Paleomiąższości, litofacje i paleotektonika epikontynentalnej kredy dolnej w Polsce

Abstract

Polski epikontynentalny basen dolnokredowy miał charakter ekspansywny, jednakże o zmiennych relacjach między dopływem silnie zasolonych wód Tetydy i Atlantyku a dopływem wód słodkich z otaczających lądów. Z analizy formacyjnej wynika, że dolna kreda reprezentuje morską asocjację terygeniczną z subasocjacją piasków i iłów na północnym zachodzie oraz z subasocjacją piasków, iłów i wapieni na południowym wschodzie. Profil o pełnym rozwoju kredy dolnej ograniczony jest do bruzdy środkowopolskiej, gdzie miąższość osadów dochodzi do 650 m. W centralnych strefach bruzdy występują liczne paleostruktury w przewadze salinarne. Na zewnątrz bruzdy stwierdza się duże redukcje litologiczno-stratygraficzne i stopniowe cienienie osadów, które tworzą monokliny i tarasy strukturalne. W ich obrębie występują synsedymentacyjne rowy z profilami typowymi dla bruzdy środkowopolskiej. PALAEOTHICKNESS, LITHOFACIES AND PALAEOTECTONICS OF THE EPICONTINENTAL LOWER CRETACEOUS IN POLAND The Early Cretaceous sedimentary-palaeotectonic evolution of platform areas in Poland is presented on the palaeothickness and lithofacies maps of Riasanian/Lower Valanginian  Platilenticeras (Fig. 1), Lower Valanginian – Polyptichites (Fig. 2), Upper Valanginian (Fig. 3), Hauterivian (Fig. 4) and Barremian/ Middle Albian (Fig. 5) as well as on the palaeotectonic map (Fig. 6). The palaeothickness and facies maps have elaborated by the quantitative analysis of thickness and facies using the coefficient combination. The detailed analysis of present and primary extents of sediments, the reconstruction of thickness and facies in the zones of epigenetic erosion, and the analyses of subsidence zone limits and sedimentary gaps were carried out.The palaeotectonic map that was elaborated by superposition of the particular palaeothickness and facies map, presents the palaeostructural pattern of the base of Lower Cretaceous at the end of the Middle Albian sedimentation. It presents the results of the thickness analysis subsidence, gaps and unconformities, analysis of lithological associations and palaeotectonic regionalization.The Lower Cretaceous basin was strictly connected with the Zechstein-Mesozoic Mid-Polish Furrow formed at the contact between the Precambrian East-European Platform and the Paleozoic Mid- and West-European Platform. The furrow connected two palaeogeographic provinces: the Atlantic in the west and north-west and the Thetys in the south and south-east. The Lower Cretaceous Polish epicontinehtal basin was of expansive type with variable relations between the incursions of the Thetys and Atlantic waters with high salinity and fresh waters from surrounded lands.It is evident from the facies analysis that the epicontinental Lower Cretaceous in Poland representsthe early transgressive stage of tectonic development marked by a marine terrigenous association. The sub-association of sands and clays (clastic), occurs in the north-western part of the basin, and the sub- association of sands, clays and limestones (clastic-calcareous) in its south-eastern part.The fully developed Lower Cretaceous sequence is limited as rule to the Mid-Polish Furrow, where the continuity between Jurassic and Cretaceous is observed and thickness of Lower Cretaceous amounts to 650 m (Fig. 6). The central part of the furrow was characterized by distinct mobility expressed by numerous low-amplitude palaeostructures, mostly salt anticlines. They were formed mainly in the Upper Valanginian, at the Valanginian/Hauterivian boundary and also in the Barremian and Lower Albian.In the south-eastern extension of the Mid-Polish Furrow the Lower San elevation, was formed in Hauterivian; elevations broke the connection (l)f the Lower Cretaceous epicontinental basin, with, the Thetys in Barremian, Albian and Lower Albian. Large lithological-stratigraphic reductions and gradual thinning out of sediments are stated outside the Mid-Polish Furrow.The Upper Valanginian, Hauterivian, partly Barremian and Aptian and particularly Middle Albian sediments distinctly exceeded the furrow. These sediments form monoc1ines and structural terraces with small and low-differentiated subsidence.Synsedimentary troughs, connected with block tectonics, which developed within the terraces in boundary zones of the Mid-Polish Furrow are noteworthy. They are characterized by a more complete lithological-stratigraphical profile and thickness of sediments typical for the most subsident zones of the Mid-Polish Furrow.