Devonian

Author of the elaboration: prof. dr hab. Stanisław Skompski

Devonian exposures are extremely common in the Holy Cross Mountains, occurring as natural outcrops, quarries, road and railway cross-cuts. The abundance of fossils, resulting in a very detailed stratigraphy (Fig. 5),

Fig. 5. Simplified lithostratigraphic scheme of the Devonian in the Holy Cross Mountains (after Szulczewski 1995, Tabela Stratygraficzna Polski [Stratigraphic Chart of Poland] 2008; Narkiewicz and Narkiewicz 2010; Fijałkowska-Mader and Malec 2011) 1 – Kadzielnia Limestone Mb., 2 – Laskowa Góra B., 3 – Manticoceras Limestone Mb., 4 – Crinoid-cephalopod condensed complex

based mainly on conodonts, the high variability of sedimentary settings and completeness of the succession have caused the Devonian of the Holy Cross Mountains to be unique in Europe. Its exposures are eagerly visited by Polish and foreign geologists.

The development of Devonian sedimentation corresponded to the rhythm of transgressions caused by eustatic sea level oscillations, and thus recorded in successions around the world, particularly in Europe and North America. Caledonian movements at the Silurian/Devonian boundary resulted in the closure of the Iapetus Ocean and collision of the Laurentian Craton of North America and the East European Craton, which formed a new continent known as Laurussia or Euramerica. It is also commonly referred to as the Old Red Continent, from the traditional English term for the Old Red Sandstone, a typical Devonian continental deposit covering the continent interior. During the Devonian and Carboniferous, the continent was subjected to a northward movement; nevertheless, throughout this time, its southern shelf, exemplified by the Devonian of the Holy Cross Mountains, was located within the tropics. Due to this fact, sedimentation of thick successions of carbonate continued in this area for many million years.

The beginning of the transgression, which took place approximately in mid-Emsian times (the youngest part of the Early Devonian), is marked by sandstone facies. The best exposures are located in the Klonowskie Range near Barcza and Bukowa hills. The Lower Devonian, whose thickness is estimated at several hundreds of metres, was initially deposited in fluvial, possibly lagoonal, settings (Czarnocki 1936; Łobanowski 1990; Szulczewski 1995). An unequivocally marine setting is represented by the Zagórze Formation, perfectly exposed in Bukowa Góra Quarry and also accessible in rocks on the top of Bukowa Hill, rising to the south of the quarry. According to Szulczewski (2006) and Szulczewski and Porębski (2008), the formation was formed in a shoreface depositional system, in which sandy barriers separated calm lagoons from more dynamic offshore environments. The succession has abundant fossils of marine organisms and traces of their life activities. Towards the south, the thickness of Lower Devonian formations rapidly decreases and in the vicinity of Chęciny the entire Emsian is represented by only several metres of red-violet continental sandstones (Głazek et al. 1981). In other parts of the Kielce Region, for example in the vicinity of the town of Daleszyce, occur Placoderm Sandstones, a facies characteristic of the Lower Devonian, filled with the remains (imprints) of early vertebrates, living in lagoons or in a shallow shoreface zone. Unfortunately, these unique and rare strata are currently poorly exposed. A characteristic feature of Lower Devonian sequences is the presence of bentonite beds, thin, greenish clay horizons that were formed due to submarine weathering of volcanic ashes. Due to their wide lateral range, these beds can serve as correlation horizons. As in the Łysogóry Region, the Lower Devonian of the Kielce Region also yields numerous trace fossils (Tarnowska 1981).

A gradual sea level rise at the end of the Early Devonian resulted in successive floodings of the southern zone. In particular parts of the shallow “Łysogóry” sea appeared localized sedimentary settings with various clastic and carbonate deposits, of which some, such as the Dombrowa Limestones, were formed in relatively deep, offshore settings (cf. Wójcik 2015). Variable lithological members that formed in the Łysogóry Region at the Early/Middle Devonian boundary are sometimes referred to the Grzegorzowice Formation (Malec 2005), but this proposal does not seem justified due to the extremely diverse lithology of its individual units. In the Kielce Region contemporaneous strata include the offshore Dombrowa Limestones with brachiopods and tentaculoids (Studencka 1983).

Progressive levelling of the sea floor topography, coupled with sea level fall at the beginning of the Middle Devonian (Eifelian Age), is marked in both regions of the Holy Cross Mountains by a thick series of dolomites. A summary of many years of research on the origin of these rocks can be found in Narkiewicz (1991, 2006). These deposits, exploited in numerous quarries and extremely valuable in road construction, are rather monotonous for geologists; however, in a few cases they yield exceptional structures and fossils. Skała Quarry near Nowa Słupia exposes part of the dolomite formation with beautifully preserved sedimentary cycles, typical of shallow marine, tide-dominated settings (Skompski and Szulczewski 1994), among which occur unique horizons with large brachiopods of the genus Bornhardtina. In turn, the succession in Zachełmie Quarry near the village of Zagnańsk has recently become famous for traces of the world’s oldest tetrapods (Niedźwiedzki et al. 2010; Narkiewicz and Narkiewicz 2010, 2014) that crawled and swam in shallow lagoons separated from the open sea

The dolomite episode is the time of the largest facies unification in both Palaeozoic regions of the Holy Cross Mountains. From that time onwards, the succession of the Łysogóry Region began to differ in its evolution from the Kielce Region, in which a sub-division into two blocks, a stable northern block and a rapidly subsiding southern block was periodically evident (Szulczewski 1977).

The northern part of the Kielce Region became the basement of the Central Carbonate Platform, which rapidly increased its thickness but at the same time restricted its lateral range from a bank to reef stage (Racki 1993; Szulczewski 1995). The most characteristic lithology linked with this sedimentary episode includes stromatoporoid-coral limestones, encompassing numerous members, such as the Sitkówka Beds and Kadzielnia Limestone (Fig. 5). They are well known from numerous exposures in the south-western part of the Holy Cross Mountains (Zamkowa Hill in Chęciny, Zelejowa Hill, Miedzianka Hill, Kadzielnia Quarry in Kielce). Impressive stone cladding tiles made from this material, exploited for almost 300 years in the Bolechowice-Panek Quarry (commercially known as the “Bolechowice marble”), can be observed in numerous modern and historical buildings throughout Poland. Facies differentiation within the platform and its range are difficult to determine precisely due to Variscan folding, uplift and later erosion of Devonian rocks, although it seems rather clear that the central part of the platform was the area of the Dyminy Anticline, uplifted during Variscan deformation and currently devoid of Devonian rocks (present-day Posłowickie and Dymińskie ranges). Thus, the stromatoporoid-coral complex is often referred to as the “Dyminy reef” (Narkiewicz 1988), or the “Dyminy reef complex” (Racki 1993). Such a lateral distribution is interpreted from the lithological analysis of Devonian successions surrounding the Posłowickie and Dymińskie ranges (Kaźmierczak 1971; Racki 1993), and to a certain degree is shown in a specific type of cyclicity, appearing on the platform margins.

Development of the carbonate platform was rapidly terminated by events at the boundary between two Late Devonian ages: Frasnian and Famennian. The nature of these events still remains unclear (compare Szulczewski 1989 and Narkiewicz 1990), but their influence on the evolution of organisms and sedimentary conditions within the shelf of Laurussia was far-reaching. A global biotic crisis led to the extinction of 50% of genera and impoverishment in the number of species in many animal groups, including the stromatoporoids responsible for the construction of large carbonate build-ups, determining the structure of carbonate platforms in the Ordovician, Silurian and Devonian around the world. The Upper Devonian succession of the Holy Cross Mountains was additionally influenced by minor tectonic block movements that uplifted and sank the sea floor. Parts of the sea floor with the most profound relief (usually organic build-ups) were often elevated above sea level, and their tops were subject to erosional shearing or even karstification. Subsidence of the sea floor induced the appearance of characteristic cephalopod and crinoid facies on these blocks, in which the increase of sediment thickness was compensated by submarine erosion. Low increases in total thickness resulted in highly condensed successions, in which the thickness of the sediment preserved per unit of time was several times lower than in successions in the surrounding, deep basins. Exceptional examples of such successions can be observed in the escarpments of Ostrówka Quarry near the village of Gałęzice in the south-western part of the Palaeozoic of the Holy Cross Mountains or in Kadzielnia Quarry in the city of Kielce. The deepening trend continued through most of the early Carboniferous (Szulczewski et al. 1996).

The total thickness of the Devonian platform facies in the Kielce Region is estimated at 800-1000 m. At that time, a similar thickness of strata was deposited in the Łysogóry Region but in completely different environments. From approximately the beginning of the Middle Devonian, this area was covered by a basin that was much deeper compared to the southern basin. The boundary zone between the basin and platform comprised a distinct slope, on which bioclastic material was transported through sliding, slumping and flows to form carbonate turbidites in the northern basin.

As mentioned at the beginning of this chapter, facies evolution of the Devonian in the Holy Cross Mountains was determined by eustatic sea level oscillations. Although the correlation between the global eustatic curve and the bathymetric curve for particular Middle Devonian formations differs in minor details among various authors (Skompski and Szulczewski 1994; Racki 1997; Malec and Turnau 1997; Narkiewicz and Narkiewicz 2010), it is rather clear that the bathymetry of the succeeding formations reflects well the rhythm of eustatic oscillations. In the younger, much deeper complexes of the Upper Devonian and lower Carboniferous, this rhythm becomes unclear. Particularly effective examples of dynamic sedimentary phenomena encompassing transport of material from shallower to deeper parts of the basin and their deposition on the slope and at its base can be observed in the Kostomłoty Beds (Szulczewski 1971), perfectly exposed in the north-western part of the Palaeozoic inlier of the Holy Cross Mountains (quarries on Kostomłoty Hills). Analogous sedimentation developed on the southern side of the platform, in the vicinity of an isolated Palaeozoic exposure known as the Zbrza Anticline. The area generally lacks good exposure, but recent research ditches have supplied new data on the presence of the Kostomłoty facies in this zone also (Wójcik 2009). The palaeogeographic outline shows that the northern margin of the Małopolska Block with the Kielce Region was not a stable zone at the end of the Devonian but rather comprised a series of uplifted blocks and subsiding basins. Gradually these blocks sank and the extent of the deep Łysogóry basin successively moved to the south. Carboniferous sedimentation supplied further arguments in support of this palaeogeographic pattern.