The Geopark Bohemian Paradise is situated in the northern part of the Czech Republic, formerly Bohemia (Fig. 1). It is extraordinarily rich in various geological and paleontological features. The eastern part of the area is covered by upper Paleozoic deposits of the Krkonoše Piedmont Basin that are historically well known for their rich fossil record. One of the most interesting phenomena is petrified stems. Their frequency and esthetic value have always attracted the attention of local and foreign researchers (e.g., Březinová 1970). They were first described more than 150 years ago (Goeppert 1857, 1858; Renger 1858), and rarely superficially mentioned in the next decades (e.g., Feistmantel 1873 a, b, c; Feistmantel 1883; Frič 1912). However, they have never been studied in detail. Only in the last few years have they been studied intensively by researchers from the Geopark Bohemian Paradise in cooperation with the Charles University in Prague, Czech Geological Survey, and Czech Academy of Science, as well as colleagues from institutes abroad, e.g., Chemnitz Museum of Natural History, IRD/AMAP Institute Montpellier (e.g., Matysová et al. 2008; Mencl et al. 2009; Sakala et al. 2009; Matysová et al. 2010; Mencl et al. 2013a, b).
The result of a long and rich geological history of the Geopark area are many geological sites attractive for visitors. However, the petrified stems are not widely known and are often ignored. Based on modern research and new data, we can demonstrate the attractive geological history of the area much better.
The informal center of petrified tree trunk research is the town of Nová Paka, the “Eastern Gate to the UNESCO Geopark Bohemian Paradise”. It is situated in the heart of the Krkonoše Piedmont Basin and there are many places to find petrified stems around it. The most famous fossil site is probably Nová Paka - Balka, where petrified trunks were historically found. Currently, a large collection of about 2500 petrified stems is in the Municipal Museum in Nová Paka, and more than 600 specimens are presented in its exhibition, which is the largest in the Czech Republic.
Except for the Bohemian Paradise Geopark, late Paleozoic petrified stems are quite common in deposits of several coeval basins in the Czech Republic. The Krkonoše Piedmont Basin, however, shows the most complete fossil record (Mencl et al. 2009; Sakala et al. 2009; Bureš 2011; Mencl et al. 2013a, b; Opluštil et al. 2013). In Europe, stems of similar age are known from e.g., Sachsen, Germany and Autun, France (e.g., Rößler 2001; Rößler & Noll 2006; 2010, Rößler et al. 2012; Trümper et al. 2018).
The Bohemian Massif, the easternmost part of the Variscan orogeny, was formed as a result of the closure of Rheic oceanic basins between Gondwana and Laurussia, and the collision of both continents at the end of Paleozoic (Dallmeyer et al. 1995; Žák et al. 2014). Faulting related to collision and Gondwanan rotation formed dozens of continental basins, with significant deposition since early Westphalian (e.g., Matte 1986, 2001; Pešek et al. 2001), which was several times interrupted by tectonic activity (Schulmann et al. 2014).
The late Paleozoic Krkonoše Piedmont Basin, which covers the eastern part of the UNESCO Global Geopark Bohemian Paradise (Fig. 1), represents the widest stratigraphic range among all others coeval basins in the entire Bohemian Massif. Purely continental deposits in the basin are Moscovian to Triassic in age, and their maximum thickness is about 1800 m. It is mostly composed of fluvial and lacustrine siltstones, sandstones and conglomerates, with sporadic content of volcanics (Pešek et al. 2001). According to the sedimentary and fossil record, the climate during the Pennsylvanian (Late Carboniferous) and Cisuralian (Early Permian) oscillated between wet and dry periods, and generally shifted from humid to arid (Opluštil et al. 2013).
Figure 1. Geographic position of the UNESCO Global Geopark Bohemian Paradise in Central Europe and its schematic geological map with sites.
The stratigraphic succession of the basin contains a very rich fossil record. Among researchers and private collectors, it is widely known for the presence of impressions of various flora, insects, fish, and amphibians, as well as ichnofossils. Moreover, there is also a well-known and rich occurrence of petrified stems, so-called silicified wood. In the past, these fossils were generally considered to be Early Permian (e.g., Feistmantel 1873a, b, c), but according to current research they cover a wider stratigraphic range, from Kasimovian to Sakmarian, and they occur in at least four stratigraphic levels (Pešek et al. 2001; Mencl et al. 2009; Mencl et al. 2013a, b; Opluštil et al. 2013; Fig. 2). They are most abundant in the so-called Ploužnice Horizon, which is part of the Semily Formation and is placed today very close to the Carboniferous / Permian boundary (Opluštil et al. 2016).
Figure 2. Stratigraphy of the Krkonoše Piedmont Basin with stratigraphic positions of petrified stems.
Petrified Stems of the Krkonoše Piedmont Basin
In this region, fragments of petrified tree trunks are most often found as loose pieces on hillsides, without any relations to the original deposits. They are less commonly preserved in outcrops, but always lying horizontally as allochthonous material, and never upright (Purkyně 1927; Mencl et al. 2009). They are usually preserved only as fragments of secondary xylem. Branches and bark with leaf scars are extremely rarely preserved (Renger 1858, 1863; Mencl et al. 2009, 2013b).
Based on present research and detailed anatomical studies of the plant tissues, various types of petrified plants were described in the basin, including arborescent calamitaleans and lycophytes, arborescent and climbing ferns, and gymnosperms as well (e.g., Mencl et al. 2009; Mencl et al. 2013; Fig. 3, Fig. 4).
Figure 3. 1. Cross section of calamite Arthropitys bistriata, scale bar = 5 cm, 2. Cross section of Psaronius tree fern stem, scale bar = 7 cm, 3. Cross section of Agathoxylon sp. (conifer or cordaite), scale bar = 10 cm, 4. Polished section of silicified peat with plant fragments as a lycopsid cone, scale bar = 5 cm. Courtesy of Museum Nová Paka, adopted from Mencl et al. 2013 (1) and Opluštil et al. 2013 (2, 4).
Arborescent calamitaleans of the Krkonoše Piedmont Basin can be attributed to two species: the common Arthropitys cf. bistriata and the rare Calamitea striata (Mencl et al. 2013a). In the area, calamitaleans of various sizes are found, from small axes a few centimeters in diameter, to big stems with diameters of almost 50 cm.
Most of the ferns are Psaronius sp., an arborescent marattialeans. The biggest specimen ever found in the Krkonoše Piedmont Basin is about 50 cms in diameter. Inside the root zone of their false stems are rarely found climbing axes of the fern Ankyropteris brongniartii, and small epiphytes of the fern Tubicaulis sp. (e.g., Frič 1912; Rößler 2000). The newest findings also prove the presence of Asterochlaena laxa, a small arborescent zygopterid fern (unpublished).
Figure 4. Detail of secondary xylem of arborescent calamite Arthopitys bistriata (left) and conifer Agathoxylon sp. (right). Scale bars = 0, 5 mm.
Stems of gymnosperms include two types: “seed ferns” (Medullosas) and stems of the genus Agathoxylon sp. of two types, cordaites and conifers (Mencl et al. 2013b). Medullosas are extremely rare in the basin. They are usually up to 20 cm in diameter and are currently studied intensively. On the contrary, stems of other gymnosperms are the most common and the largest among all petrified stems of the Krkonoše Piedmont Basin. They were described from four fossiliferous levels (mentioned above, Fig. 2). The biggest trunk, more than 8 m long and up to 1 m thick, was found in Nová Paka in 1953 (Fig. 5).
Other plant remains are known from large nodules, similar to coal-balls, preserved inside the fossiliferous deposits, and so-called “silicified peat”. This unique material presents the most complex view of floral assemblages of the area. It is composed of fragments of various plant tissues, including roots, small branches, reproductive organs, seeds, etc.