The occurrences of Upper Permian marine sediments – Zechstein – in the Leszczyna region are very interesting for geology enthusiasts. Zechstein is a traditional name associated with the German term for copper-rich rocks exploited in mines in the Mansfeld Basin. One can connect geological, biological, and historical observations here. The geological aspect is interesting here due to the opportunity to independently interpret the history of environmental changes recorded in the rocks of the former marine basin.
In the lower part of the exposed layers, in a narrow trench at the lower level of the quarry, we observe the alternating arrangement of gray limestones and brownish-gray marls. Among the lowest layers, we can notice red, irregular spots, while in the higher layers – green and blue deposits. Above the layers with spots (hence the name spotted marls), an elevated gold content has been observed, reaching up to 0.3g/ton. The layers with colorful deposits are copper-bearing marls. The name reflects their compositional significance: the green deposit is malachite, and the blue one is azurite. Both of these minerals are hydrated copper carbonates, formed through the influence of water and oxygen from weathering copper sulfides. The copper content in the rocks exceeds 1% and was attractive to miners in previous centuries.
The layers of gray limestones above have a significantly greater thickness than the marls that separate them. Due to the presence of lead and zinc sulfides in their lower part, they were classified in mining and stratigraphic nomenclature as lead-bearing marls.
The higher level of the quarry is represented by a large rock wall. At its lower part, we can observe the highest part of the lead-bearing marl layer. The complex of marl-limestone rocks was formed on the sea bottom, which was not directly affected by the action of even the strongest storms. Only the limestone layers are a trace of the restless activity of the Zechstein sea. They were formed as deposits of bottom currents, loaded with particles washed near the shore and transported to deeper parts of the sea. Often in their lower part, we can find shells of mussels and snails, sometimes crushed during transportation.
The majority of the exposure of the upper level consists of limestones and dolomitic limestones. The beds have a thickness ranging from a few centimetres to over two metres. In the lower part of the exposure, the beds have uneven upper (roof) surfaces, and within them, there is a clear diagonal layering visible. If we could trace the appearance of these beds in a cross-section perpendicular to the wall, we would see a similar structure – the limestones here appear as flat domes several metres in length. This is the so-called hummocky cross-stratification. They form during the deposition of small mounds on the seabed between the depressions eroded in the bottom by storm eddies. Such structures are a record of the shallowing of the water reservoir. They document its state when the base of the turbulent waves during the storm touched the bottom of the reservoir. So we can see the effects of the elements preserved from millions of years ago!
Over the storm layers, we can see thick beds. They formed in shallow water, on the continuously waved bottom. These consist of accumulations of shells, mainly mussels and snails, too heavy to be lifted to deeper water. They are usually crushed and rounded; often, it is necessary to prepare a microscopic slide from the rock to examine them closely. Some of these thick banks contain crushed remains of bryozoans – colonial animals with calcareous skeletons living on the seabed. Their colonies resembled moss branches, from which they borrowed both the Polish and Latin names (Bryozoa).
The limestones and dolomitic limestones containing fossils formed a barrier that separated the shallow lagoon stretching from here towards Stanisławów and Kondratów. The sea was becoming shallower, and fewer organisms lived on the bottom. The main component of the sediment being formed was grains created in the moving water through the chemical precipitation of calcium carbonate. In cross-section, they resemble small spheres or beans, composed of many very thin layers. The grains, which were carried in the water, have diameters of up to two millimetres. We call them oolites. Larger ones growing on the bottom are oncoids. The grainy layers are usually found in the upper parts of the barrier. We can also observe them in the rock blocks at the foot of the wall.
Above the formations of the barrier lie white and pink sandstones. Diagonally layered, often at a fairly large angle, they document the next stage in the history of this region – the retreat of the sea and the arrival of the beach.
