Epikontynentalne baseny permu i mezozoiku w Polsce

Abstract

Na podstawie map litofacji i miąższości, zamieszczonych w serii artykułów w n-rze 1/88 Kwartalnika Geologicznego, dokonano analizy zmian cech rozwojowych basenów permsko-mezozoicznych na Niżu Polskim. Wyróżnionych sześć pulsów ekspansji basenów jest na ogół zgodne z etapami transgresyjnymi, wywołanymi eustatycznym podniesieniem poziomu wód oceanów. Najtrwalsza komunikacja międzybasenowa istniała w przedłużeniu bruzdy środkowopolskiej ku NW i SE, najbardziej epizodyczna zaś - ku wschodowi. Zauważono również w dynamice ekspansji basenu na NE i SW od bruzdy. Położenie bruzdy ustabilizowało się dopiero od wczesnej jury, garby wewnątrz basenu w jego części SW ulegały również przemieszczeniom. Tempo akumulacji osadów z biegiem czasu malało, wzmagając się tuż przed fazami raptownej ekspansji basenu i znacznie słabnąc w czasie tych faz. Transport materiału klastycznego z południa dominował w zasadzie tylko w początkowych okresach. Rozwój facji węglanowych związany był wyraźnie ze swobodną komunikacją z basenami Tetydy. Sedymentacja, począwszy od późnego triasu, była lokalnie silnie modyfikowana przez wzrost antyklin solnych i formowanie rowów synsedymentacyjnych. Stadia rozwojowe basenu (perm-trias, wczesna jura, środkowa -późna jura, kreda) definiowane są przez zespół analizowanych cech i wykazują silne związki z etapami ewolucji sąsiednich basenów oceanicznych.

EPICONTINENTAL PERMIAN AND MESOZOIC BASINS IN POLAND

A set of 46 litbofacies and thickness maps, published recently in Kwartalnik Geologicny (1988, papers denoted by asterisks in the list of references), served as a main source for evaluating the evolution of the epicontinental basins in Polish territories during the Permian and Mesozoic. Following features have been analyzed: primary extents of the basins (sometimes modified in comparison with the source maps - see Fig. 1); trans-and regressive pulses; interbasinal connections (Fig. 2); changes in position of intra basinal depocentres (Fig. 3) and uplifts (Fig. 4); rates of sediment accumulation in the major depocentres (corrected for compaction); megafacies and clastic transport; manifestations of local diastrophism (Fig. 6). These features have been plotted against the time scale (Fig. 7).

Essential conclusions are given below.

1. Polish Basin was the eastern part of the Mid-European Basin (Fig. 2). Its axial zone with almost continuous sedimentation is called the Mid-Polish Trough (MPT).

2. There were six pulses of basinal expansion (Fig. 7). Four of them were connected with the well-known eustatic sea-level rises culminating in the Anisian-Ladinian, Toarcian, Oxfordian and Turonian. The latest Permian and Norian pulses bear a more local character.

3. The latest Permian and Early Jurassic transgressions were coming from the west while the Middle Triassic one - from the south. The greatest eustatic transgressions (Late Jurassic and Late Cretaceous) were preceded by marine incursions from the Tethys into the MPT (Aalenian and Berriasian. respectively). Connections to the northwest were then quickly restored, but the seas spread rapidly outside the MPT only some 20-30 Ma later (Late Bathonian and Late Albian, respectively) and reached their maxima after the next to - 15 Ma.

4. Southwestern part of the basin developed on the basement of partly eroded Palaeozoic orogens. northeastern part - on the Palaeozoic platform cover of the East European craton (EEP). The former was divided into a number of small blocks reflecting probably the morphotectonic zones of the orogens; the latter was more elevated and rather slightly warped. Therefore, the basin was expanding easier to the southwest. and - while shrinking - was abandoning these areas with more difficulty. However, once the sea reached a sufficiently high level (Late Jurassic and Late Cretaceous), it penetrated far into the interior of the EEP.

5. Northwestern connection with the Danish Basin was most permanent (Fig. 7) which stressed the role of the MPT. Its southeastern extension towards the Tethys opened probably in the Norian and remained free ever since except for the late Early Cretaceous. On the contrary, the eastern connection was opened only during the maximum high-stands of the sea.

6. During the Permian and Triassic the position of the MPT was not yet fixed (Fig. 3); especially in the Middle and Late Triassic the depocentres were dispersed and frequently situated outside it. Since the Early Jurassic the MPT has been firmly stabilized along the edge of the EEP. In the times of maximum basinal contraction the sedimentation was confined to the MPT which was also strongly accentuated just before the rapid basinal expansions when the sedimentation rates were about an order greater within the MPT than beyond it.

7. Position of intrabasinal uplifts with reduced sedimentation (Fig. 4) as well as the configuration of the basinal shorelines (examples in Fig. 5) were evidently conditioned by the geology and tectonics of the sub-Permian basement.

8. Clastics were shed mainly from the south until the Late Scythian (Fig. 7) due to relatively fresh relief of the Hercynian mountains. The northern influx prevailed later, indicating the permanent rejuvenation of the Scandinavian Shield topography. Development of carbonate sedimentation depended clearly from the free communication with the warm Tethyan seas.

9. Sedimentation rates in general decreased with time (Fig. 7). They were high (more than 150 m/Ma) until the Carnian, except for the Middle Triassic carbonates. Two peaks in the latest Permian (520 m/Ma) and Late Scythian (640 m/Ma) were different in character, the first being accompanied by precipitation of salts and anhydrites, the second - by clastic accumulation. From the Norian to the Toarcian the rates were moderate (50-120) m/Ma). During the great eustatic transgressions the rates decelerated distinctly in the phases of rapid basinal expansion (5-30 m/Ma) while they were moderate in the initial and maximum phases.

10. Local diastrophism was expressed by the growth of salt anticlines and by the formation of narrow synsedimentary graben. Flowage of the Zechstein salts started in the Middle Triassic, graben formation - in the Late Triassic. Both features have been since intermittently active, especially in the limes of basinal contraction. It resulted in a system of graben bordering the MPT and in a group of abOut 90 salt antyclines. both strongly modifying the Mesozoic thickness and facies pattern.

11. Subdivision into evolutionary stages is defined by the diastrophic. climatic and eustatic events hat induced the changes in sedimentary environments. The 1st stage (Permian and Triassic). before the break·up of Pangea, was characterized by the predominant redbed sedimentation with several evaporitic episodes and with two relatively short marine transgressions. In the 2nd stage (Early Jurassic) there were also a few short-lived marine incursions from the west into the otherwise paralic-continental basin. The 3rd stage (Middle-Late Jurassic) was strongly influenced by the Tethys domain which experienced then a climax of the openings of small ocean basins. During the 4th stage (Cretaceous) the western influences were again more noticeable due to the onset of spreading in the southern North Atlantic and the coeval incipient closure of the western Tethys.