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High resolution paleoenvironmental analysis of the marine flooding in the southern Pannonian basin (Middle Miocene, NE Bosnia and Herzegovina)

Open mine pit Bogutovo Selo near Ugljevik in NE Bosnia and Herzegovina is positioned at the southern margin of the Pannonian Basin System. The Oligocene lignite bearing lacustrine deposits are superposed there by Middle Miocene marine sediments. The measured section is about 60 m thick and beside marine sediments includes the uppermost part of the lake deposits, comprising clays, sands and coal seams. The marine ingression is marked by the first occurrence of the marine macrofauna in a sandy marl horizon comprising mollusk accumulations. About 20 m of gray marls with two dark clay intercalations follow, superposed by a massive, 10 m thick carbonate body. Its sedimentary structures and lower contact point to deposition by debris flow. The topmost 40 m of the logged succession include in its lower part two subordinate, 1m thick carbonate beds, followed in very top by subordinate diatomite intercalations, volcanic ash layers and fossiliferous beds with well preserved, marine fish remains. The assemblages of benthic and planktic foraminifera allow the biostratigraphic correlation of the marine transgression horizon with the Lower Badenian. Benthic foraminifera record with 84 detected species shows distinct changes in composition and diversity across the section, reflecting changes of the paleoenvironment., i.e. depositional depth, oxygen content and organic matter content. Within the first meter of the marine part of the section shallow water assemblage predominates, reflecting highly oxic environmental conditions. The most common are Asterigerinata planorbis, Elphidium rugosum, Lobatula lobatula and Ammonia viennensis. Gradually, deep water middle to outer shelf assemblage installs, showing a stable moderately oxic bottom environment. The characteristic representatives are Valvulineria complanata, Cassisidulna laevigata, Heterolepa dutemplei together with Bulimina elongata. Slightly preceding the first dark clay intercalation the environmental perturbation starts pointed out by repetitive change of diversity index, species dominance and bottom water oxygen content. Some sea level fluctuation is present with distinct shallowing peak directly above the thick carbonate body. Within following meter distance the most distinct change develops with installation of low oxic near-bottom conditions remaining stable until the section top. The benthic fauna of that latter interval is characterized by low diversity and high dominance fauna comprising oxygen depletion indicators such as Praeglobobulimina pyrula or Bolivina ssp. The benthic foraminifera assemblage allows a good correlation of the marine deposits with the Badenian regional stage. The lowermost part of the section Ugljevik comprising the lake sediments is barren of autochthonous calcareous nannoplankton. Continuous occurrences of zonal marker Sphenolithus heteromorphus and the absence of Helicosphaera ampliaperta in the middle and the upper part of the section allow stratigraphical attribution to the nannoplankton Zone NN5. 218 13th Congress RCMNS - 2nd - 6th September 2009, Naples, Italy Based on the last occurrence of S. heteromorphus, NN5/NN6 boundary can be placed about 10 m below the top of the section below the diatom bloom event. Together with blooms of small reticulofenestrids, which are characteristic in Badenian marine deposits of the Central Paratethys, nannoplankton assemblages contain also: Braarudosphaera bigelowii, Coccolithus pelagicus, Coronocyclus nitescens, Coronosphaera mediterranea, Cyclicargolithus floridanus, Helicosphaera carteri, H. walbersdorfensis, Holodiscolithus macroporus, Reticulofenestra gelida, R. pseudoumbilica, Sphenolithus moriformis and Umbilicosphaera jafari. Blooms of small reticulofenestrids and very rare or absent discoasterids in all investigated samples point to nearshore paleoenvironmental conditions. Stable isotopes of carbon and oxygen as well as the content of total organic carbon (TOC), carbonate and sulphur have been analyzed within a geochemical study on bulk sediment samples. The lake sediments at the base of the section are characterized by very low isotopic values (mean: -8.2‰ for 􀄯13C, -5.4‰ for 􀄯18O), low TOC values (mean: 0.23%), a variable carbonate content ranging from 1-35% and a sulphur content between 0.3-1.3%. The overlying marine sediments show in their lower part a mean isotopic value of -0.9‰ for both, the 􀄯13C and 􀄯18O signals. TOC values of 0.4-3.9%, carbonate content between 19-85% and a sulphur content ranging from 0.8-3.7% have been evaluated for the whole section. These preliminary results are consistent with a transgressive setting ; increases in TOC and sulphur content up-section reflect a decrease in bottom-water oxygenation favoring organic carbon preservation as a result of the deepening environment.

paleoecology; environmental evolution; marine transgression; Badenian;

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