PRE-BALATON EVOLUTION

The Pannonian pre-Balaton area was characterised by a shallow inland sea with changing coastlines, intense oscillations of the water level, narrower and wider sand beaches, planes of abrasional coasts, plateau remnants and horsts of the Transdanubian Mountains or central range (Keszthely Mountains, Balaton Uplands, South Bakony) in the form of islands and their groups, sea channels, narrower and wider embayments.

The Balaton Basin had subsided into Pannonian sediments so the geomorphic evolution is outlined from this moment. Position of the basin along the mountain margin provides an explanation for the events in the history of evolution of its wider environ­

ment (formation of the Görgeteg-Babócsa Depression, southward shift of the Upper Pannonian inland lake) even if in an indirect way, played part in the formation of the pre-Balaton topography.

There might be a hiatus of the Lower Pannonian formations along the coastline, but toward south they become thicker and in the area of the Görgeteg-Babócsa geo­

physical maximum they exceed 2000 m (Szentes, T. 1943, Kertai, Gy. 1957, Vadász, E.

1960, Körössy, L. 1963, Bartha, I. 1959).

There are spatial and temporal differences in the regression of the Upper Pan­

nonian inland lake, in changes of the bottom configuration. The beginning of this proc­

ess can be best studied in the sequences of the high bluffs at Balatonakarattya, Balatonk­

enese and Balatonföldvár, where oscillations of the inland lake, coast line shifting, lake level fluctuations are referred to by the presence of marshy layers. The termination of this regression in some places is indicated by traces of erosional-denudational activity and the appearance of erosional unconformities and sand deposits (even in exposures such as Fonyód Hill /Figure I/, Gomba-puszta, Zamárdi), while in other places thin clay benches appear closing the Pannonian formation (product of desiccation at e.g. Hollád, Balatonszentgyörgy, Kőröshegy, Zamárdi, Szigliget, Arács), where the long lasting inland lake state terminated in a calmer period and/or at a protected place. Resulted from the erosional-denudational activity the upper level of layers having contained Upper

Fig. 1. Sketch bloc-diagram o f the Nagy-Várdomb in Fonyód from the north. - 1 = Late Pannonian series (sand with clay and lignite beds); 2 = Late Pliocene cross-bedded sand; 3 = Late Pliocene clay bed; 4 = Pleistocene fluviatile sand with silt and debris stripes; rising and wedging from the rim to the south; 5 = wind blown sand; 6 = slope deposit o f sandy loess; 7 = collapsed material

Pannonian Congeria balatonica have been removed in considerable thickness; older members situated deeper survived until a new stage of evolution between the Pannonian lake and Quaternary continental evolution began (Marosi, S. 1970).

The effect of slow, stadial dictiogenetic movements leading to the uplift of the bottom of the basin and its turning into land and, simultaneously, the shrinking o f the Pannonian lake and its decay into partial lakes resulted in a southward shift of the inland lake, accompanied by the mentioned oscillations; finally it had been restricted to the Slavonian or Zagreb Basin. It should be emphasised, however, that this process had been very slow, gradual, even stadial.

Rivers flowing northward, transported sediment toward their base lines perma­

nently.

This activity actually was similar to the effect of the uplift, because the fluvial sediments deposited along the margin of the inland lake which shifted southward from the water body. Slowly flowing waters and shallow channels between the residual parts of the disintegrating lake were typical. These riverines sedimented sands and clays with Viviparus and Unio wetzleri. These layers are characterised by oblique and cross bed­

ding (fluvial bedding; Balatonaliga, Zamárdi, Balatonföldvár, Lengyeltóti, Szigliget, Szent György-hegy) or strong dip (delta-like bedding; Balatonberény). There have been an extensive discussion about the genesis of this sediment considered widespread in the western and southern parts of Transdanubia (from E. Szádeczky-Kardoss’s /1938/

standpoint to that of M. Pécsi /1962/) summarised and evaluated by J. Szilárd (1967) and S. Marosi (1970) and only referred herewith.

The Upper Pannonian transgression brought about profound changes also in the morphological features of the Transdanubian Mountains. Prior to the Miocene the area probably was the pediment of the southern crystalline mountains and of the emerged horsts of the Bakony (Pécsi, M. 1969, Juhász, Á. 1995, Korpás, L. 1981), i.e. it consists of a series of pediplanated transitional types of horsts. The residual forms, in some places traces of red clays in the cracks of rocks, manganese and bauxite gravels occur­

ring along the plateau margins and red clay mantles of 2-3 m thickness evidence to the

NW SE

During the Upper Pannonian the Balaton Uplands, Keszthely Mountains and South Bakony elevated above the inland sea as islands, while the present intramontane basins (Tapolca Basin, Nagyvázsony Basin) constituted embayments. Sediments cover­

ing the marginal plateaus and horsts in lower position were partly inundated, partly transformed by abrasion; in many places travertine was deposited on them (Szentkirályszabadja /Figure 21, Nagyvázsony).

The Upper Pannonian sediment formation was accompanied with basalt vol­

canic activity of the basic type, and the ancient surface had became covered by material from more than 40 centres of eruption. This volcanism took place in three phases; on the Tihany Peninsula it started in the period of marine sedimentation and terminated at ca. 2 M yr B.P. during pediment formation. Lava has conserved former pediment surfaces, giving this way an opportunity for dating pedimentation. According to K/A dating this latter took place between 3.3-4.7 M yr B.P. in the Keszthely Mountains, 2.8-3.5 M yr in the Tapolca Basin and 4.5-6.0 M yr on the Balaton Uplands.

Pediment planes formed along the southeastern mountain margins simultane­

ously to Pannonian regression were the initial surfaces of the emergence of the Balaton Basin.

With the growing differentiation in the elevation of the mountains and their foreland, during the transition from the Pliocene subtropical to the Pleistocene climate, the former fluvio-lacustric hydrography had gradually given way to a river network producing gravelly sediments.

At the same time, subtropical semiarid climate and the growing energy of relief created favourable conditions for the emergence of torrents, consequently, for the dis­

section of the plateaus in the Transdanubian Mountains. During this period of evolution the highest abrasional terraces (Szentkirályszabadja, Vörösberény, Balatonfüred,

Eder-ics, Keszthely Mountains) got started to be dissected and torrents began to build alluvial fans in the foreland areas (Juhász, A. 1988).

Taking into account the paleoclimatic data and geomorphic evolution of the closer and broader environment, wind action (deflation) must have played a significant role (Lóczy, L. Sen. 1913; Cholnoky, J. 1918; Bulla, B. 1962; Schweitzer, F. 1997). The appearance of fine grain sediments was attributed by M. Pécsi to a savanna-like climate.

Recently F. Schweitzer provided confirming paleoclimatic data for the desert theory evolved by Lóczy and Cholnoky (Schweitzer, F. 1994, 1997).

Simultaneously to the subsidence of the Little Plain, to the uplift of the Trans- danubian Mountains (central range), to the emergence of the Siimeg-Gleichenberg wa­

tershed, the Balaton area and the whole South Transdanubia lost a major part of their previous northern catchment, and the Upper Kapos-Kalocsa Depression (the middle i.e.

second member of the Transdanubian generation of catchments) which had functioned as the principal base line since the Plio-Pleistocene boundary, from this time collected the sediments transported by water courses running southward from the central range (Marosi, S. 1960, 1970). Meridional valleys of north-northwest-south-southeast orien­

tation had emerged, adjusted to the structural lines, gradually incising into the gently sloping surface and separated from each other by elongated fluvial ridges. Material of nappe sediment covering the Transdanubian Mountains, products of physical weathering and deluvium originated from the fluvial ridges were transported through these valleys to the Upper Kapos-Kalocsa Depression (Szilárd, J. 1962, 1967; Marosi, S. 1970).

During the Lower Pleistocene in the western part of the basin the formation of meridional valleys was substituted by the formation of alluvial fans within a wide strip of subsidence. This formation of fans and a uniform spatial pattern of the meridional valleys from the central range watershed up to the Upper Kapos-Kalocsa Depression continued through the Middle Pleistocene. This is evidenced by the system of terraces and valley shoulders of the meridional valleys (Figures 3 through 5), by an undisturbed buildup of the fan and its surface not being covered by loess-like deposits.

The actual landforms have also inherited features of the pre-Balaton topogra­

phy: meridional valleys being structurally preformed, can be traced as the direct

con-Fig 3. Longitudinal section o f the Kőrös Hill meridional valley. - A = valley watershed; vt = valley floor; avi = lower step o f a low valley shoulder; avn = higher step o f a low valley shoulder; mv = high valley shoulder

Fig 4. East-west section across the western slope o f the meridional valley between Somogytúr and Orci with low (avi, avu) and high (mv) valley shoulders and with derasional steps (dl). - 1 = finely stratified slope deposits in the loess and sand fraction; 2 = sandy loess; 3 = medium-grain fluviatile sand with debris lenses; 4 = slope deposit in the loess and sand fraction with debris strings, redeposited from the previous layer by derasion; 5 = fluviatile sand with debris; 6 = overlying Late Pliocene clay; 7 = Late- Pliocene cross-bedded sand

tinuation of valleys cutting into the Balaton Uplands, while the mirror-image of the spacious Tapolca Basin on the southern side of the lake is the embayment of Nagyberek and the fluvial network, represented by valleys in the final phase of alluvial fan forma­

tion. A very similar picture is characteristic for the meridional strip of the Little Balaton.

The predominantly quartz material of the sediments transported by the pre- Balaton watercourses evidently contains some debris originated from the Transdanubian Mountains. These deposits are getting more rounded and fine grained southward; in the Inner Somogy alluvial fan they acquire normal stratigraphic sequence becoming finer vertically upward and turning into sand fraction (Marosi, S. 1962, 1970, Figure 6). They also occur on the high valley shoulders of the meridional valleys in the elevated Outer Somogy Hills, on the former Lower and Middle Pleistocene terraces, 70-80 m above the

a s j Balaton Karád-ridge S -0u ter Som ogy Loess Plateau Kapos-river

Fig. 5. Longitudinal section o f the meridional valley between Somogytúr and Orci. (For legend see Fig.

3.)

Fig. 6. North-south section of Inner Somogy from Lake Balaton to the Dráva River. - I = sandy-clayey Pannonian deposit, at several places with Late Pliocene cross-bedded sand o f various thickness in the upper layer; 2 = Pleistocene clayey, silty, sandy, gravelly fluviatile, partly lacustrine deposits with wind-blown sand and in spots loess-lik e deposits on the surface; 3 = clay disclosed in boreholes; 4 = loess, sandy loess disclosed in boreholes; 5 = hypothetical fault zones; 6 = Plio-Pleistocene boundary

present medium level of the Balaton (Szilárd, J. 1962, 1967). In secondary position the major part of the contemporary filling of the valleys consists of them.

Naturally, the fluviatile sequence reaches maximum thickness in the Upper Ka- pos-Kalocsa Depression, where the gravelly sediments constitute several layers with a total thickness of 20-25 m in boreholes. In Outer Somogy a nearly complete Würm loess sequence can be traced underlying the sandy fluviatile series. The age of the gravelly sediments (Günz, Villafranca stage) are also supported by findings of vertebrate fauna from the lower layers (Rhinoceros etruscus Falc., Elephas meridionalis Nesti, Elephas antiquus Falc., Hipparion', Lóczy, L. Sen. 1913, Kretzoi, M. 1953).

The above described processes started on a gently sloping surface, stretching from the foreland zone of the present Transdanubian Mountains (northern shore of Lake Balaton). There was a differentiation of the relief in a double sense; on the one hand the central range experienced a steady uplift (300-400 m for the Quaternary; Láng, S. 1958, Pécsi, M. 1956, 1959) and simultaneously South Transdanubia saw an elevation of slower rate; on the other hand the local base line had shifted northward (from the Croa- tian-Slavonian Depression to the Upper Kapos-Kalocsa Depression; Marosi, S. 1960), besides, the surface had been dissected by meridional valleys (Szilárd, J. 1960, 1967).