Słodkowodna fauna utworów mioceńskich z Bełchatowa i jej znaczenie stratygraficzne

Abstract

FRESHWATER FAUNA OF THE MIOCENE DEPOSITS FROM BEŁCHATÓW, AND ITS STRATIGRAPHICAL SIGNIFICANCESummaryThe Belchat6w trough, situated about 40 km south of Lódź, constitutes an elliptical, strongly elongated form, narrowed in the middle, that runs in a WE direction. It is fairly strongly disturbed tectonically, and cut by numerous faults. The lowermost beds of the Tertiary formations are built up of green sands and muds, probably of Bartonian age. It is possible that the underlying grey quartz sands correspond in age to the Toruń clays. These sands lying on the Cretaceous or Jurassic deposits, are covered with a bed of limestones, marls or siltstones with plant detritus, brown coal beds, locally also with fauna remains. Higher up there rests a brown coal bed (20-80 m in thicknes) with intercalations of lake marls, siltstones and clays. These, in turn, are overlain with sands, siltstones and clays, 40 m in thickness, with a green clay bed that contains some siltstone partings. This series is covered with 30–126 m thick Quaternary formations.Among 31 fauna species encountered in the Miocene deposits within the Bełchatów region, 12,7% belong to the Lower Miocene forms, 26,8% are Middle Miocene species, 6,4% represent species known to occur in the entire Miocene, 3,2% of the species are characterized by their extent from Lower Miocene to Pliocene, 26,8% of the species are referred to the Upper Miocene, and 6,4% to the Lower Pliocene ones. In addition, 22,2% of the Miocene species found in the Bełchatów area, are in common with those of the Miocene freshwater formations of Podolia, described by M. Łomnicki.About 15 km south of the Bełchatów trough, near Smotryszów, there occur freshwater limestones bearing planorbis forms, resembling those from the bottom part of the Bełchatów beds. This may evidence that they were formed at the same time maybe also in the same lake basin. In the Bełchatów trough, above the series with the planorbis limestones, a bed of marls, and clays occurs, crowded with spores of Cypris althi and Chara escheri. This bed strongly resembles the Miocene freshwater marls from Podhajce in Podolia, also described by M. Łomnicki. On account of the predominance of the Middle and Upper Miocene species in the Tertiary deposits of the Bełchatów area, and due to their great similarity to the formations from Smotryszew and Podolia, the age of which is thought to be Helvetian – Tortonian boundary, this age may be referred to the brown coal deposits from the Bełchatów vicinity, as well.It is worthy to be stressed, too, that the Miocene deposits from the Bełchatów trough contain, at the bottom, mainly lacustrine fauna; towards the top, continental species gradually appear, thus proving that, initially, the basin was a lake which little by little underwent a shallowing process, and consequently changed into a vide moor.