Towards an understanding of southern peri-Pannonian lacustrine depositional cycles: Interplay of sediment delivery and shifting intrabasinal height, a case study of drilled Neogene sediments from northwest Toplica Basin (Central Serbia)

A multidisciplinary approach allowed the reconstruction of the shallow, highly complex Neogene lacustrine-type sedimentological interplay between the peri-Pannonian (sub)basin subsidence and its seafloor topography. The current study further discusses the mechanism of localized uplift and subsidence by analyzing depositional cycles of middle Miocene sediments drilled in a northwestern or shallower Toplica Basin depocenter (borehole BL4, depth up to 630 m; Central Serbia). Supported by recent constraints on a deeper basinal section of eastern and western subbasin depocenters, the composite study of the segmented Toplica Basin involved geological, sedimentological, mineralogical, inorganic, and organic geochemical analysis, as well as constraints on interchanging geodynamic drivers. The data were extracted from thirty-one selected samples from four sedimentary lithomembers: A, B, C, and D. The architecture of these deposits reflects an intricate pattern influenced by complex lake bottom and subsurface geology (dis)connecting the two depocenters. The investigated Neogene deposition, as a whole, was dominantly controlled by a lithospheric-scale extensional graben system (involving the Jastrebac core complex-type tectonic exhumation) developed on top of the underlying Serbo-Macedonian basement unit. The crustal extension allowed rapid material inflow from other exposed sequences of the juvenile basin, including the abutting surface exposures. During the initial stretching and basin subsidence stage, sediment inflow towards the eastern Toplica depocenter was hindered. The influx of surface-eroded material was interrupted by a natural “obstacle”. In turn, such a configuration facilitated voluminous material transport into the western depocenter of the basin, thereby controlling the deposition of lithomembers A and B. After the deposition of lithomember B ceased, the “barrier”, or likely intrabasinal structural high, contributed to a reversal of the former westward-directed transport. In that manner, the vertical movements of the structural high enabled material transfer typical for the eastern basin segment, consequently prompting a sedimentary development of the lithomembers C and D.

In this context, mineralogical and geochemical differences between the sequences are used as tracers of depositional changes affected by tectonic events. Sequences of sand and gravel layers of upper lithomembers C and D pointed out that alluvial processes strongly influenced their depositional cycle. On the other hand, a more pronounced presence of sulfide minerals (pyrite concretions) in lithomembers A and B correlated with a calm and anoxic paleoenvironment. The elevated trend of mixed terrigenous and/or microbiologically reworked organic matter (higher carbon/nitrogen (C/N) ratio and terrigenous/aquatic (TAR) ratio, the lower sum of steroids/sum of hopanoids (S/H)) ratio, deposited under anoxic–dysoxic freshwater lacustrine conditions (higher pristane/phytane (Pr/Ph) ratio, absence of squalane (i-C₃₀) and β-carotane), is observed towards a shallower section of the borehole (lithomembers C and D).