A detailed stratigraphic and facies reconstruction of a bentonitized acid-tuff succession, deposited within the transgressive systems tract of the Upper Miocene–Sarmatian Ser-3 eustatic cycle, at Sajóbábony, northern Hungary, was performed via petrographic, mineralogical and geochemical analyses. The purpose of the work was to analyze the degree of alteration of the volcanogenic sediments, as an indicator of the relative volcanic sedimentation rate. This may have an important role in indicating volcanic periods synchronous with sedimentation or reconstructing the volcanosedimentary paleoconditions. Sample pairs were collected from each bentonite and tuff layer, and, to facilitiate microstratigraphic relations, samples were collected every 10 cm within bentonite layers. Mineralogical analyses were performed by X-ray diffraction and geochemical analyses by inductively coupled plasma-mass spectroscopy.

The CaO/K2O and Eu/La ratios correlate with each other and with a montmorillonite/X-ray-amorphous phase ratio, reflecting Ca and Eu incorporation associated with devitrification and smectite formation. In accordance with the current literature, these mineralogical and geochemical proxies can be related primarily to the weathering processes. Considering vertical distributions in a sequence-stratigraphic context, the Ca content and Eu/La values show that local peaks and Eu anomalies characteristic of acid tuffs show minima at flooding surfaces (FS). Within a bentonite layer, representing a single transgressive period, the repeated events of dust-tuff accumulations have been determined by K2O/CaO and La/Eu peaks, confirmed also by the Eu anomalies in the rare earth element (REE) patterns, thus leading to the conclusion that the level of alteration is closely correlated with the elimination of terrigenous input and a minimum in volcanic sedimentation rate allowing more intensive alteration of the deposited volcanic material. In the case of fine tuff beds, Eu anomalies on REE patterns reflect limited alteration at the bottom and more intensive alteration in the upper parts of the beds, reflecting the effect of infiltration of sea water into the pores.

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