Głubczyce Plateau

Research methods of the fine-grained alluvial fans
(an example of alluvial fan in Borucin); Głubczyce Plateau
Edyta Zygmunt
University of Silesia, Faculty of Earth Sciences, Department of Quaternary Palaeogeography and Palaeoecology;
ul. B dzi ska 60, 41-200 Sosnowiec, Poland
[email protected]
Introduction: Alluvial fan is a slightly oblique form of terrain, characterized of convex transverse section,
reminding open cone (Klimaszewski, 1981). The highest part of the cone is called inner fan, the middle – mid
fan, and the lowest – outer fan (Miall, 1996) or proximal, medial and distal fan respectively (Blair & McPherson,
1994). Alluvial fans are the common forms occurring at the mouths of valleys. There are numerous papers
concerning their inner structure, morphology and hydrology (Blissenbach, 1954; Rachocki, 1981; Harvey, 1988;
Miall, 1992; Blair & McPherson, 1994; Calvache et al., 1997). So far, majority of investigations of alluvial fans
were conducted in mountain and pre-mountain areas, where forms built of coarse-grained sediments, derived
from debris flows, were analysed (Chamyal et al., 1997; Blair & McPherson, 1992; Sorisso-Valvo et al., 1997;
Blair & McPherson, 1998; Harvey et al., 1999; Anderson et al., 2000; Gómez-Villar & Garcia-Ruiz, 2000).
Gómez-Villar & Garcia-Ruiz (1997) give direct and indirect factors, favouring and limiting the development of
alluvial fans. It is considered, that three factors control the evolution of alluvial fans. Their progradation may be
caused by changes of base level (Miall, 1992; 1996; Harvey, 1999 a; 2000), climatic variations (Wasson, 1977;
Chamyal et al., 1997; Harvey, 1999 b) or land use changes (Ballantyne, 1991; Teisseyre, 1995; Klimek et al.,
2001; Coulthard, 2002; 2001; Zygmunt, 2004a). Many times, the alluvial fans forming in different climatic
conditions, undergo renewed activation caused by anthropogenic changes of vegetation cover (Brazier, 1987).
The loess cover of the Głubczyce Plateau is densely cut by syncline in shape, erosional – denudational
valleys (Fig. 1), at which mouths the alluvial fans were deposited. The development of these forms is related
with colonization of the Głubczyce Plateau by the agricultural-farming tribes (Zygmunt, 2004b). The
prehistorical farmers, wanting to gain the ground for cultivation and pasturage, and the material for building their
settlements, they started intensive clearances. Interference into natural forest stand, caused activation of sheet
erosion on the loess slopes (especially after intensive rainfalls and snow melts). The eroded material derived
from cultivated slopes, was deposited in the valley bottoms, and partly at their mouths as alluvial fans.
The main purpose of the present paper is presentation of the methods of palaeoenvironmental
reconstruction on the basis of analysis of morphometric-sedimentological features of alluvial fan at Borucin,
deposited at the mouth of periodical inflow of the Psina River (Fig. 2).
Characteristics of the study area: Głubczyce Plateau (Opavská Pahorkatina) is the transitional area
between Sudetes Mts and Silesian Lowland (Dylikowa, 1973).From the east, it is bordered by the Upper Odra
Valley.
This region was entered two times by the continental glacier of the San and Odra stages. The periglacial
climate, prevailing during the last glaciation in the Upper Odra Basin, was favourable for accumulation of loess.
(Jary, 1991; Jersak, 1991; Jersak i in., 1992; nieszko, 1995). The loess cover, up to 9 m thick, rests upon older
clayey-gravel Pleistocene deposits, or rarely upon Tertiary deposits. The Pleistocene in the western part of the
Plateau, is underlied by the Carboniferous Kulm deposits of the Sudetes, represented by shales, mudstones and
greywackes. In the east, under the Pleistocene deposits, the Miocene clays occur.
The loess rolling cover of the Głubczyce Plateau reaches 300 m above the sea level, at the base of the
Sudetes, where from it gently falls to 250-270 m above the sea level toward NE. The cover is densely dissected
by the permanent or episodic drainage of the valley, passing into the main river valleys, draining the Plateau (of
Osobłoga, Stradunia and Psina together with Troja). At the mouth of the valleys, numerous alluvial fans were
deposited (Fig. 3), which inner structure preserves the record of the human impact on the primary landscape of
the area, as well as climatic changes, especially the moist periods, of the late Holocene.
Research methods: In order to define the time of alluvial fans development, as well as their correlation
with the time of settlement of the Głubczyce Plateau by individual communities, various field investigations
were carried out (Fig. 4). They comprise the geomorphological mapping and alluvial fans levelling, in order to
define their range and shape. In order to recognition of the inner structure of the fans, the set of drillings have to
be made; whereas in order to investigation of the sedimentary structures, the geological sites have to be exposed
by the use of excavator and dozer. In order to define the thicknesses of peat underlying the mineral deposits of
alluvial fan, the geophysical electro-resistance measurements were carried out. These measurements were used
in order to find and trace the changes of the physical parameters, occurring in shallow (up to 10 m) superficial
layers.
The samples collected during the field works were subjected for laboratory analyses. In the case of
mineral samples, the grain size analyses (by the sieve and aerometric methods) must be done. The organic
samples are subjected for radiometric dating using 14C method (the organic samples from the alluvial fan from
Borucin were dated at the Kyiv Radiocarbon Laboratory).
The important stage of the research is an interpretation of sedimentary structures of alluvial fans,
because on their basis we can mark the phases of enhanced erosion occurring in the drainage area of the fan.
These episodes can be linked either with human interference into natural environment, or rapid climatic events,
or both these factors occurring together, intensifying reciprocally the erosional effects.
On the basis of the inner structure of the fan and obtained radiometric data, the chronostratigraphy and
chronology of events that occurred in the past within the analyzed valley, can be established.
Having the data about the time of enhanced erosion caused by deforestation and introduction of
agriculture, the essential is correlation of obtained data with published and unpublished archaeological data,
concerning the time of settlement of the Głubczyce Plateau by the prehistorical and historical communities with
agricultural-farming system.
The all gathered informations tend to palaeogeographical reconstruction of evolution of alluvial fans
deposited at the valley mouths.
Research results: At the mouth of a dry valley near Borucin over 2.5 kilometres long and with a
gradient of 10.5 metres per kilometre, an alluvial fan is situated (Fig. 3, 5), the morphological features of which
make it clearly visible. Electro-resistance measurements showed that the alluvial fan prograded toward the peats
filling the bottom of the Psina River Valley up to 5 m of thickness. The area of the valley catchment is 0.45 km2
and the area of the fan is around 0.13 km2 (Fig. 5). The fan is asymmetrical, indicating a variable direction of its
progradation, which had progressed in several phases (Fig. 5). This is also indicated by flattened areas visible
along the longitudinal profile of the fan. The thickness of sediments at the base of the fan reaches up to 2.5
metres. The sediments/deluvia forming the fan are only 40–80 centimetres thick at a distance of around 450
metres from the valley slope.
Despite the fact that the loess layer was the source of the alluvial fan, grain size analysis has indicated
that the sediments forming the fan are of two types. The bottom part of the fan is formed by clayey silts,
sometimes of organic origin, which indicates the long range of the silty material eroded from the valley,
probably due to the gradual forest clearance of the latter. The silty clays covering the former layer were probably
deposited during a period when the valley catchment was utilised in a less intensive way and the forest cover
partly regenerated. The differences in sediments forming the fan are probably due both to climate changes that
occurred during the Holocene and to changes in land cultivation, particularly the degree of deforestation.
The results of palaeogeographical research, based on geomorphological and sedimentological analyses
and supported by radiocarbon dating, confirm the results of archaeological research conducted to date. The
alluvial fan in Borucin started to form at the second part of the Subboreal period during the Early Bronze Age at
the latest (3650 ± 70 BP; Fig. 5) and the maximum progradation of the fan (1670 ± 70 BP; Fig. 5) is linked to the
most intensive settlement in this area during the Late Roman Period. This coincides with intensive agricultural
activity during the period of the Przeworsk Culture. The next stage of fan progradation occurred in the Early
Middle Ages (after the “settlement depression” during the Migration Period, which lasted for two hundred
years). The much lower number of Slavs in the area and thus the decreased agricultural activity prevented the
silty clays washed from valley slopes from reaching the outer fan zone, which is indicated by its convex
longitudinal profile.
Research results show that the deforestation of the slopes of side valleys on the loess Głubczyce Plateau
significantly influenced the changes in slope processes, resulting e.g. in the washing of the soil cover formed on
the loess bedrock and the formation of alluvial fans. The phenomenon of the movement of erodable loess as a
result of the destruction of the plant cover through human agricultural activity is also typical for other loesscovered areas in Poland ( nieszko, 1995; Kruk et al., 1996; Klimek et al., 2001).
Therefore, it may be stated that from the Neolithic Age onwards, humans have become a force shaping
the slope and valley systems on the loess Głubczyce Plateau. Their influence on erosion processes was larger
than that of climate conditions. Climate, and especially humid periods, may only modify processes caused by
(deliberate or involuntary) human activity.
This research has been funded by grant No. KBN PB-508/NoZ/2003 from the State Committee for Scientific
Research.
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Fig.1. Location of the Głubczyce Plateau and geomorphic sketch map showing the study area
Fig. 2. The sample of erosional – denudational valley on the loess Głubczyce Plateau
Fig. 3. The sample of the alluvial fan which was deposited at the mouth of the small episodic drainage
valley
Fig. 4. Scheme of the geoarcheological approach to reconstruct of the alluvial fans
Fig. 5. Morphological sketch of the drainage basins of the analyzed alluvial fan