The Quaternary Development of the Dravinja Hills (NE Slovenia) and of the Neighbouring Fringe Lands

Abstract

Studies of the Quarternary development of the Dravinja hills (NE Slovenia, near Maribor) and of the neighbouring fringe land have shown that the erosion was by far the most predominant process during that period. Rivers deepened their valleys
both in the mountainous Pohorje region and in the mountain range Konjiška gora Boč as well as in the area of Dravinja hills. It was only during this period that the area of Dravinja hills had first lost its former character of a flat plain and developed into a minutely dissected hilly region. For the formation of Dravinja hills, the brooks that flow into the Dravinja river from the Pohorje mountain were of special importance. With the exceptional quantities of their waters and of the material they carried even in the oldest periods they pressed the flow of the Dravinja river to the uttermost southern fringe of Dravinja hills. Dravinja river was pressed into the same direction also by the Drava river which reached the area of these hills at their extreme eastern end. Due to this general pressure on the Dravinja river to take a course as far south parallel to the mountain range Konjiška gora—Boč we find, e. g., that the important Ljutomer fault east of Loče remained north of the river, and that the oldest terraces are not cut merely into the poorly resistant Pliocene sediments in the centre of the syncline, but also — and that regardless of the tectonic conditions —
through the more resistant Miocene and even Oligocene strata at the very foot of the folded and tectonically uplifred mountains Konjiška gora, Boč, and Haloze. The morphological facts illustrate well these conditions of the development. It has been
established already by A. M e l i k that the altitude of Dravinja hills decreases from west to east and south, parallel to the direction of the present day hydrographic net (11). This direction is indicated not merely by the remains of the oldest surface,
but also by the lower terraces, including the bottoms of the valleys along the Dravinja-river and its tributaries which are reached by inundations. Thus the Dravinja hills show very simple traits of development that can be explained with fluvial processes
and with a general rising of the land which in all probability during the Quarternary extended to the region of the Dravinja hills and to the neighbouring higher fringe of the Pohorje, Konjiška gora, Boč, and Haloze mountains. On the basis of this inclination and of the congruity of all terraces in the area here investigated we may conclude that the processes which had formed them must have been very similar. This is also proved by the gravel preserved in the terraces which shows in its granulation and rounding, as well as in its petrographic characteristics, close similarity. This is true for the material deposited along the Dravinja river and its tributaries, as well as for the corresponding material deposited by the Drava river along the eastern fringe of Dravinja hills. All these facts point to a
comparatively young character of the relief of the Dravinja hills which in all probability does not go beyond the Quarternary age. If it had been older, we would have to take into consideration another climate, and other processes, together with other forms of the relief and sediments in them. We have been confirmed in our suppositions with our study of terraces. It has been possible to identify 5 to 6 terraces along the Dravinja river, along its tributaries, and along the Drava river. Of these terraces the upper three represent the uppermost parts of the Dravinja hills. The gravel in them is — as we have already stated — very similar. In the lower three terraces we could establish a connection between the accumulated material and the thick strata of the periglacial solifluctional scree which had literally filled in the upper parts of the valleys. A part of this material
was carried by rivers Dravinja, Oplotniščica, Ložnica, and Polskava, as well as by the brooks of the Konjiška gora—Boč mountain range, further down the valleys; it formed wide fans at the entrance into the area of poorly resistant Terciary rocks out of which the Dravinja hills are built. The fact that the depositing process had actually taken place during the cooler parts of the Quarternary is proved — besides by the already established connection with the solifluctional material — also by the considerable thickness and by the comparatively slight rounding of gravel stones: these are considerably less rounded than the recent gravel (cf. the diagram showing the rounding of the gravel). Important additional support for this supposition is given by trunks of coniferous trees preserved in the gravel and by the characteristic composition of the pollen found in loams that had been deposited directly on the gravel during the period following each of the fluvioperiglacial depositions. In all specimina of the loam in which pollen has been preserved we have been able to identify samples belonging to the trees that grow in the cool northern taiga. In the uppermost strata only we could find among such samples also pollen that betrays a partially warmer climate. The fluvioperiglacial character of these accumulations in the Dravinja river system is alsoi proved by the contemporaneity of these depositions with the accumulations formed by the Drava river which are, judging by all their characteristics, of a fluvioglacial origin and continued — similarly as those along the Sava river and its tributaries — into late glacial periods when the fluvioperiglacial deposition had already ceased (1; 9; 10; 11;
14; 16; 29; 32; 35). Eeach deposition was accompanied by a strong lateral erosion which was especially intensive in the area where the brooks passed from the Pohorje mountain into the poorly resistant Terciary rocks of the Dravinja hills. This erosive power decreased rapidly further down the valleys simultaneously with the decreased size of gravel stones. For this reason the Pleistocene terraces and valleys are particularly wide at the transition from the Pohorje mountain into the Dravinja hills. Here we can find among the gravel large quantities of rounded quartz which came fom the eroded bedrock. Thus this widening of the valleys at the foot of Pohorje mountain was caused by an increased lateral erosion, and not by any tectonic reasons as this has been suggested so far (11; 19). Each such fluvioperiglacial deposition was followed — similarly as this was the case after the last Glacial Age — by a vertical erosion which produced differences between individual accumulated terraces. The Holocene erosion was in the Dravinja river system more intensive in the upper parts of the valleys (up to 8 m), where the valleys were filled especially high during the former period of the glaciation, and along the lower flow of the Dravinja river, where the erosion was caused by the exceptionally strong cutting activity of the Drava river. In the central sector of the Dravinja river, and in the lower parts of its tributaries which here flow into it, the Holocene erosion was comparatively insignificant: here the floor of the Würm accumulation lies generally only about 1—3 m over the Holocene plains that consist of sand and loam, while in some places it lies even below the latter. In spite of the fact that the Holocene erosion can be distinguished very clearly, nevertheless with its effect it considerably falls behind the older phases of erosion with which the older terraces differ one from the other. It has already been mentioned how the terrace 2 rises ca 8—15 m over the terrace 1, while the height of the higher terraces reaches even 15—20 m (terrace 3), 30 m (terrace 4), 30—40 m (terrace 5), and the remains of the oldest surface reach up to ca 40 m over the lower lying surfaces. In our interpretation of these differences in the intensity of erosion and of the processes of erosion we have so far above all referred to the fact that these phases of erosion had always followed each of the fluvioperiglacial deposition. Each process of erosion — with the exception of the Holocene erosion — was interrupted only with
a renewed periglacial deposition, and for this reason it has been considered as rather probably that the intensity of each phase of erosion corresponds with the duration of individual warmer intervals. For the sake of a support of this scheme we have given as a comparison the last Glacial Age and the period of erosion which had followed it and for which it has been possible to prove beyond doubt that it was entirely of a climatic origin and that it was begun immediately after the start of a warmer climate when the ground was again covered with forest and when the rapid inflow of the periglacial material into the valleys was ceased. In our study of the Dravinja hills we had to put into the foreground this climatic concept. It has always been possible to establish that each fluvioperiglacial deposition was followed by an erosion, and that each of these changes in the process was entirely caused by a change of the climate. We have made several critical observations only in connection with the subsequent course of individual phases of erosion. In the study of the Holocene period it has been possible to establish that so far we have limited ourselves too much to the investigation of the erosion while at the same time we have much neglected the appearance of the contemporary deposition with which the process of erosion was in many respects modified and which for shorter periods at least even completely stopped the erosion. Here we think above all of the accumulation of the most varied Holocene loam and sand sediments in which large quantities of wood, leaves, pollen, and other organic remains of the warm Holocene climate can be found preserved. We have examined them now in detail also in the area of the Dravinja hills. Our finding that the accumulation of these loams and sands was conditioned
entirely by the climate — at least before a more intensive encroachment by the man on the nature in this area — calls our attention to the fact that in the older warmer parts of the Pleistocene, too, we must reckon with similar situations. In our country, however, discoveries of such sediments from warmer periods are very rare. They are limited almost exclusively to the areas with younger tectonic sinking (e. g., the basin of Zalog near Novo mesto, the Ljubljana Moor), while in the Dravinja hills, as much as this has been established so far, they are completely absent. Here — as well as in other parts of Slovenia — we could establish on the terraces a thick
and poorly assorted fluvioperiglacial gravel only. In loams, too, found locally under or over the gravel, we could identify the pollen, seeds, cones, rind, and wood only belonging to plants of the cool northern taiga (e. g., in the Wiirm terrace 1 near Konjice and Spodnja Polskava, and in the older terraces at Pobrez, near the village of Jurovci, and at Podlehnik). Our present views regarding the erosion cannot give a satisfactory explanation of these facts. Above all the present interpretation can give no satisfactory answer to the question why in the area of Dravinja hills the sediments belonging to warmer periods are so completely absent and why do the sediments belonging to the cooler periods lie directly on the eroded bedrock? We could namely expect — in case that the erosion had really ceased immediately after the end of each warmer period — that the loamy and organic sediments from warmer periods would still be preserved in larger quantities and that they would lie directly on the bedrock or under the gravel from the cooler period which had stopped each phase of erosion. The facts, however, are really different, and the gravel from the cooler period lies directly on the bedrock, and because of this we are becoming more and more convinced that the development had taken another course. Above all, the interpretation suggests itself that with the end of warmer periods the erosion was not stopped, and that instead it continued into the period of glaciation. During such periods of glaciation the sediments from warmer periods had been perhaps removed to such an extent that the fluvioperiglacial gravel had been deposited directly on the bedrock. It seems that a more intensive vertical erosion was probably interrupted only during the coldest and driest climaxes of the periods of glaciation when the fluvioperiglacial deposition was most intensive. This took place first in the upper parts of the valleys where the inflow of finer scree material from the flanks of the
valleys was greatest. With the increasingly drier climate the process of accumulation became widely prevalent. Large fans were deposited by the brooks in those places where they passed from the mountainous area into the Terciary region of Dravinja
hills. This deposition and the shifting of the riverbed from one side of the fans to the other was combined with a strong lateral erosion with which the earlier basins formed by erosion were much widened. In spite of this the vertical erosion was in all probability not completely stopped even during this period. In our area it has been namely possible to notice that the stratum of gravel in the terraces was on the whole not too thick. It is thicker in the fans in those places only where the brooks leave the Pohorje mountain. Further down the valleys this stratum of gravel becomes rapidly thinner reaching a thickness of even less than one metre. Thus this gravel
was still carried by high waters and because of this the vertical erosion could still be continued in spite of the tendencies for deposition and lateral erosion. This interpretation has been supported by the finding that exactly in those places where the stratum of gravel becomes thinner, the quantity of stones originating from the bedrock basis — especially of a finely rounded quartz gravel which is so much prevalent in the Dravinja hills — increases considerably. All these facts prove that during the transport of the gravel from the Pohorje mountain to the Dravinja hills, and further down along the valleys, a partial deepening of the valleys had still continued. This
could occur especially at higher waters which in all probability were particularly frequent in the periglacial climate. The outflow of rain water and of the water from the melted snow was, because of the humble vegetation, in all probability very rapid. This led to high inundations and to the transportation of gravel. With high inundations we could well explain the comparatively poor assortment of the Pleistocene gravel which shows above all signs of a quick transport, perhaps even by torrents. All this agrees well with the findings made by J. Biide l in the Spitzbergen where these processes are active even in the present time. J. Büdel has established for this area that the deepening and widening of valleys is a very rapid process, much quicker than under all other climates. The main geomorphological changes occur when the surface of the ground on the flanks and at the bottom of the valleys, which is permeated with ice, becomes deeply thawed. During similar considerable increases in temperature, which are frequently accompanied by rain,
waters grow very rapidly and inundate the wide bottoms of the valleys, they undercut the banks, and even carry away the material from the bedrock which had been much loosened by the freezing. All this speeds up particularly the deepening and
widening of the valleys (4). This comparison is obviously only broadly valid. The researches that have been made so far have shown that in our region the climate had never reached such extremes. The surface covered by forest was more widely limited and the forest
thinned when also the solifluction became more intensive only during the extreme climaxes in the periods of glaciation. For this reason traces of the fluvial reshaping of the relief have been better preserved, they are less deformed, and the traces of a selective erosion are more clear, while in the Spitzbergen and in the Central Europe where during the Pleistocene periods of glaciation the climatic conditions had been more extreme, they could almost not be observed by J. Büdel those areas the decomposition did not extend to the poorly resistant rocks only but also to the strata of rocks with great resistance. All this, however, does by no means change our basic concepts that in the Dravinja hills the erosion was a strongly predominant process during the Quarternary and that it continued from the warmer periods of the Pleistocene even into the cooler ages. During these large quantities of sediments that had been deposited during warmer periods had been removed. The vertical erosion passed only during the extreme climaxes of these periods into lateral erosion while with the entrance of a warmer climate the linear cutting into the ground of the valleys started again. Such processes explain the genesis of all the main Pleistocene terraces and with them we can better understand the exceptionally strong vertical erosion which in a comparatively short time changed the originally flat area along the Dravinja river into a minutely dissected hilly region. As regards the tectonic structure it must be emphasized that no trace of a distinct tectonic movement during the Quarternary could be found in the whole region of the Dravinja hills. It is, however, very likely that during the Quarternary this area was also raised parallel to the general rising of the land, and that this contributed to the development of the lateral and vertical erosions, as an additional reason to the predominant climatic influence. The beginning of the dissection of the Dravinja hills, too, seems to have been
decisively influenced by climatic factors. It is certainly not a mere coincidence that the beginning of the cutting of this terrain falls exactly into the period with great changes of the climate which took place towards the end of the Pliocene and at the transition into the Pleistocene. The tropic or subtropic characteristics of the climate and of the corresponding processes were replaced by the processes of the cooler Pleistocene age which was essentially different from the preceding climate. In opposition to the conditions of the tropic and subtropic climate, the new conditions led to the formation of large peneplains and pediments at the foot of the mountains. Parallel to the accelerated and dynamic processes of the Pleistocene climate wen the accelerated lowering of the flat area and the gradual transition into the erosion (4; 5; 12; 13; 21; 22; 23; '31; 33). In this respect the terrace 5 seems to be especially interesting. It has a remarkably dominating position in the whole region of the Dravinja hills. It bears, withits inclination and with the sediments that are preserved on it, characteristics that are purely Pleistocene. Because of the exceptional width of this terrace we suppose that it is the product of the first exceptionally deep fall of temperature in the Pleistocene. The flat area of the Dravinja hills was at that time in all probability still entirely uncut. For this reason the deposition of the material, the transportation of the gravel and the parallelly developing lateral erosion which is so typical of the cooler periods extended over a very wide surface. Very few remnants only of an older surface still rise over the terrace 5. Because of the exceptional width of this terrace (terrace 5) we suppose that the processes of the pediplanation continued at the foot of the Pohorje mountain into the Pleistocene itself. Quite naturally detailed investigations of this development must be left to later researches which will not be limited to the Dravinja hills only but will include also the eastern fringe of the valley Dravsko polje and the hills of Slovenske gorice, as well as the hills along the Mura river where traces of a deposition older than from the Pleistocene will perhaps also be found: such traces are in the Dravinja hills already removed due to the exceptional erosion caused by the brooks floving from the Pohorje mountain. The development of the Dravinja hills as explained in the present study shows clearly that we cannot agree with older researchers who supposed that this hilly region was dissected already during the Pliocene in a way as it is now (e. g. 11; 19). Closer to our views stands the concept proposed by A. Winkler ; yet in opposition to him we date the formation of the gravel terraces that are covered with loam from a glacial and not from an interglacial age (36).