GEOCHEMICAL RECORDS OF A BENTONITIC ACID-TUFF SUCCESSION RELATED TO A TRANSGRESSIVE SYSTEMS TRACT À INDICATION OF CHANGES IN THE VOLCANIC SEDIMENTATION RATE Z. P U ¨ SPO ¨ KI 1, *, M. K OZA ´ K 1 , P. K OVA ´ CS -P A ´ LFFY 2 , J. S ZEPESI 1 , R. M CI NTOSH 1 , P. K O ´ NYA 2 , L. V INCZE 1 , AND G. G YULA 1 1 Department of Mineralogy and Geology, University of Debrecen, Egyetem te ´r 1., Debrecen, H-4032, Hungary 2 Geological Institute of Hungary, Stefa ´nia u ´t 14., Budapest, H-1142, Hungary Abstract—A detailed stratigraphic and facies reconstruction of a bentonitized acid-tuff succession, deposited within the transgressive systems tract of the Upper MioceneÀSarmatian Ser-3 eustatic cycle, at Sajo ´ba ´bony, 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/K 2 O 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 K 2 O/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. Key Words—Acid-tuff Sedimentation, Bentonites, Geochemistry, Sediment Condensation, Sequence Stratigraphy. INTRODUCTION The determination of volcanic rhythms and micro- rhythms using the distal facies of a pyroclastic airfall succession may have great importance for stratigraphic and paleogeographic perspectives, in Cenozoic, Paleozoic, and Mesozoic strata alike, since facies analysis, basin research, and volcanic and palinspastic reconstructions rely equally on the accurate recognition of the volcanic events. Assuming submarine accumula- tion with a more or less complete preservation, the appearance of the volcanogenic material in sedimentary sequences is controlled by the interaction of episodic volcanic activity and sedimentary processes such as condensed sedimentation during transgression and pro- gradation of terrigenous input. Episodic occurrence of volcanic activity simulta- neously with a eustatic transgression is a rare but not unique situation. Sediment condensation during trans- gression may shut off terrigenous input into the system, resulting in exclusive deposition of airfall volcanic ash, depending on the rate of tuff fall relative to changes in accommodation space. Relatively short periods of low to no net ash deposition may enable the weathering of the volcanic material and thus the consequent formation of smectite. The bentonite layers forming in this way, as confirmed mineralogically by using flooding surfaces, are useful key beds in geophysical, i.e. well-to-well correlations and sequence stratigraphic reconstructions (Cant, 1992). However, even under these relatively homogeneous conditions, a variation in the level of alteration can be assumed related to the changes in the volcanic sedimentation rate. The periods of decreased volcanic input allow for greater alteration, while greater tuff accumulation can inhibit weathering; therefore detailed analysis of the alteration may indicate changes in volcanic sedimentation rate. Analysis of early diagenetic alteration of volcanic glass in volcanogenic sediments requires conventional petrographic, sedimentological, mineralogical, and geo- chemical data. However, in numerous cases, the limits of petrological and/or sedimentological methods enhance Clays and Clay Minerals, Vol. 56, No. 1, 23–38, 2008. Copyright # 2008, The Clay Minerals Society 23 * E-mail address of corresponding author: puspokiz@gmail.com DOI: 10.1346/CCMN.2008.0560103