Truncated higher order sequences as responses to compressive intratectonic events superimposed on eustatic sea-level rise

High resolution sequence stratigraphic analysis of the lignite-bearing Miocene siliciclastic sequence of the Salótarján Lignite Formation (SLF) has been performed by lithostratigraphic dissection of more than 1350 boreholes and by detailed analysis of well logs from more than 150 boreholes. Biostratigraphic data indicate that the SLF represents a single 3rd order eustatic sequence, namely the Bur-4 of Vakarcs et al. [Vakarcs, G., Hardenbol, J., Abreu, V.S., Vail, P.R., Várnai, P., Tari, G., 1998. Oligocene – Middle Miocene depositional sequences of the central paratethys and their correlation with regional stages. SEPM Special Publication vol. 60, 209 – 231]. As for facies conditions, facies models for sandy shorefaces and/or wave dominated deltas can be applied. Twenty-six parasequences have been identi fi ed and mapped in the adjacent sub-basins from well-log correlations. The sharp-based sand bodies of ps. 10 and 17 were interpreted as the falling stage systems tract (FSST) of higher order sequences. The subsequent transgressive systems tracts (TST) are represented by retrograding sets of 2 – 3 parasequences (ps. 11 – 13, 18 – 19). Based on the regional unconformity at the base, and the regionally extended sharp-based sand bodies, three sequence boundaries (SB) were determined that dissect the 3rd order sequence into three higher order sequences. One of the special features of the sequence is that the FSSTs interrupt the rapid relative sea-level rises (ps. 9 and 16) and lie directly on the silty material of nearshore environments, leading to the lack of the higher order highstand system tracts (HST) ( ‘ truncated transgressive semi-cycles ’ ). Another characteristic is the striking basinward fore-stepping of the sedimentary depocentres and that of the facies belts of the lignite seams. The rapid relative sea-level falls implied by the two FSSTs are unexpected in the context of an overall eustatic rise, thus the possibility of tectonic origin of shallowing was investigated. This veri fi cation was based primarily on the observations that (1) the basin was united when parasequences 1 to 13 were developed, whereas (2) by the end of the sequence development the Palaeozoic basement protruded to the surface, dividing the basin, re fl ecting intense tectonic events simultaneous with sequence development. The FSSTs with the SBs above and the formation of higher order sequences can thus be regarded as sedimentary responses to syn-sedimentary tectonic elevation and tilting of the basement. This elevation was induced by tectonic compression and associated imbrication along a regional reverse fault that was known as the Darnó Line. The imbrication could have controlled not only the repeated relative sea-level falls but also the striking fore-stepping of the sedimentary depocentres and facies belts of the associated lignite seams. Tectonic tilting in fl uenced not only the FSSTs, but the subsequent early TSTs as well, thus lignite seams associated with the tectonically counteracted TSTs are characterized by multiple seams with several accessory seams (Va, IIIa, b and Ia), while those associated with tectonically quiescent periods are aerially extensive and solitary. Comparison with other coal bearing formations enhanced the importance of the FSST in the interpretation of stratigraphic truncations, coal seam geometry conditions, facies shiftings and stratal geometry, enabling a more accurate description of syn-sedimentary tectonic events

Magnetic susceptibility as a possible correlation tool in Quaternary alluvial stratigraphy

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Magnetic susceptibility as a possible correlation tool in Quaternary alluvial stratigraphy

Correlation of the fluvial magnetic susceptibility (MS) record of borehole Devavanya-1 in the Kőrős Basin (eastern Hungary) with Chinese aeolian MS records (Jingbian, Lingtai) and the marine d18O record from the Equatorial Pacific (V28-239) is established here based on cross- orrelations and singular spectral analysis. A basin-scale well-to-well correlation based on magnetic susceptibility records was also performed involving unpublished cores. To refine the age model, a Monte Carlo simulation was conducted using the Chinese Jingbian section as a tuning target. Spectral analysis of the tuned record revealed c. 400, c. 100 and c. 41 ka cycles over the 2.5 million years of the Quaternary fluvial succession. To ensure a complementary palaeoclimate proxy, the full width at half maximum of smectites was measured as a facies-independent indicator of weathering intensity. This investigation was carried out on a subset of samples involved in MS measurements representing a c. 400 ka time interval across the top of the Olduvai subchron. A phase-shift between MS and weathering intensity recorded in the clay mineralogy indicates different response times of the considered proxies. The fluvial MS record is determined by the climatic control on delivery and preservation of magnetic minerals, mainly of magnetite. Under cold-and-dry climate these minerals were released owing to frost shattering in the adjacent hinterlands and were transported to alluvial plains in the early postglacial periods thanks to the increasing discharge of rivers. With further warming the weathering-sensitive magnetic minerals soon disappeared from the soils of the catchment area and thus from the fluvial load. As a result, in fluvial successions early postglacial warmings are expressed by the occurrences of MS maxima (magnetic episodes), while the palaeotemperature maximum and the subsequent cooling remain concealed within the tract of low MS values. The early postglacial magnetic episodes may serve as ideal stratigraphical markers in regional and global correlations

Tectonically controlled Quaternary intracontinental fluvial sequence development in the Nyírség–Pannonian Basin, Hungary

The Quaternary fluvial succession of the Nyírség (NE Hungary), a proximal sub-basin of 4000 km2 in the intracontinental Pannonian Basin, was studied based on log facies analysis. Regional mapping of sequences was established by analysis of fully cored boreholes and high scale local correlations in densely drilled areas. The age of the sequences was determined by correlating the magnetic susceptibility (MS) record of the fully cored boreholes with that of the reference Hungarian boreholes dated paleomagnetically (Dévaványa-1 and Vésztő-1 in Cooke et al., 1979). To give the Hungarian data global perspective they were correlated to the MS curve of the Chinese Loess (Ding et al., 2005) that are in turn correlated with the Marine Isotope Record (Lisiecki and Raymo, 2005 and Gibbard and Cohen, 2008). The ages of the mapped sequence boundaries are 2.62, 2.26, 2.12, 1.22, 1.04, 0.58 and 0.34 Ma respectively and can be related to the transitions from cool to warm conditions. Regional unconformities at 2.26–2.12, 1.22 and 0.58 Ma also coincide with activity maxima of the radiometrically dated Quaternary volcanism. This high frequency of climatically controlled erosive sequence boundaries in the structurally active periods indicates that the sedimentary record of climatic erosion is better expressed in times when structural changes generate instability in the drainage network. The occurrence of packages of regional unconformities in relation to volcanic activity enables the geochronological dating of episodes in the Quaternary compression of the Carpathian–Pannonian region. The role of tectonic control on the climatically induced changes in the drainage network has been explained by a structural development model based on seismic, gravity and magnetic data. The changes in the local paleohydrology were triggered by a compression related elevation of the basement and the associated occurrence of a local transtension-related subsidence