Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 14. 1989. (Budapest, 1989)

applied name is Szőc Limestone Formation. The field-work and the study of several hundred thin sections revealed remarkable vertical changes within this formation. The Szőc Limestone is rather thick: in boreholes crossing the whole formation, the thickness is more than 70 m, but locally (e.g. Devecser, Dv-4) it may reach 200 m. Strong pressure-solution (stilolitization) was recorded in the field and in thin sections (Plate I; 1,2). This may result in a 10-30% compaction and, considering this value, the original thickness of the Szőc Limestone might have been much greater. The limestone is very pure, almost totally avoid of terrigenous material; in the higher part of the sequences even the finest quartz grains and extraclasts are missing. The majority of the biogenic grains are larger Foraminiferids : mainly Nummulites (Plate I-II), sometimes Alveolina, Assilina or Discocyclina (Plate I: 2) but Corallinaceae and Dasycladaceae are also common. Traces of biological erosion and encrustations are frequent (Plate 11:1,2)1 Bioturbation must have been very intensive; this gives the characteristic, nodular bedding of the limestones. Turning to the vertical changes, going upwards the calcareous algae disappear gradually (first the Dasycladaceae, later the Corallinaceae). Synchronously, the biological erosion and encrusting activity decreases. In the highest part of the sequences an increasing amount of glauconite can be recorded. From the above facts we may draw the conclusion, that the southwestern part of the TCR was covered by a shallow, but gradually deepening sea during the Middle Eocene. The intense limestone production kept pace with the subsidence (or with the sea-level rise), but near the end of the Middle Eocene the bottom reached a critical depth, where the environment became unfavourable for a rich benthonic community. It is remarkable, that even in this phase of subsidence coarse elastics did not reach the basin; this implies very remote shore lines and terrestrial sources, i. e. a practically pelagic environment. SUBSIDENCE HISTORY The quantitative analysis of the subsidence history is essential for a sedimentological model, or for a paleogeographical synthesis. Therefore, we have to construct subsidence curves, even if we known that the result will carry serious errors. One of the main source of these errors lies in the measuring of geological time: stratigraphical correlation is sometimes unreliable or not precise enough, and the 'absolute" (radiometric) time scale changes from year to year. Another, perhaps even greater source of error is in the reliability of "bathymétrie markers" used to measure the paleo-depth of seas. In this chapter, subsidence curves of some typical Middle Eocene sequences of the TCR will be presented. The following principles have been applied in the construction of the curves : (1) The Nummulites- zones of KECSKEMÉTI (1982) have been used to measure the geological time. In the TCR these biozones have been correlated with the plankton-foraminiferal and nannoplankton zones by KECSKEMÉTI (1982); the latter zones have been calibrated to a radiometric scale recently by HAQ, HARDENBOL and VAIL (1987) (Fig. 2). These data appear on the time-axis of the subsidence diagrams. (2) Three "markers" appeared suitable for measuring the deepening of the Eocene seabottom: Dasycladaceae, Nummulites and glauconite. Disappearance or appearance of these marks a relatively well-defined depth value. In present-day seas Dasycladaceae are practically restricted to the shallow part of the euphotic zone, and do not live deeper than 40 m (MILLIMAN 1977). The living rotaliid foraminifers, the closest relatives of Nummulites occur in the deeper part of the euphotic zone, but they are not known in depths more than 150 m (HAYNES 1981). The formation of glauconite depends on temperature and on chemical circumstances, rather than directly on depth (EHLMANN 19 78), but recent observations show that a great amount of glauconite is present in the outer shelf, below 100 m. These three values (40 m: disappearance of Dasycladaceae, 100 m: mass appearance of glauconita, 150 m: disappearance of Nummulites) are shown on the vertical axis of the subsidence diagrams.