Stratygrafia jury środkowej regionu bełchatowskiego

Abstract

Przedmiotem badań były profile kilkunastu płytkich otworów wiertniczych, wykonanych na S i W od Bełchatowa, ukazujące fragmentarycznie nawiercone osady jury środkowej (bajos, baton i kelowej). Przedstawiono Zwięzłą charakterystykę litologiczną tych utworów, ich korelację oraz podział stratygraficzny oparty na szczegółowej analizie znalezisk amonitów, małżów, otwornic i małżoraczków.

STRATIGRAPBY OF THE MIDDLE JURASSIC OF THE BEŁCHTÓW REGION

About a dozen boreholes entering the Middle Jurassic directly underlying the Tertiary in the Bełchatów region (S of Łódź) were studied in the years 1964-1965. The studied sections are situated in ,two neighbouring areas: northern, representing a part of the Łękińsko anticlline (Bełchatów trough zone), and southern, comprising the Dąbrowa Rusiecka anticline (Polish Jura Chain).

The oldest Middle Jurassic strata penetrated by the drillings are represented by white, coarse-grained, faunistically barren sandstones belonging to either Lower Bajocian or the lowermost Upper Bajocian. They are overlain by 200 m series of siltstone-clay rocks with intercalations of sandstones, chamosite oolites, clay ironstones and oyster lumachelles, belonging to the Częstochowa ore-bearing clay formation (Upper Bajocian - Mdddle Bajocian, see Fig. 1). Upper parts of the Middle Jurassic (Upper Bathonian - Callovian), recorded in one drilling only, were strongly faulted which precluded any more accurate characteristics. Lower parts of the Upper Bajocian (? Lower Kuiavian) did not yield any guide fauna. Such fauna (ostracodes, foraminifers - see Table 1) was not recorded below ;the Middle Kuiavian but even there cephalopods were too scarce to carry out zonation of these strata.

Ammonites, including representatives of index species, are somewhat more numerous in the Upper Kuiavian (Lower Bathonian sensu anglico), especially in its upper parts which may be divided into two uppermost Kuiavian zones: the yeovilensis and tenuiplicatus zones. In the foraminifer assemblage of the latter zone there is recorded gradual disappearance of Ophthalmidium carinatum terquemi P a z d r o (the end of continuous range), giving further support Ito its stratigraphic value.

Ammonites are moderately frequent in ,the Procerites spp. zone, Lower Bathonian (Middle Bathonlian sensu anglico, pars), being represented by specifically unidentifiable Procerites (Gracilisphinctes) sp:, Oecotraustes (Paroecotraustes) formosus A r k e l l  and Eohecticoceras costatum (R o e m er). In this zone also disappear some Miliolidae: Ophthalmidium carinatum porai P a z d r o, O. sp. 1 and Palaeomiliolina rawiensis (P a z d r o).

In the Middle Bathonian (Morrisiceras morrisi and Cadomites bremeri zones), ammonites are fairly rare, being represented by Eohecticoceras costatum (R o emer), E. sp. and Wagnericeras sp. Rocks become more sandy here, which results in impoverishment of foraminiferal assemblage in all but agglutinating forms, quite common in some places. The Upper Bathonian, similarly as the Callovian, is fragmentarily known but faunistic record of its lower parts (Oecotraustes heterocostatus zone) is fairly unequivocal: Oecotraustes (Paroecotraustes) cf. paradoxus R o e m e r), O. (P.) densecostatus L i s s a j o u s, ?Tragophylloceras bathonicum W e s t e r m a n n. No guide fossils were found in the Callovian.