Exploring the Jurassic at Zalas Quarry, southern Poland
Tomasz Borszcz and Dr Michał Zatoń (Poland)
The area of southern Poland is well known for its widespread Jurassic deposits, in particular, Middle and Upper Jurassic sedimentary rocks that outcrop in a belt running from south-east to north-west in the area known as the Polish Jura Chain (Fig. 1). This area owes its name to the occurrence of spectacular klippes (outliers formed by thrusting) of by white, massive limestones deposited in the northern shelf of the Tethys Ocean during the Late Jurassic (Oxfordian). Because of their resistance to erosion, the rocks form a picturesque element in the surrounding upland landscape.
As well as these, the Middle to Upper Jurassic deposits (in the form of glauconitic sandstones, marls, platy limestones and sponge-dominated reef-like structures called bioherms) occur in several natural and artificial exposures along the whole Polish Jura Chain. They are, and used to be, a real Mecca for professional researchers since the 19th century, and also amateur collectors from both Poland and elsewhere. This not surprising, as the deposits contain abundant and diverse fossils, including nearly all the fossil groups characteristic of this geological period.
In this article, we will focus on the spectacular Middle to Upper Jurassic sequence that is exposed at the famous Zalas Quarry, which has been actively explored since the 1870s. It is located in the southern part of the Polish Jura Chain.
Locality and geology of Zalas Quarry
The quarry is located in the eastern part of the village of Zalas, situated about 30km west of the city of Kraków and about 8km south of the town of Krzeszowice (Fig. 1). This is a large, active quarry, where the extraction of rock mainly focuses on volcanic rocks called rhyodacites from the early Permian. These rocks are overlain by Middle to Upper Jurassic sediments that are the main focus of this article. The Jurassic sedimentary rocks at Zalas Quarry, covered by loess and other Quaternary deposits, are clearly visible and easily reachable along the quarry wall that runs from east to west. However, if you ever decide to visit the quarry to hunt for fossils (which we really do recommend), permission from the quarry owners must be obtained first.
The oldest Jurassic deposits exposed at the quarry are sands of near-shore marine origin, intercalated with quartzite sandstones and conglomerates. The sands are overlain by fossiliferous, sandy, crinoidal limestone, the name of which is derived from the remains of stalked sea-lilies (crinoids) appearing in and making up the sediments. Based on ammonites, the sands and sandy crinoidal limestones are dated as being Early Callovian (the fourth and last stage of the Middle Jurassic).
The top of this limestone is covered by a spectacular stromatolitic layer. Stromatolites are biosedimentary structures formed by tiny, blue-green microorganisms called cyanobacteria and associated assemblages of other micro-organisms. Such structures were very prolific in Precambrian times. After that they declined, and today they can only be seen in a few places of the world, mainly in shallow-water environments. The stromatolitic layer of Zalas Quarry is dated from the late Callovian and is considered to have been formed in a deeper, open shelf environment. Above the stromatolitic layer, there are pink limestones from the beginning of the Lower Oxfordian (the first and one of the three stages of the Upper Jurassic). The bulk of the Oxfordian strata (up to the lower part of the Middle Oxfordian) exposed at Zalas Quarry is represented by both grey-bedded limestones and marls, and also spectacular huge bioherms built by sponges. It is worth noting that, currently, the architecture of the bioherms here is the best example that can be seen of such structures in the Polish Jura Chain.
Fossils
Despite a dozen visits to Zalas Quarry, it still amazes us that we still find hundreds of different types of fossil. In most cases, the fossils are very well preserved and represent a broad spectrum of marine animals that lived in the shallow, epi-continental sea that covered the area during the Middle and Late Jurassic period. The fossil creatures represent the whole trophic structure, including tiny, planktonic foraminifers and radiolarians, benthonic molluscs (gastropods and bivalves), sponges and corals, echinoderms (for example, sea-urchins, sea-lilies and sea-stars), to free-swimming, nectonic animals such as belemnites, ammonites, nautiloids and those that preyed upon them – sharks and marine reptiles. (These latter groups are mainly known from their fossilised teeth.) Indeed, you can find all the characteristic fossils of the Jurassic that are known elsewhere from marine deposits of that age. And their abundance and variety make every trip a fruitful one!
After obtaining permission to visit, you should focus first on the Middle Jurassic (Callovian) crinoidal limestones that are full of different fossils, from tiny colonial bryozoans, fragments or even nearly complete skeletons of sea urchins, ossicles of sea-lilies, large bivalves (Pholadomya), gastropods and ammonites. The latter group is represented by the characteristic and common Macrocephalites. If you are lucky, you can find crabs, which are preserved as casts of isolated carapaces or chelae.
Above this section, you can find a characteristic horizon full of different fossils that are overlain by an undulating layer of stromatolites. However, the fossil horizon below the stromatolites is very interesting because it represents the so-called hardground. This represents a very slow period of sedimentation (or even a break in sedimentation) during which various shelly fossils (especially the large bivalve Ctenostreon, but also nautiloid, ammonite and other mollusc’s shells) served as a hard substrate for other different, small-sized creatures to cement to it. They include colonial bryozoans, various tubes of serpulid worms, tiny brachiopods and oysters. Because of their tiny sizes, they do not receive as much attention as larger fossils, for example, the ammonite Macrocephalites, which has a diameter of 30cm or more.
However, such fossils are very interesting for palaeoecological research, because, due to cementation, they are preserved in life positions. The stromatolite layer, covering the hardground, is another interesting thing. Formed by one-celled microorganisms, they serve as a window into the microbiological world existing in a deep Jurassic sea. Over a period of many years and without any serious disturbance, lamina-by-lamina, bacteria and fungi built an extensive, wavy, carbonate structure.
It is worth noting that the stromatolites from Zalas quarry represent one of the most beautiful and best developed of such structures in the Middle Jurassic deposits in Poland. Higher in the section, grey and thick, wavy structures are visible. Built by siliceous sponges of different morphologies, they form huge ‘sponge-reefs’ with a characteristic, undulating shape. Within this framework, you can also find different fossils, including brachiopods, ammonites, belemnites, fragments of sea urchins (especially their spines), sea-stars, brittle-stars and sea-lilies.
The first group allows us to date the bioherms as being Oxfordian in age. Originally, the sponge bioherms might have built immense organic domes in the Late Jurassic sea, harbouring a vast number of animals. Indeed, such Oxfordian bioherms were extensive structures, ranging from Iberia in the west, to the Caucasus in the east, along the northern shelf of the Tethys Ocean. In the opinion of some experts, their origin may be related to volcanic intrusions below them, which occurred in the Cracow region. Evidence for this is the fact that Jurassic deposits in Zalas quarry directly overlay volcanic rocks. The sponges are also encrusted by tiny bryozoans and serpulids, forming the external construction of the ‘reef’.
And finally…
Although the Jurassic deposits are widespread in the Polish Jura area, Zalas quarry is somewhat unique and possesses something that make us return there again and again. Not only can you find the best Jurassic stromatolites and the most spectacular sponge bioherm in the region, but you can also easily find plenty of very well-preserved fossils. The only problem is getting permission to collect as you have to get this before entering the quarry. And, once you have permission, remember that this is an active quarry and such things as safety helmets and high-visibility jackets are very important.
Further reading
Matyja, B. A. & Tarkowski, R. 1981. Lower to Middle Oxfordian ammonite biostratigraphy at Zalas in the Cracow Upland, Acta Geologica Polonica, Vol. 31, No. 1 – 2: 1 – 14.
Nawrocki, J., Polechońska, O., Lewandowska, A. & Werner, T. 2005. On the palaeomagnetic age of the Zalas laccolith (southern Poland). Acta Geologica Polonica, Vol. 55, No. 3: 229-236.
Radwańska, U. 2005. Callovian and Oxfordian echinoids of Zalas, Volumina Jurassica, Vol. 3, s.: 63 – 74.
Trammer, J. 1982. Lower to Middle Oxfordian sponges of the Polish Jura, Acta Geologica Polonica, Vol. 32, No. 1 – 2: 1 – 39.
Wierzbowski, A. et al. (eds.). 2006. Jurassic of Poland and adjacent Slovakian Carpathians. Field trip guidebook of 7th International Congress on the Jurassic System, Poland, Krakow, September 6-18, 2006.