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

Abstract

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 identified 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 verification 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, reflecting 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 influenced 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.