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.