subsided relative to their surroundings. As a result, they conserved sedimentary successions which were abraded and removed by erosion from the neighbouring areas. This occurrence pertains to the so-called Jihlava Graben, which is hidden beneath younger rocks over most of its extent, and is documented by deep boreholes. The graben begins near Hradec Králové and proceeds across Chotěboř towards Jihlava.

What happened at this site in the past?

– 350–300 million years

A major orogenic cycle called Variscan Orogeny comes to its end in the latest Carboniferous (the fifth period of the Paleozoic era) and earliest Permian (the sixth period of the Paleozoic era). It was responsible for the origin of a huge mountain range in the territory of the Czech Republic – the so-called Variscides. This mountain range gave rise to the solid basement of the Bohemian Massif.

This area became incorporated in a single supercontinent of Pangea, formed by the collision of two smaller lithospheric plates: Laurasia in the north and Gondwana in the south (this area was located on the margin of Gondwana). The mountain range was built by rocks which underwent complex metamorphic processes and tectonic deformation. Rock masses were bent, fractured and shifted. The orogenic processes induced the ascent of hot magma which either flowed onto the surface or formed large, loaf-shaped bodies hidden under the surface. This area was located approximately on the Equator, being governed by warm and dry climate. Mountains were constantly abraded and the material was transported to depressions where it was deposited in high thicknesses. Gravel, sand and clay beds were formed. The dry desert climate gave the sediments a typical rusty to reddish colour caused by iron compounds originated during arid weathering. After some time, the sediments became hardened. They occasionally hosted lava flows produced by volcanic activity.

What does the site display today?

This site lies in a prominent, fault-bounded depression among the villages of Prachovice, Seč and Lichnice, called the Kraskov Basin. The basin is filled with rusty-, red- and grey-coloured sediments, generally attributed to the Permo-Carboniferous. A collective name for the two periods is justified by the fact that their terrestrial rocks can be hardly distinguished from each other.

The basin is of tectonic origin: the rocks of the basin fill subsided along faults. The basin fill comprises subarkoses or subgreywackes to conglomerates, i.e. consolidated sedimentary rocks composed of a wide variety of rock clasts of various sizes. Thickness of the sedimentary fill is hard to estimate: geological exploration has not provided sufficient data yet.

What was affected by man?

In this landscape, the old Paleozoic basement is hidden beneath younger sedimentary cover originated in the Quaternary. The occasional red coloration of soil on the fields is caused by the presence of iron compounds in rock fragments brought to the surface by deep ploughing.

Small natural outcrops of Paleozoic rocks are not man-made but mostly induced by stream erosion: they are mostly located in the valleys of the Počátecký potok and Zlatý potok streams and in the slope above the Peklo Reservoir. Other outcrops of these rocks were induced by humans. The rocks were exposed at several construction sites in the 1950s, such as the construction of retention dams and the water-supply system of the Prachovice cement plant.

What was discovered?

The first geological studies of Upper Paleozoic rocks were conducted in the late 19th century, within the compilation of the first geological map of the Iron Mountains. Further research dates to the 1940s to 1950s, when the principal rocks types and several sections were described. These Upper Paleozoic sediments were documented to overlie granitic rocks of the Nasavrky pluton near Kraskov. This suggests that they are demonstrably younger than their westerly-lying neighbour body.

Paleontological evidence of the precise age of the sediments was, however, still missing. It was found in the year 1965: three araucarite fragments were found in sediments ca. 500 m NE of Seč within a geological field mapping course for students of the Faculty of Science, Charles University. Araucarites are fossilized parts of tree trunks of Cordaites – the ancestors of conifers, tall, arborescent plants with prop roots and big, strap-like leaves, found mainly in the piedmont region of the Giant Mountains. This first piece of paleontological evidence suggested a probable pertinence of the local rocks to strata of Carboniferous age.

“The Iron Mountains – a geologically significant region” project of 2014

An information panel was manufactured within the project of “The Iron Mountains – a geologically significant region”. It was erected in the centre of the Kraskov village, by Road 337 from Třemošnice to Seč. The geosite is located some 500 m to the west of the information panel, in the northern vicinity of the Horní Peklo and Dolní Peklo reservoirs. The actual outcrops of Permo-Carboniferous rocks are covered with talus today. Therefore, a drilling survey was undertaken at this site to reveal the character of these Paleozoic sediments.

Conglomerate, borehole KR-1, depth 24 m, 2014 (Daniel Smutek)