Dr Michał Zatoń (Poland)
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’.
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.
Interesting localities and fossils
Bathonian ore-bearing clays outcrop in several localities scattered over the whole Polish Jura area (Fig. 1). They are mainly exposed in the local brickyards, the majority of which still use them for brick production. However, some of the brickyards, which were interesting to geologists in the past, are now closed and are in the process of being returned to agriculture use. As the clay facies in the whole Bathonian exposed is the same, I will focus on a few localities that are worth exploring. Other than ammonites, the benthic fauna is rather similar in the whole Bathonian succession.
(a) The Gnaszyn and Kawodrza brickyards
The first and most interesting sites are the Gnaszyn and the neighbouring Kawodrza localities (Fig. 1).
In this small area, almost the whole uppermost Bajocian to Upper Bathonian clay sequence is exposed. It is also the only locality where there are currently seven active brickyards. The ‘Gnaszyn’ brickyard (Fig. 3), located at Gnaszyn Dolny (Fig. 1), is the largest in the whole of this upland region.
This is a site that is often explored by professional geologists and palaeontologists, as well as amateur collectors, and access to the sediments is quite easy. All you have to do is go to the caretaker’s lodge and ask him, who will call his boss who almost always agrees to allow access. The section is impressive as it consists of about 20m of clays, intercalated with a few horizons of carbonate nodules. It was dated as Middle to Upper Bathonian. The fossils are numerous, either in the host clay or in nodules. In the clay, they are almost invariably flattened, but in nodules they are preserved three-dimensionally. However, both in the clay and the nodules, the fossils have preserved the aragonitic, pearly (nacre) structure of the shells.
Macroscopically, it is possible to find plenty of bivalves, ammonites, belemnites, gastropods and even fish teeth. If you split the nodules, you can sometimes find several specimens in one go. However, if you do split them, it is quite common not to find fossils, but rather beautiful minerals (calcite, siderite) inside.
Bivalves are very often represented by well-preserved and large Pholadomyaand smaller Goniomya, as well as oysters. Ammonites are represented by oppeliids (Oxycerites, which are common), perisphinctids (Procerites and Choffatia), tulitids (Tulites, which are rare and Morrisiceras, which are very common) and rare stephanoceratids (Cadomites). Belemnites, preserved as calcitic guards, belong to the Hibolites and Belemnopsis genera. Nautiloids (belonging to the Procymatoceras genus) also occur, but are usually poorly preserved merely as phragmocone fragments and isolated chambers.
In the brickyard, you can also find plenty of fossil conifer wood fragments and, if lucky, you may even find the leaves of seed-ferns, but these are extremely rare. Sometimes, when the relevant sediments are dry enough, it is possible to find characteristic clumps of various crushed fossils, representing the meal of marine vertebrates, probably fish. For collectors, a spoil heap just above the clay-pit, where all the nodules are transported to, is very attractive. There, the majority of fossils and minerals can easily be found by simply hammering the nodules.
(b) The Leszczyński brickyard
The second site is the so-called Leszczyński brickyard (named after the owner), located in Kawodrza Górna, south-east of the Gnaszyn brickyard mentioned above (Fig. 1). Here, we can see the Lower Bathonian clay sequence, intercalated by two massive siderite horizons. The most interesting is the second and highest horizon, where an enormous number of ammonites of the Asphinctites–Polysphinctites dimorphic group can easily be found. Other ammonites that can be found are those belonging to Procerites (quite common), Oxycerites (common), Calliphylloceras (rare) and Lissoceras (very rare) genera.
With a bit of luck, nautiloids can also be found. Other fossils consist of belemnite guards, different types of bivalves and gastropods. Ammonites are numerous, but their state of preservation is not as good as some of the other fossils that can be found. Generally, they are flattened and their innermost whorls are so weak that they have become crushed, as a result of which, you mainly find only their last whorls. However, such delicate elements as lappets in microconchs are still preserved. The shells of ammonites are preserved as chalky, calcareous dust that can make your fingers dirty.
(c) The village of Faustianka
The next interesting locality is the small village of Faustianka, situated quite far north-west of Częstochowa (Fig. 1). Here, you can see the uppermost part of the Lower Bathonian and lowermost part of the Middle Bathonian. The most interesting part of the clay section exposed here is the lowermost level of brown nodules, which corresponds with that described above at ‘Leszczyński’ brickyard.
Some time ago, beautifully preserved ammonites, especially belonging to the Asphinctites–Polysphinctites dimorphic group (could be found there in large numbers, along with Oxycerites, Wagnericeras and Procerites. Their uncompacted shells displayed a multi-coloured iridescence as a result of the nacre that is attractive to collectors. This was also the main reason that the owner of the brickyard collected fees from those who wished to explore his clays. Unfortunately, for a few years, the ammonite-bearing horizon has been covered up. However, the brickyard is still active and it is simply a matter of time before this horizon is re-excavated again.
(d) The Krzyworzeka brickyard
A further site can be found at the Krzyworzeka brickyard, situated in the northernmost part of the Polish Jura (Fig. 1), where Upper Bathonian clays are exposed. Here, as well as fauna found in the host clay and one level of large nodules containing ammonites (Oxycerites and Cadomites), belemnites and bivalves (large Pholadomya and Goniomya), there are two concretionary horizons of a different kind. These small concretions are calcitic and oval to irregular in shape.
They have been intensively bored and encrusted and are referred to as the hiatus concretions, as they underwent episodes of exhumation on the seafloor after their formation within the sediments. This allowed various biotas to bore and encrust them. In appearance, the concretions look like Swiss cheese, as they are full of holes that are assigned to Gastrochaenolites ichnofossils, and made by the boring of bivalves.
The empty borings are often colonized by another bivalve, Hiatella, or different encrusting biota, such as serpulid worms (Serpula and Dorsoserpula), cyclostomatous bryozoans and bivalves (Plicatula). What is clear is that this locality yields not only fauna that lived on the soft bottom of the sea and are preserved in the clays, but also the hard-substrate communities, colonising a completely different substrate during the episodes of re-excavation of concretions and sedimentation pauses (or hiatus).
If you can get the permission from the boss of the local brickyard to enter, there is no problem in getting the collected fossils from the site. It is worth noting that if you want to retrieve more faunal groups, you will have to sieve large clay samples. If you do, you can find a diverse variety of echinoderm skeletal elements (sea-stars, brittle-stars, sea-urchins, sea-cucumbers and sea-lilies), tiny gastropods, bivalves, sharks’ teeth, ostracods and foraminifers.
However, apart from a rich invertebrate fauna, some of which still lack a proper scientific study (especially the bivalves, brachiopods and crabs), and fish teeth, no vertebrate skeleton has been found in the Bathonian clays so far. This may be because of the nature of the soft sediment that has impeded proper research. However, it is more probable that, up until now, collectors have been more focused on ammonites embedded in nodules. Maybe someday, an amateur collector will excavate an interesting Jurassic sea monster, one of those known from the famous Callovian Oxford Clay.
About the author
Dr Michał Zatoń is an assistant professor at the Faculty of Earth Sciences, University of Silesia, Sosnowiec in Poland. He is interested in palaeobiology, especially from the Middle Jurassic biota of the Bajocian-Bathonian siliclastic deposits of Poland.