Mikrobiostratygrafia osadów kredowych południowej części synklinorium łódzkiego

Abstract

Microbiostratigraphy of the Cretaceous deposits in the southern part of Łódź synclinorium (Central Poland) In the southern part of the Łódź synclinorium, the epicontinental Cretaceous deposits, from 200 m to 1000 m in thickness, overlie the Upper Jurassic deposits, mostly the Upper and Middle Portlandian and locally the Upper Kimmeridgian ones. The Cretaceous deposits are overlain, in turn, by the Tertiary and Quaternary ones, ca. 20—100 metres in thickness. The present paper discusses the micropaleontological stratigraphy and the correlation of the Cretaceous deposits in the boreholes localized in the Bełchatów and Tuszyn region (fig. 1). In the area in question lithostratigraphic studies have permitted to assign the Lower Cretaceous deposits, poor in fossils, to the Valanginian — Albian (Mrozek, in press). Their sequence, from the bottom, is : dark-grey shales; medium- and fine-grained sandstones with claystone intercalations; fine-grained sands with intercalations of mudstones and sandy claystones with scarce arenaceous Foraminifera 1 ; finegrained sandstones and mudstones or claystones with scanty macrofauna 2. The Middle Albian deposits are represented by glauconitic sandstones and mudstones with phosphorites. These beds contain soairce arenaceous Foraminiifeira (eg. Arenobulimina, Textularia). The overlying Upper Albian — Senonian deposits contain as a rule, numerous Foraminifera3. Fourteen foraminiferal assemblages, denoted with the symbols „ I—XIV” 4, have been distinguished; they allow the stratigraphy (fig. 2) and correlation of beds in the profiles of the investigated boreholes (figs. 3 and 4). The stratigraphie sequence (the Upper Albian-Campanian) established in the area under investigation on the basis of Foraminifera is as follows: Upper Albian. Three foraminiferal assemblages have been distinguished. In the region of Bełchatów in sandy glauconitic marls there are: Assemblage „ I ” (arenaceous Foraminifera with prevailing Arenobulimina), and Assemblage „ II” (arenaceous Foraminifera and scarce calcareous ones e. g. of the genera Gavelinella, Cibicides, Hedbergella). In the Tuszyn region in clayey marls there is rich Assemblage „ III” (dominant planktonie Foraminifera and Radiolaria). Cenomani an. Three foraminiferal assemblages have been distinguished. In the area of Bełchatów in sandy marls with glauconite there is Assemblage „IV” — most likely of the Lower Cenomanian age (the prevalence of arenaceous Foraminifera with the substantial content of Gavelinella among the calcareous forms). In the area under examination in light pelitic limestones intercalated with green marls Assemblage „V” — most likely of the Middle Cenomanian age (numerous Hedbergella and Rotalvpora) — has been established. In the Tuszyn region in grey clayey marls there is Assemblage „VI”, of the Upper Cenomanian age (prevailing Hedbergella). Turonian. Three foraminiferal assemblages have been distinguished. In the Bełchatów and Tuszyn regions in white limestones intercalated with greenish marls Assemblage „VII” of the Lower Turonian age (dominant Hedbergella and Praeglobotruncana), has been found. In marls and light limestones there is Assemblage „V III” , of the uppermost Lower Turonian or the Middle Turonian age (numerous and well preserved Globotruncana, Praeglobotruncana and Hedbergella). In grey marls and limestones with cherts Assemblage „ IX ” , of the Upper Turonian age (prevalent Globotruncana), has been recorded. Coniacian. In grey spotted mairls there is rich Assemblage „X ” (prevailing Globotruncana and Stensioina). Coniacian-Santonian In grey marls a well preserved Assemblage „XI” (numerous Globotruncana, Nodosariidae and Stensioina) has been found. Sant oni an. Two foraminiferal assemblages have been distinguished : in grey marls with glauconite there is Assemblage „XII” of the Lower Santonian age (numerous Nodosariidae, Globotruncana, Gavelinella, Stensioina). Above, in light-grey, partly sandy marls there is Assemblage „XIII” , probably of the Upper Santonian age (numerous Spiroplectammina, Gaudryina, Globotruncana, Gavelinella, Stensioina). Campanian In the region of Bełchatów in white marls from the outcrop Czarny Las (see fig. 1) there is Assemblage „XIV” (almost exclusively benthonic arenaceous and calcareous Foraminifera with the index species of Stensioina and Bolivinoides). The investigated boreholes are situated in anticlinal zones therefore they lack the Campanian and Maesitrichtian deposits that had been eroded. On the other band, Campanian and Maesitrichtian sediments fill vast synclinal zones (see fig. 1). The foraminiferal Assemblages „I—XIV” , discussed above, have allowed a detailed correlation of beds in profiles of the examined boreholes (figs. 3 and 4). Photographs of the index Foraminifera are shown in plates I—IV.