Sedimentological and geochemical characterisation of the Lower Oligocene Menilite shales from the Magura, Dukla, and Silesian nappes, Polish Outer Carpathians - A new concept

Abstract

The Oligocene Menilite Beds (Formation) represent the most important hydrocarbon source rock in the Carpathian Belt. The formation is laterally uniform across long distances but it shows strong internal heterogeneity that reflects changes in depositional environments, which controls the source rock potential. The age of the Menilite Formation in the study area is Lower Oligocene – Rupellian - Lower Chattian (nannofossil zone NP22-NP24). Generally, Menilite Beds are dominated by fine-grained (claystone, mudstone, and siltstone) facies, but in some areas they are replaced by thick sandy facies or both types are present. In the study area, both types of facies are present; however, only fine-grained facies have been sedimentologically analysed here because these types can also provide good analytical material for organic geochemistry. The conducted research was aimed at verifying the concepts discussed in the literature that the Menilite Beds were formed in one or more different sedimentary basins, and thus also different palaeogeographic conditions in a small area of the Polish Carpathians, which is located on the border of three tectonic units, precisely in the research area. Therefore, a comparison of these areas was carried out both in terms of facies interpretation and geochemical characteristics. The sedimentological features of 11 different sections of the Menilite Beds from seven localities have been examined: two connected to the Magura Nappe, six to the Dukla Nappe (including tectonic windows) and three to the Silesian Nappe, and total of eight facies (F1 to F8) have been distinguished. Facies F1 from Magura Nappe shows aerobic deposition conditions for sediments with a decisive majority of terrestrial material originating from both angio- and gymnosperms, but from Dukla Nappe facies F1 biomarkers indicate a mixed type of marine-terrestrial matter, with a majority of the terrestrial substance and low-oxygen conditions of the sedimentation environment, with an indication of a lacustrine or marine-lacustrine environment, while in Silesian Nappe facies F1 shows marine and typically anoxic features of sedimentation environment. Facies F2, which is presence in all units, characterise a high content of terrestrial-type substance or there is no clear domination of a terrestrial, marine, or lacustrine component. Facies F2 in Magura Nappe has many features in common with facies F1 but differs higher share of angiosperms (high content of terrestrial-type substance), compared to in the Dukla and Silesian units. It does not show clear features or biomarkers that can be unambiguously interpreted to indicate one specific type of sedimentation condition. Facies F3, which was found in the Silesian unit, stands out due to its high deposition environment anoxicity index and a lack of markers proving a share of angiosperms with higher content of algae-type material. The structures found within these facies shows clear lamination, which frequently accompanies algae mats, hence its sedimentation environment may be similar to lacustrine conditions. Facies F4 is present only in the Dukla unit with all its geochemical features and indicates the deltaic and marine shallow-water aerobic conditions of its deposition environment, which has a share of organic matter of marine and terrestrial types. Facies F5, which is only present in the Silesian Nappe, originated in an anaerobic environment. A high input of organic matter from freshwater and terrestrial origins, indicated by biomarkers, is consistent with shallow-water deposition. Facies F6, present in the Magura Nappe, similar like in Dukla Nappe in most cases stands out due to its low generation potential and high deposition environment aerobicity indices. The origin of organic matter is typically terrestrial originating from angiosperms matter that was deposited probably in lacustrine environment, contrary to the domination of gymnosperms observed in F1 and F2 in the Dukla unit. Stratification of this facies with light and dark colours of fine-grained deposits can indicate cyclically oxidation changes in the sedimentary basin. Generally, biomarker data suggest varied types of organic matter from marine in the Silesian Nappe through mixed in the Dukla Nappe to typically terrestrial in the Magura Nappe which may indicate a deposition in separate basins with their periodical joining. Most of the geochemical parameters indicate also varying redox conditions during deposition in shallow water. Additionally, a variety of the sedimentary structures within accompanying thin-bedded sandstone beds were also recognised. These sedimentary structures provide a possibility to interpret the sedimentary environments of those facies. Most of them are connected to wide-type shallow-water sedimentary environments and are found in the zones between the storm-wave base and the fair-weather water wave base (facies F2, F4, F5 and F6). The analysis confirmed the variability of the sedimentation conditions of the Menilite Beds in terms of the deposition conditions, which can also be used to infer the paleogeographic variability of the original deposition areas.