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Palotai M.,ELTE Altalanos es Alkalmazott Foldtani Tanszek | Mindszenty A.,ELTE Altalanos es Alkalmazott Foldtani Tanszek | Kopecsko K.,Budapest University of Technology and Economics | Poros Z.,MTA ELTE Geologiai
Foldtani Kozlony | Year: 2012

Low water levels of the Danube during the late autumn of 2011 exposed rocks in the riverbed along the Gellért Hill (Budapest, Hungary). In contrary to common belief, these rocks, called the Ínség-ko{double acute}, are not Triassic dolomites, but silicified Eocene sandstones. Similar formations are known from the Gellért Hill. The significant secondary porosity of the sandstones comes from the almost complete dissolution of the carbonate material of the original marl clasts. Silica cementation preceded the dissolution, and/or the two processes were coeval. The fault pattern in these rocks fits earlier observations on the Gellért Hill, where east-west trending faults are crosscut by a younger, north-west-south-east striking dextral-normal fault system.

This paper presents the results of the geological mapping and structural geological analysis done in a small part of the Aggtelek-Rudabánya Hills (Northern Hungary, Figure 1). As a result, cross-sections, a structural map and a geological map were made and these illustrate the pre-Pannonian formations (Figures 2-4). The aim of the work was to understand the age and kinematics of the main tectonic boundaries and to create a comprehensive structural model for their formation. A critical review of previous concepts is also given. The area is located on the border of the Aggtelek and Rudabánya Hills and is significant for its complicated structures. A total of five structural units we distinguished as follows from top to bottom: Aggtelek nappe, Lászi, Henc, Szo{double acute}lo{double acute}sardó and Bódva units (Figure 4). Previous models have suggested contrasting interpretations of the Lászi Unit: either below, or above the Aggtelek (Silica) Unit, or as a megabreccia within the Darnó shear zone. The work presented in this paper suggests that all the units were thrust on each other, first to the SE, then to the S. These movements correspond to the D1 and D2 phases of the structural evolution (Figure 7). They could have occurred around 90 Ma, based on the extrapolation of geochronological data from the surrounding areas (KÖVÉR et al. 2009). This deformation could also have involved incipient salt tectonics near Alsótelekes, where Permian - lowermost Triassic gypsum and anhydrite form a dome (ZELENKA et al. 2005); the latter arose from below the Szo{double acute}lo{double acute}sardó unit. The D2 phase was particularly marked by a young-on-older (out-of-sequence) thrust which formed a duplex in the Henc unit (Figure 3, 10, 11). This type of deformation could have completely rearranged the original nappe stacking and this makes its recognition difficult without carrying out a thorough sedimentological study of Triassic slope deposits. These deposits occur widely in the Lászi Unit, as demonstrated in the Szo{double acute}lo{double acute}sardó Szo{double acute}l-2 borehole and outcrops (Figures 5, 6). SE vergent thrusts were reactivated in the early Miocene (before the late Ottnangian) during the D3 phase. The NW-SE compressional direction indicates that the Rudabánya Hills were probably not a positive flower structure at that time. On the other hand, the subsequent D4 phase was a strike-slip event in the late Ottnangian to early Badenian. Left lateral slip could have reactivated earlier thrust planes and contributed to the final shaping of the Alsótelekes gypsum dome. The final D5 phase resulted in normal or oblique-normal faults (Figures 8d, 9) which affected the late Miocene (Pannonian) sediments.

Toro B.,Altalanos es Alkalmazott Foldtani Tanszek | Toro B.,University of Saskatchewan | Sztano O.,Altalanos es Alkalmazott Foldtani Tanszek | Fodor L.,MTA ELTE Geologiai
Foldtani Kozlony | Year: 2012

The prograding shelf margin of Lake Pannon reached and ran across the Northern Somogy area between 8.8 and 8 Ma in an overall NNW-SSE direction. At about the same time, the slope crossed in the deep-water area of the Zala Basin. However, as a result of the shallow water depth, in the area of the Transdanubian Range and near the zone of Lake Balaton the slope could not develop. South of this area, at Northern Somogy, the slope progradation became uniform again. The aim of this study was to understand the controlling effects of relative lake-level changes and structural movements on slope sedimentation, based on the interpretation of approximately 1800 km of 2D seismic reflection. The retro-deformation of the seismic sections revealed that the characteristic morphological features (deep basins and elevated highs) of the present-day pre-Pannonian basement existed before and fundamentally influenced the Pannonian sedimentation. This dissected morphology is a result of the complex structural evolution of the area, which can be subdivided into four main phases. During the Late Palaeogene to Early Miocene a transpressional phase with strikeslip and reverse faults resulted in the juxtaposition of the distinct basement units. This phase was followed by syn-rift extension and the development of the deep sub-basins bounded by normal faults during the Karpatian to the Middle Miocene. Before and during the slope progradation the structural elements exhibit a complex transpressional-transtensional deformation. This phase is responsible for the uneven basement morphology before the slope progradation. This influenced the thickness variations of the basin-filling marls and turbiditic sandstones and, also the direction of slope progradation locally. Later, during the fourth phase, the older structures were reactivated and the Pannonian sedimentary succession uplifted and folded during the neotectonic inversion of the area. As a consequence of the highly irregular basement morphology, the Pannonian strata vary significantly over short distances. The inherited sub-basins drew the prograding slope, while the elevated edges acted as a barriers and deflected it. The uneven basement relief also influenced the local water depth of the lake and this is reflected by the varying height of the slope: at the basinal areas, the thickness of the slope sediments is greater compared to that of the elevated ones. Based on the regional seismic mapping, the area studied was filled by two slopes prograding from different directions. The north-western slope prograded from the area of the present-day Tapolca Basin towards the S-SE into the Mezo{double acute}csokonya Trough. The north-eastern slope prograded towards the SW and its progradation was influenced by the Ozora Trough and its elevated south-eastern margin, the Tamási Edge. These slopes were merged around the Igal High. In general, the slope prograded towards the S and the slope advanced 35 km within ca. 0.7 million years. The relative lake-level changes during the progradation are marked by the shelf-edge trajectory of the advancing clinoforms: the shelf margin is constructed of alternating aggradational and progradational units, which indicates repeated rising and stagnant lake levels. These cycles can be explained by climatic changes with a periodicity of ca. 100 ky. At one location it was possible to identify two overlying slope sequences which indicates a major relative lake-level rise, however it was not traceable in the neighbouring sections. Features indicating relative lake-level fall were not apparent on the studied sections. However, unconformities could be identified and these are the results of the superposition of two slopes, prograding from different directions.

Sztano O.,ELTE Altalanos es Alkalmazott Foldtani Tanszek | Magyar I.,MTA MTM ELTE Paleontologiai Kutatocsoport | Szonoky M.,Szegedi Tudomanyegyetem | Lantos M.,Magyar Foldtani es Geofizikai Intezet | And 4 more authors.
Foldtani Kozlony | Year: 2013

Revisiting the Tihany, Fehérpart section, overviewing archive data, comparison with successions of nearby wells, well-logs, stratigraphic data and results of the high-resolution seismic surveys on Lake Balaton resulted a coherent picture on the depositional environment, age, stratigraphic correlation and palaeogeographic connections of the Tihany Formation. In addition to former analyses of grain-size distributions, carbonate and clay content, the sedimentary structures were investigated, a pilot study of gamma-ray measurements on the field was carried out, and several orders of cyclicity were demonstrated. Palaeontological data from earlier studies were collected and analyzed, and magnetic polarity of the rocks was measured. The Tihany, Fehérpart section is correlated with the Spiniferites tihanyensis dinoflagellate, the MN11 micromammal and the Lymnocardium decorum littoral mollusc biozones. With the exception of the lowermost few metres, it shows normal magnetic polarity. It is underlain by open lacustrine, reverse polarity shales of the Congeria praerhomboidea zone, and is overlain by layers indicative of the Prosodacnomya zone. The latter is well definied by the radiometric age (7.9 Ma) of the overlying volcanosedimentary suite. Therefore the Fehérpart section was deposited either 8.1-8.0 Ma (C4n.2n) or 8.3-8.2 Ma (C4r.1n chron) ago. The Tihany Formation was deposited in a variety of palaeoenvironments related to deltas entering Lake Pannon. It consists of parasequences, i.e. shallowing up successions from below wave base to lake level, generated by sediment accumulation. Parasequences were formed on the delta front or in inter-distributary bays to delta-plain swamps and distributary channels. Beyond the high frequency lake-level and partly autocyclic environmental fluctuations, most likely climatically induced fourth-order lake-level changes of about 15-30 m amplitude occurred, resulting in minor transgressions followed by repeated progradation of deltaic lobes. Although the Tihany (as well as the very alike Somló) Formation is currently found along the rim of the hills, it was originally deposited in the same way as the Újfalu Formation known only from the subsurface of deep basins. The dynamics of deltaic settings feeding to Lake Pannon can be understood by studying the Tihany Formation in outcrops. The only difference between the two formations might be in the number of overlying delta cycles and their thickness; both were determined by rate of subsidence, being smaller at basement highs where Tihany Formation accumulated than at basin areas where Újfalu Formation was defined. It is suggested here to include the Tihany (and Somló) beds as members of the Újfalu Formation. Fourth-order sequence boundaries were recognized between the overlying progradational deltaic bodies. In the vicinity of Tihany no evidences of lake-level drops were revealed, but elsewhere small incised-valley fills point to minor lake-level drops. The overall regression, interrupted by transgressive events, continued in the study area until the shelf edge of Lake Pannon shifted as far to the south as 50-60 km, i.e. at about 8 Ma ago. After that, flooding events became rare and small in amplitude, then the area became a terrestrial plain. Fluvial deposits are not known from the direct vicinity, but travertines formed in small freshwater ponds fed by karst springs. The transition from lacustrine to terrestrial palaeo-environments is part of the overall normal regression as a result of high sediment input to Lake Pannon. Large incised valleys or other evidences of recurring terrestrial conditions which could be related to thirdorder sequence boundaries mappable all over the Pannonian Basin were identified neither in Tihany nor in Újfalu Formation.

Barbara B.,MTA ELTE Geologiai | Laszlo F.,MTA ELTE Geologiai | Laszlo F.,Eotvos Lorand University
Foldtani Kozlony | Year: 2013

Deformation bands are widespread strain localization structures in porous media. The main aim of this paper is to introduce these deformation elements giving an overview of their basic macroscopic characteristics, classification, formation mechanisms and rheological evolution using published data and Hungarian field examples. Formation of deformation bands is influenced by numerous factors, such as porosity, rock fabric parameters (grain size, grain shape, sorting), pore water content, lithification, degree of burial diagenesis, and loading path. Following the characterisation of deformation bands a description is given of the way which faults with a slip surface can be localized along clusters of deformation bands during the mature phase of their evolution.In the literature two main types of deformation band classification are widely accepted. One of them distinguishes five types, based on their kinematic aspects from dilational through shear variants to pure compactional ones. The other classification separates four types on the basis of dominant deformation mechanism. These two types of classification reflect the stress and petrophysical conditions operating during deformation band formation.Here, particular interest focuses on a special type of deformation band, namely solution band in carbonates, where dissolution is the main deformation mechanism. This type of bands has partly different features than those ones developed in siliciclastics, this being due to the petrophysical properties of carbonates. Pressure solution is the prevailing deformation mechanism in carbonates which results in grain size and porosity reduction.Some interesting Hungarian examples will be presented. Deformation bands were investigated on macroand microscopic scales respectively, in porous sandstones and conglomerates from the Bükkalja area. Combined analysis of fault slip data and deformation bands reveals the progressive evolution of the latter, thus reflecting a changing deformation mechanism with burial depth and also with cumulative displacement. It was concluded that the identified ‘less-destructive’ type of deformation band represents the earliest phase in their evolution. Upon burial diagenesis, the deformation became more destructive and formed more intense cataclasis with increasing displacements up to the point of occurrence of the discrete slip surface (fault). This trend clearly corresponds to published data and its application should be tested in other sub-basins within the Pannonian basin system. On the applied geological side, deformation bands have considerable effect on path of fluid flow; this is due to their reduced permeability and should be considered during future studies in the Pannonian Basin. © 2014 Hungarian Geological Society. All rights reserved.

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