Biostratygrafia osadów kambru w polskiej części platformy wschodnioeuropejskiej

Abstract

W osadach kambru wyróżniono i udokumentowano 3 poziomy nietrylobitowe i 15 poziomów trylobitowych. W porównaniu ze skandynawskim podziałem biostratygraficznym stwierdzono brak osadów poziomów: Ptychagnostus punctuosus, Triplagnostus lundgreni i Goniagnostus nathorsti (A.H. Westergard, 1946 = Ptychagnostus lundgreni i P. nathorsti - J. Bergstrom, 1980) w kambrze środkowym oraz Leptoplastus i Protopeltura praecursor w kambrze górnym. Dolną granicę kambru prowadzi się w spągu utworów, w których pojawiają się gatunki Tyrasoteania podolica Gnilovskaya. T. tungusica Gnilovskaya i Sabellidites cambriensis Yanishevsky,.górną zaś wyznacza pojawienie się charakterystycznych dla tremadoku graptolitów Dictyonema flabelliforme s.l. oraz licznych ramienionogów Obolus apollinis Eichwald.BIOSTRATIGRAPHY OF THE CAMBRIAN DEPOSITS IN THE POLISH PART OF THE EAST-EUROPEAN PLATFORM The Lower, Middle. and partly Upper (in its bottom) Cambrian in Polish part of the East-European Platform are represented by terrigenous deposits, while carbonate and argillaceous rocks are representative for the remaining members of Upper Cambrian (Figs. 1, 3). Bedrock of Cambrian deposits is composed of the Proterozoic formations of different age and in places of the Archaean itself. Only in west part of the Peri-Baltic Syneclise and in the Lublin slope of the Platform a sedimentary continuity is maintained from the Vendian through the Cambrian. Stratigraphic hiatus appears both in the Middle and the Upper Cambrian. Deposits of the Eccaparadoxides oelandicus Superzone terminates the Cambrian profile over much of the area under study, whereas the Acerocare Zone (Fig. 3) is the upper-most member of Cambrian within north-western part of the Peri-Baltic Syneclise. V e n d i a n - C a m b r i a n. Within the western part of the Peri-Baltic Syneclise, both the lowermost Cambrian and the Upper Vendian deposits constitute the Żarnowiec Formation. Fauna has not been yet known in the Formation. As K. Jaworowski (197·9) suggest, continental conditions had governed the deposition of the Żarnowiec Formation in environment of alluvial fans. The crystalline basement morphologicaly differentiated had been the surface of deposition and the sedimentation of the Żarnowiec Formation had begun in that area the development of sedimentary cover of old platform. L o w e r C a m b r i a n. When sedimentation within the Polish sector of north-western part of the East-European Platform was governed by the continental conditions then the marine ones dominated over south-eastern Poland's territory. The sea basin, existing there from the beginning of the Upper Vendian, progressively expanded and reached its maximum extent in the Holmia Zone of the Lower Carnbrian. The five Zones have been distinguished on the base of fauna found in the Lower Cambrian. They are (in chronological order): Sabellidites. Platysolenites, Mobergella, Holmia, and Protolenus(Figs 2. 4, 5, 7). Lack of the trilobite fauna is a characteristic feature of the first three Zones (Fig. 4). The trilobite fauna is fundamentally important in the Holmia and the Protolenus Zone (Figs 4, 5). From a faunistical point of view the Holmia Zone is to be subdivided into two independent Subzones; this is well motivated by the expansion and the assemblage of some trilobite species. For the present the author tentatively suggests to designate the lower Subzone as the Schmidtiellus one and the upper Subzone as the Strenuaeva one. The Schmidtiellus sp. and Holmia grandis Kiaer are the base for separating the Schmidtiellus Subzone. The Strenuaeva Subzone is faunistically better evidenced, since its trilobite fauna is represented by: Strenuaeva primaeva (Brögger), ElIipsocephalus cf. gripi (Kautsky), Strenuella aff salopiensis Cobbold. and S. sp. ex gr. polonica. The Protolenus Zone within the platfonn area is fairly rich in trilobite fauna with: EIlipsocephalus hoffi (Schlotheim), Kingaspis (Kingaspis) borealis Lendzion and Protolenus sp. M i d d l e  C a m b r i a n. With regard to a common trilobite index fauna, the division of the Middle Cambrian within the platform areas has, with one exception, followed the Scandinavian scheme. Instead of stages. typical for the Scandinavian division, superzones have been established whose names have come from characteristic trilobites. Thus, the Middle Cambrian within the platform areas in Poland has been subdivided into three Superzones: Eccaparadoxides oelandicus, Paradoxides paradoxissmus and Paradoxides forchhammeri (Figs 2, 3, 5. 7). Documentation of zones distinguished within particular superzone.s is clearly shown in Fig. 5. Between the Paradoxides paradoxissmus and the Paradoxides forchhammeri Superzones an interruption appeared in deposition, which continued through the Zones of Ptychagnostus punctuosus, Triplanognostus lundgreni (= Ptychagnostus lundgreni), and Goniagnostus nathorsti (= Ptychagnostus nathorsti), well known in the Middle Cambrian of Scandinavia. U p p e r  C a m b r i a n. In the Upper Cambrian of platform area of Poland there have been separated following zones (in chronological order): Agnostus pisiformis, Olenus and Homagnostus obesus, Parabolina spinulosa, Peltura minor (= Sphaerophthalmus alatus). Peltura scarabaeoides, and Aceroeare (Figs 2, 3, 7). Fig. 6 provides the fauna assemblage characteristic for particular zones. In comparison with the typical stratigraphic profiles of southern Scandinavia a stratigraphic hiatus has been revealed in Poland, which lasted from the Leptoplastus Zone through the Protopeltura praecursor Zone. Problem of lower boundary of Cambrian appears with respect to those profiles where a marine succession continued during the Vendian and the Cambrian. In such cases the boundary is delineated in the base of deposits with the first appearance of new species of algae such as Tyrasotaenia podolica Gnilovskaya, and Tyrasotaenia tungusica Gnilovskaya accompanied by Sabellidites cambriensis Yanishevsky. Over most of the Polish part of the platform area the Cambrian - Ordovician boundary is of erosive nature, so it is distinct and sharp. On the contrary, within the north-western areas of the Peri-Baltic Syneclise there is a sedimentary continuity from the Cambrian through the Ordovician, thus the boundary is based on the biostratigraphic data. The Upper Cambrian boundary is delineated by appearance of Dictyonema flabelliforme s.l. graptolites and abundant Obolus apollinis Eichwald brachiopods, both already characteristic for the Lower Ordovician. Fauna found in the platform-type Cambrian deposits is represented in Poland by abundant cosmopolitic species, This allows for the broad biostratigraphic correlation exceeding far away the Acado-Baltic Sub-province (Fig. 7; K. Lendzion, 1983). Separate faunal feature of the Middle Cambrian in Czechoslovakia is a particular case having no univocal paleontologic basis for placing it in time interval as given in Fig. 7. Similar unreliable situation referring to the correlation of the Middle Cambrian in Morocco and the platform·type Cambrian in Poland (and particularly the Middle Cambrian of the Acado-Baltic Sub-province) results from poor development of Moroccan Cambrian and its insufficient documentation.