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  • Author or Editor: Béla Raucsik x
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In the Hungarian part of the Tisia Block, four occurrences of rhizolith-bearing pedogenic calcrete have been published, three of which are located in southern Transdanubia. Nodular calcrete with beta fabrics was documented from the lower Permian (Cisuralian) continental Korpád Sandstone Formation where the subaerial exposure section was developed on a strongly altered, volcanic shard-rich siliciclastic substrate. Additionally, two locations with Microcodium-bearing calcrete developed on a lower Jurassic carbonate substrate were published in the last few years. The scope of this study is to briefly summarize pedogenic calcrete records known from the Permian to the Cenozoic of southern Transdanubia (Tisia Block, Hungary), and to highlight their regional paleoenvironmental and paleogeographic importance.

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Redeposited Eocene calcrete gravel was found in a Miocene conglomerate sequence in the Western Mecsek Mountains, S Hungary. The purpose of this paper is to describe the micromorphological and mineralogical characteristics of these rocks. A large number of calcrete thin sections were analyzed using a petrographic microscope. This study was supplemented by microchemical staining, cathode luminescence examination and X-ray diffraction methods.  The diagnostic features identified are rhizoliths (rhizocretion, root cast and root petrifaction), alveolar textures, in situ Microcodium grains, peloids, coated grains and pedogenic voids. According to these micromorphological results the studied calcrete belongs to the group of beta calcretes. The biofabric of this calcrete reflects an extensive vegetation cover and a relatively high degree of biological activity. Micromorphology and mineralogical composition of the calcrete gravels (i.e. dominance of calcite, quartz, illite±muscovite and illite/smectite) suggest a semi-arid/subhumid climate during calcrete pedogenesis. Within the Eocene calcrete gravel two main cement types have been distinguished: a meteoric vadose cement (vadose silt) and a phreatic one (drusy ferroan calcite spar). Drusy sparite is the typical cement of near-surface diagenesis, and this, together with its ferroan nature, leads to the conclusion that the main site of cementation of the Eocene calcrete was the meteoric phreatic zone.

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The clay mineralogical and chemical compositions of Upper Carboniferous siliciclastic rocks from the western flank of the Villány Mountains (Téseny Sandstone Formation) have been investigated to determine paleoweathering conditions, as well as to appraise the influence of the post-depositional processes upon source rock signature. The clay-mineral assemblage of the samples consists predominantly of illite±muscovite, suggesting a potassium metasomatism in the Téseny clastics. Therefore the use of the Chemical Index of Alteration (CIA), which provides a consistent quantitative framework for examining weathering, leads to erroneous conclusions without correction for K-metasomatism. When considered in Al2O3-CaO* + Na2O-K2O (A-CN-K) compositional space, orthogneiss and igneous rock clasts selected from the Téseny conglomerate reflect two different weathering trends; one (including orthogneiss, quartz diorite, and andesite samples) shows an ideal trend observed for granodioritic rocks, and the other (including aplite, rhyodacite, and rhyolite samples) follows a trend from a slightly more K-feldspar-rich fresh rock composition than that of average granite. Intermediate to intense chemical weathering of the source areas is indicated by premetasomatized CIA values of 77–84 for the samples from borehole Siklósbodony-1, suggesting that these rocks have gained about 6–7% K2O (in A-CN-K space) during metasomatism.

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In this paper, results of a bulk-rock geochemical study of silty and albitic claystone samples selected from the Upper Permian Boda Siltstone Formation (BSF) in the western part of the Mecsek Mountains (Tisza Mega-unit, Hungary) are presented. The high Na2O and P2O5 contents, relative to the post-Archean Australian average shale (PAAS) and the average Russian Paleozoic shale compositions, are the most striking features of the geochemistry of the Boda sediments. The samples studied are depleted in SiO2, TiO2 and Al2O3, and they are enriched in Fe2O3, MgO, CaO and K2O relative to the PAAS. The major element relations clearly show that the geochemistry of the BSF is strongly affected by post-depositional modification, corresponding to large-scale dispersal or addition of components. On the other hand, relatively high La content, low concentrations of V, Cr, Cu and Ni, and the result of the TiO2 versus Ni plot reflect a relatively felsic provenance of the BSF. By comparison with detrital mineralogy of the heteropic Cserdi alluvial fan system deposits, the authors assume that the Boda playa lake deposits had a similar immature primary composition consisting of quartz, plagioclase, K-feldspar, muscovite, biotite, chlorite and clay minerals. During weathering and transport in a semi-arid to arid climate, detrital mafic minerals were altered to yield chlorite and clay minerals plus Fe and Mg, and trace metals in solution. Mg was built into the structure of chlorite and of secondary carbonates such as dolomite and Mg-rich calcite. Based on previous geochemical studies, the BSF contains particularly sodic sedimentary rocks (up to 8 wt%) which may represent an addition of sodium in authigenic silicates from alkaline brine and/or evaporites in the playa deposits. In claystone, after albite formed, Na-depleted, Ca, K-enriched waters reacted with the clay minerals such as smectite and kaolinite to yield K-rich illitic sediments, Ca precipitated in calcite cement. P2O5 is enriched in some samples due to phosphorus mobilization during diagenesis.

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The pre-Cenozoic basement of central Hungary is partly made up of different types of carbonate rocks. These carbonates are often good hydrocarbon reservoirs, and hydrocarbon production is significant in this region in Hungary. Nonetheless, the petrography of the reservoir rocks has not yet been investigated in detail. In this study, the results of the investigations of the lithology of a carbonate hydrocarbon reservoir from central Hungary (Gomba Field) are presented. Based on this work, two types of pure limestone, a dolomitic limestone and a polymictic breccia, could be distinguished in the study area. The limestone types are similar to the Kisfennsík Limestone Member and the Berva Limestone of the Bükk Mountains, but they contain significant amounts of framboidal pyrite and dead oil as vein fillings. The breccia is predominantly composed of angular carbonate clasts and minor metamorphic and sedimentary rock fragments in a chaotic pattern. The breccia has some grains that may be speleothems (e.g., stalactite or stalagmite) based on their structure and isotopic compositions. The fabric of the breccia suggests that it may have been formed by fluid-related processes. Cross-cutting relationships of the veins and petrography of the vein fillings suggest that there are four different fracture generations and two different hydrocarbon migration phases to be distinguished. The composition of the hydrocarbon-bearing fluid inclusions related to the second migration event is similar to the crude oil currently produced from the Gomba Field. During the Eocene, the Triassic basement was buried and brecciated. Subsequently, a primary hydrocarbon migration can be assumed, but the hydrocarbons became overmature, apparently due to the high temperatures of the burial environment. Finally, an uplift phase began and the youngest fracture generation formed, which serves as a primary pathway for the more recent hydrocarbon migration.

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