Cycles and rhythms within the Boda Claystone Formation in WELL Ib-4

Halász, Amadé (2009) Cycles and rhythms within the Boda Claystone Formation in WELL Ib-4. Central European Geology, 52 (3-4). pp. 325-342. ISSN 1788-2281

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Abstract

Abstract The present cycle-stratigraphic study is based on four main alternating lithofacies of the Boda Claystone Formation (sandstone, siltstone, claystone and dolomite). The statistical analysis of cycles/rhythms identified from the main rock types of the sequence showed that the theoretical cycle is a succession made up of all layer types and contains the entire sandstone-dolomite row. The definition of the modal cycles that are closer to reality and to practical applications was carried out in several steps. Because of changes within the formation, the modal cycle was identified separately within the major lithostratigraphic units. The more detailed analysis using seven rock types gave similar results, with the dominance of rhythms. Markov chain analysis indicates the arrangement of the Boda Formation in the form of fining-upward cycles. A complete cycle consists of sandstone or siltstone at the base sequentially succeeded by claystone and dolomite at the top. As a result two major facies transition successions can be identified from the facies relationships diagram, which are sandstone to claystone and dolomite to claystone transition. Unit One is represented by sandstone (a, b) and siltstone (c, d), mainly occurring in the basal sequence. Unit Two is dominated by transition from sandstone (a) to silty sandstone (b) and (clayey) dolomite (g, f) to siltstone (b) and/or claystone (e) facies transition successions.

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