THE EMERGENCE OF THE BALATON DEPRESSION

A decisive change in the geomorphic evolution was provoked by a further northward shift of the base line. First so called proto-depressions formed during the Middle Pleistocene, especially by the end of it. They had not stopped completely the flow of watercourses running from the Transdanubian Mountains in southeastern direc­

tion and the concomitant sediment transfer, but gradually trapped the coarser sediments;

only the relatively finer material (sand with small gravel, sand, clay) reached the Upper Kapos-Kalocsa Depression (Marosi, S. 1960, 1962, 1965, 1969, 1970; Szilárd, J. 1967,

1970).

Simultaneously with the emergence of the proto-depressions of Balaton and with the sedimentation of coarser deposits, structural blocks started their uplift along the South Balaton dislocation zone (e.g. at Buzsák and Visz).

First of all the drainage network had changed. A previous uniform southerly orientation of the flow had lost its base line and a new one came into being with the emergence of the partial basins of the Balaton which underwent a subsidence in three stages. The first of them can be put to the Middle Pleistocene. On both banks (Balaton Uplands and meridional ridges of the Somogy Hills) a step can be can be found 3-4 km from the lakeshore flanking the escarpment at 160-190 m a.s.l. and continuing south­

ward as high valley shoulders. The second stage was more restricted in areal extension (1,5-2 km from the lakeshore, at an average elevation of 120-150 a.s.l.). Remnant of the third stage of subsidence is a terrace from the Late Würm at an elevation of 110-112 m indicating the appearance of the contiguous water body, i.e. the emergence of the Bala­

ton Basin proper.

In the beginning of this stadial subsidence, formation of the alluvial fan or

N S

Fig. 7. North-south section across the southern rim o f the Balaton Basin between Balatonszéplak and Jódiszőlőhegy. - 1 = bar sand; 2 = loess with embedded angular dolomite debris strings and calcareous slope deposit in the fine sand fraction; 3 = sandy loess; 4 = Pleistocene medium-grain fluviatile sand mixed with debris or with debris lenses; 5 = Late Pliocene cross-bedded sand overlain by clay

Fig. 8. North-south section across the southern rim o f the lake basin at Balatonlelle. - 1 = Late Panno­

nian clay; 2 = Late Pannonian sand; 3 = lignite layer; 4 = Late Pliocene cross-bedded sand with over- lying clay; 5 = Pleistocene medium-grain fluviatile sand with debris lenses; 6 = sandy loess with cal­

careous concretions at the bottom; 7 = calcareous slope deposit in the loess and fine sand fraction with debris strings; 8 = silty sand; 9 = bar sand; 10 = peat layer; 11 = layers with slides

sediment slope adjoining the Balaton Uplands only intensified, then, with the advance­

ment of sinking, the Balaton Basin, northernmost member of the generations of Middle- and South Transdanubian depressions, came into existence. The basin became not only the catchment of waters from the central range but also of the northern reaches of those running northward from Somogy region. In the surroundings of Marcali and Öreglak, along the northern margins of the meridional ridges, deposits of fluviatile and other origin became eroded and transported northward. Both from the north (central range) and south material transport was oriented toward the sinking catchments.

At that time no uniform lake existed, it came into being only during the third stage of subsidence. Watercourses sedimented deposits on the step of the meridional ridges at an elevation of 160-180 m, up to the southern rim of the depression. These deposits consist of material of varied grain size, 2-4 cm in diameter, overwhelmingly limestone and dolomite, partly red sandstone gravel and debris (Figures 7 and 5).

No traces of this material can be found even in the form of deposits of finer granulometry, which may be explained by the elimination of the uniform orientation of drainage toward South Somogy by then, and the still existing opportunity for the flow across the present area of the lake, up to the southern rim of the Balaton Basin and the northern reaches of the meridional valleys. Together with other data this should be con­

sidered an argument for the stadial subsidence of the Balaton Basin (Marosi, S. 1960, 1970; Szilárd, J. 1960, 1967).

An extremely important evidence in the calculation o f the rate of subsidence and its chronological stages are the low valley shoulders of the meridional valleys. Be­

side a lower valley shoulder of post-glacial origin with 4-8 m relative height and another one of Würm age with 15-20 m elevation, sloping toward the lake, there are remains of the initial (Middle Pleistocene) subsidence, such as fluviatile deposits containing debris

Fig. 9. Section across the northern end o f the Marcali Ridge at Balatonberény. - 1 = Pannonian depos­

its (clay, sand); 2 = Late Pliocene cross-bedded sand; 3 = Pleistocene fluviatile sand with small grains o f quartz and Permian Red Sandstone debris; 4 = Pleistocene fluviatile, partly slope deposit with dolomite debris (0.5-3 cm in diameter); 5 = Upper Pleistocene calcareous deposits in the loess and sand fraction with dolomite debris (0 .3 -1 .0 cm in diameter); 6 = Upper Pleistocene loess, sandy loess;

7 = alluvium; V = fault zone

originating from the northern bank, that can be traced on the southern bank on the steps at a maximum altitude 180 m a.s.l., underlying loess-like sediments (Figures 9 and 10), but they do not continue southward.

The spatially and temporally stadial structural movements, having occurred during the Middle and Late Pleistocene resulted not only in the geomorphic levels men­

tioned above but also in freshwater limestone (travertine) horizons formed by springs related to the actual base line. Their most prominent occurrence can be found on Tihany Peninsula, along the northern bank of the lake in the vicinity of Balatonfiired,

Bala-Figure 10 Sketch bloc-diagram o f the abraded steep side o f the lake basin at Zamárdi. 1 = slope de­

posits (filling the valley) in the loess and fine sand fraction with debris strings; 2 = slope loess with fossil humus-carbonate soil; 3 = medium-grain fluviatile sand with debris lenses (material of an allu­

vial fan from times before the existence o f Lake Balaton); 4 = Late Pliocene cross-bedded sand; 5-6 = Upper Pannonian grey micaceous medium- and coarse-grain sand with overlain clay

tonalmádi, Vörösberény (Fig. 2), on the southern bank in the environs of Szántód, Ba- latonföldvár, Balatonszemes, at the earlier indicated levels. It should be mentioned that travertines also form part the „initial surface” e.g. in Tihany Peninsula at altitudes 230- 300 m a.s.l. and lower, at 180-200 m; on Papvásár Hill at ca. 180 m a.s.l. mean altitude adjoining to higher levels of Balaton Uplands (Scheuer, Gy. and Schweitzer, F. 1974).

They occur most frequently, however, at 180 and 150 m and traverines can be met be­

tween 120-135 m altitudes and occasionally even lower (Balatonfüred, Tihany Penin­

sula).