site 11

Where is the site located?

GPS: 49° 58′ 39.05″ N, 15° 36′ 14.00″ E

The site lies in the northern part of the Iron Mountains National Geopark, in an area where the mountain range passes to a flat region of the East Bohemian Table.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the marginal area of the Central Bohemian Region. Sedimentary and volcanic rocks of Paleozoic age and sedimentary rocks of Upper Cretaceous age are exposed here. Rocks at this site witnessed the presence of two marine floodings of different ages and volcanic activity.

What happened at this site in the past?

– 490 million years

In the Ordovician, early in the second period of the Paleozoic era, this area was lying on the Southern Hemisphere. It was a part of a shallow sea not far from the supercontinent of Gondwana. The climate was temperate to cold, with marked fluctuations, which affected the sea level. Sea floor was not only the site of sediment deposition but also the site of submarine volcanic activity. Typical volcanic effects included the emplacement of hot lava parallel to bedding planes or into fissures.

– 95 million years

In the latest Mesozoic, this area drifted to the Northern Hemisphere, becoming a part of the Eurasian Plate. It became progressively flooded by Late Cretaceous sea, which was also shallow but – unlike the previous sea flooding – warm. This site was located on a seashore, where shoreline cliffs protruded from sandy beaches, breaking the waves.

What does the site display today?

Rocks of the Lipoltice Formation have been preserved in a narrow strip between the villages of Lipoltice and Horní Raškovice. They date to the earliest Ordovician and enter a wider rock complex preserved since Paleozoic times – the so-called Chrudim Paleozoic. In fact, the original rocks were unconsolidated marine sediments: various sands and muds. These became affected by later geological processes, which turned them into solid rocks mostly grey in colour. Sands were changed for sandstones and quartzites, muds were changed for siltstones and shales. This rock complex hosts an elongate lens-like body of doleritic diabase, which is an intrusive magmatic rock of greyish-green colour. The whole complex was subjected to later orogenic processes including low-grade metamorphism of the original rocks and tectonic deformation of the original magmatic body.

Younger Upper Cretaceous sediments belong to the edge of the Bohemian Cretaceous Basin. At this site, they are preserved in pocket-like depressions – remains after old channels originated during individual floodings, earlier freshwater and later marine. In their age, the sediments are attributed to two stages of the Upper Cretaceous: the Cenomanian and the Lower Turonian ones. They belong to the so-called surf facies.

What was affected by man?

A shelf quarry was opened in a lens-shaped body of diabase, and was considerably enlarged in 1970s. The extracted stone is still in use as aggregate and gravel. In the quarry itself, stone is crushed to meet the desired particle size.

In the past, stone was occasionally used for the manufacture of small stonemasonry products, such as tombstones. Stone from Chrtníky can be easily distinguished in cemeteries: typical whitish plate-like crystals of feldspar occur in greenish-grey groundmass.

The company who operates the quarry is obligated to monitor the environmental impacts of mining activities, particularly the effects on water-saturated rock environment in the quarry and its close surroundings. This is achieved by using a network of monitoring boreholes in which groundwater levels are constantly measured and groundwater samples are regularly acquired for analyses. The quality of surface waters and of the discharged minewaters and waste waters is monitored as well.

Mining activities disclosed several remarkable geological phenomena, which became the subject to scientific study.

What was discovered?

Some quarry faces display effects of intense tectonic processes, which deformed the original diabase body during subsequent orogenic processes. These effects include rock fracturing, steep dips and extensive slickensides (tectonic mirrors, where rock faces are striated).

Rock extraction in the topmost part of the quarry exposed a unique system of pocket-like depressions where surf-facies sediments of Upper Cretaceous age have been preserved. The fill of these depressions consists of diabase pebbles and boulders max. 2 m in size. Matrix is formed by biodetrital limestone (mass of cemented minute shell fragments) with abundant fossils. Remains of organisms living in the surf zone are typically present: hard-shelled sessile organisms capable to withstand strong currents and wave surge. These include, among others, shells of oysters and other bivalves, shells of brachiopods, echinoid spines, remains of corals and worms, bryozoan mats and shark teeth. Surfaces of the diabase boulders are locally covered with various coatings. Even stromatolites – fossilized coatings of algal colonies – of Upper Cretaceous age have been discovered here. These sediments are attributed Cenomanian age. Higher up in the stratal succession, traces of a younger, Early Turonian sea flooding have been documented. Early Turonian sediments are primarily characterized by the presence of white calcareous siltstones with a massive occurrence of sponges. The sponges are so abundant here that the environment of deposition can be referred to as sponge reefs. Since such massive occurrence of sponges is exceptional in Bohemia, these deposits are earnestly studied by paleontologists.

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

Two information panels were manufactured within the project of “The Iron Mountains – a geologically significant region”. They are located in the centre of the village of Chrtníky, in a small park around stone menhirs from the local quarry (these were installed by the local community before). The quarry lies some 500 m to the southwest of the information panels; it can be visited only after a prior agreement by the company who operates the quarry. A metallic plate bearing the name DIABASE has been mounted on a stone menhir (GPS: 49°58’55.094″N, 15°36’21.552″E).

Echinoids (Daniel Smutek)