Zdarzenia na późnodewońskim szelfie południowej Polski i ich znaczenie stratygraficzne

Abstract

Analiza sekwencji facjalnych górnego dewonu i rejonu Dębnika-Zawiercia, regionu kieleckiego i Lubelszczyzny wskazuje na występowanie pięciu ważnych zdarzeń geologicznych na obszarze zbiornika epikontynentalnego południowej Polski. Są to: podstawowa transgresja frańska (dolny? podpoziom asymmetricus), szybka transgresja poprzedzona regresją (dolny podpoziom gigas), enigmatyczne zdarzenie na granicy franu z famenem (środkowy podpoziom triangularis) i puls regresji (środkowy podpoziom crepida). Wymienione epizody (niektóre o wymiarze globalnym) charakteryzowały się krótkotrwałością w czasie geologicznym, porównywalną do trwania jednego podpoziomu konodontawego. Miały one ponadto czytelny. choć zróżnicowany w poszczególnych strefach facjalnych. wpływ na sedymentację. Zdarzenia te mogą stanowić zatem dogodną podstawę do precyzyjnej korelacji czasowej odległych sekwencji węglanowych, co najmniej w obrębie badanej części basenu sedymentacyjnego.EVENTS ON THE LATE DEVONIAN SHELF IN SOUTHERN POLAND AND THEIR STRATIGRAPHICAL SIGNIFICANCE The present study is based on a comparison of facies sequences in the following three areas of the Late Devonian shelf in southern Poland: the Dębnik-Zawiercie area near Cracow, the Kielce region in the Holy Cross Mts., and the Lublin area (Fig 1). These areas represent different tectonic and paleogeographic settings. This made possible sorting out a common developmental pattern from some purely local effects. The interpretation of the facies superposition within a framework of the standard conodont zonation allows one to distinguish five important geological events which occurred over the whole investigated shelf portion. 1. The basal Frasnian transgression (probably Lower asymmetricus Zone). This event is generally expressed as the contact of dark, dis-to anaerobic marly subtidal deposits with the underlying stromatoporoid·coral biostromal complex (localities Dębnik, Klucze near Zawiercie and Minkowice 4a borehole; Fig. 2, Tab. 1). Locally, isolated organic buildups developed over biostromal platform; they range from small bioherms (Kowala) through kilometer-sized reefs (Dyminy Reef –l ocality Ostrówka, Bystrzyca 2 borehole 1). 2. The regressive pluse (Lower gigas Zone). The lithological evidence is variable: thick, coarse-grained intercalations within the basinal marly mudstones (Klucze, Kowala); rapid grain·size coarsening in the detrital units (Dębnik); and karsted surfaces (Ostrówka). The fact that similar phenomena have not been so far described from the Lublin area is probably due to insufficient sedimentological and biostratigraphical analyses. 3. The transgressive event (Lower gigas Zone). Typically, this episode is evidenced by the marly mudstones with a pelagic fauna overlying the detrital deposits of the earlier regressive pulse (Dębnik - Zawiercie, Kowala, Kadzielnia). Submerged reef-complexes were the sites of either nondeposition (Ostrówka ?) or condensed sedimentation of cephalopod limestones (Lublin area - Bystrzyca 2) and organodetrital limestones of a reef·cap type (Psie Górki in Kielce). 4. The Frasnian/Famennian boundary event (Middle triangularis Zone). In the sections displaying continuous basinal development in late Frasnian to Famennian, the F/F boundary is either difficult to discern within a monotonous marly sequence (Kielce region) or it is marked by the abrupt decrease in carbonate to clay ratio (Zawiercie). The dead organic buildups display nondeposition (Ostrówka, Kadzielnia) which is reflected in stratigraphical gaps spanning an age interval ranging from two conodont subzones to more than a stage. 5. The regressive pulse (Middle crepida Zone). This event is marked by an appearance of intraformational erosion levels along with benthic skeletons, and by an increase in a carbonate content. Those phenomena were observed in the marginal parts of the basin (Dębnik - Zawiercie, Lublin area) while they are hardly discernible in the Kielce region. The lithological record associated with the above events may be clearly distinguished in sections representing contrasting paleogeographical and facies settings (Fig. 2, Tab. 1). This ubiquity seems to be controlled mostly by a pattern of sealevel changes which is common for at least the investigated shelf area (Fig. 3). In fact, both transgressive episodes most probably had a world-wide occurrence (J.G. Johnson et aI., 1985); also the regressive pulses may have had an eustatic character. A global extent may be also attributed to the F/F event (D.J. Mc Laren, 1982). From the present analysis it is concluded that the major geological events may provide a useful basis for detailed chronostratigraphic correlations between various paleogeographic and facies areas of the Upper Devonian. The future application of the event·stratigraphic scheme should, however, involve refining of sedimentological and facies analysis of key sections.