Fossils from the Polish Bathonian clays

The Middle Jurassic Bathonian stage, which is preceded by Aalenian and Bajocian and overlaid by the Callovian, was established on the basis of oolitic limestones outcropping at Bath in Somerset. This historical and English connection is a major reason I have chosen the Bathonian as a topic for Deposits Magazine. The Bathonian clays in Poland, like the English classic Kimmeridge Clay or Callovian Oxford Clay, are characterised by their rich fossil content. Although some years ago, the Bathonian clays from Poland were not as well known as these two English formations, today they have become progressively more recognised outside of Poland. This is due to an increasing number of publications dealing with different aspects of the clays and the 7th Jurassic Congress held in Kraków (southern Poland) in 2006, during which scientists from all over the world had the chance to meet and actually look at the Bathonian clays.

Geological and palaeogeographical background

The best outcrops of Bathonian clays are in southern and south-central Poland, in an area called the Kraków-Częstochowa Upland (Fig. 1). Here, the Jurassic rocks, and especially Oxfordian (Upper Jurassic) limestones, form a distinct belt stretching approximately in a south-east to north-west direction. That is why the late Professor Stefan Zbigniew Różycki in 1960, when comparing the area with such classic areas as the Swabian and Franconian Jurassic, called it the ‘Polish Jura’.

Fig.1Fig. 1. A. Map of Poland without the Cenozoic cover: 1. Pre-Jurassic, 2. Jurassic, 3. Cretaceous; PJ – Polish Jura. B. Polish Jura area; 1. Upper Triassic, 2. Lower Jurassic, 3. Middle Jurassic, 4. Upper Jurassic, 5. Cretaceous, 6. Miocene, 7. Pliocene.

Actually, the Bathonian sediments under discussion are a form of claystone and mudstone, but the simple name ‘clay’ is commonly used to refer to these rocks that form a part of the so-called ore-bearing Częstochowa Clay Formation. The sediments are lithologically monotonous, dark-grey and sometimes black in colour. When freshly excavated, they are very soft. On the other hand, when dry, they can easily be split, lamina by lamina, by a knife. However, the most characteristic features of the sediments are carbonate nodules. They may occur as single bodies or as more or less continuous horizons. In addition, massive, grey siderite beds occur. These siderite bodies used to be exploited in several iron-mines that operated in the Kraków-Częstochowa Upland. Carbonate nodules and/or siderite beds may form a few horizons within a particular section. Using ammonites, the Bathonian clays have been subdivided into standard zones and subzones. These clays are underlain by the uppermost Bajocian ones and overlain either by Quaternary sands and/or gravels or by Callovian condensed glauconitic marls and limestones. However, in comparison to the Bathonian exposures, the Bajocian is scarcely represented in the area under discussion.

During Bathonian times, Poland was covered by an epicontinental sea forming the Polish Basin. This was surrounded by a large area of land to the north (referred to as ‘Fennoscandia)’, by Belorussian and Ukrainian lands to the east, and Bohemian land to the south-west. To the south, the basin was bordered by the pre-Carpathian region. It is believed that the main connection with the Tethys Ocean, which was situated to the south, was possible only through the so-called ‘East-Carpathian Gate’ south-east of the basin. To the west, the basin was well connected with Germany and other western and north-western areas, including England (Fig. 2). The large land areas surrounding the basin on almost every side provided a large amount of siliclastic material from weathering, which served as a substrate for many organisms living there.

Fig.2Fig. 2. Palaeogeographic sketch of the Polish Basin and surrounding areas in Jurassic times.

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