Dependence of Gully Networks on Faults and Lineaments Networks

Open Geosci. 2017; 9:101–113

Research Article Open Access

Libor Burian*, Michal Šujan, Miloš Stankoviansky, Jakub Šilhavý, and Ashie Okai Dependence of Gully Networks on Faults and Lineaments Networks, Case Study from Hronska Pahorkatina Hill Land DOI 10.1515/geo-2017-0008 1 Introduction Received August 10, 2015; accepted January 19, 2017

Abstract: In this paper we analyse the dependence be- The process of gully erosion in the area of Central Europe tween gully networks, networks of valleys and faults in the has ceased. There are numerous examples from the past area of Hronska pahorkatina Hill Land. The work is based indicating that the formation of gully network resulted in on the analysis of directions and densities of these three the abandonment of land by farmers. Nowadays, this hap- networks in the study area and subunits of lower hierar- pens rarely due to the ability of man to change the land chical level. The coincidence of all three networks is rare. surface with heavy machinery. At the moment, we have al- This scenario occurs when networks of gullies are situated most no evidence of the formation of new gullies or gully on the bottom of shallow valleys which are conditioned by systems in the area of Central Europe. However, there are the presence of faults. More often scenario is only the co- still numerous examples of the reactivation of this pro- incidence of network of valleys and gullies. The last sce- cess in existing gullies. The process of gully formation ac- nario appears in areas with low fault density. The most tivates secondary processes, which are often more danger- specific scenario is perpendicularity between network of ous than the erosion process itself. For example, the for- gullies and network of gullies. Gullies are situated on steep mation of muddy floods or flash flood is often directly con- slopes of incised valleys in this case. The last scenario ap- nected to areas with permanent gullies. This is one of the pears the most frequently and was also proven by findings reasons why it is necessary to identify and study factors from other studies. We propose three comprehensive ex- influencing formation and reactivation of gullies. planations of the possible dependence between network The exact set of drivers of gullies is still question- of gullies and faults. We also suggest the draft of the pos- able although there is a wide range of hypotheses which sible dependence between network of valleys and gullies. help to identify the majority of these drivers. One uncon- firmed hypothesis assumes the dependence of gully networks on networks of tectonic faults (dislocations). The verification or falsification of this hypothesis through the analysis of network of gullies, valleys and faults on Hron- ska pahorkatina Hill Land is the primary goal of this paper. This hypothesis was verified mainly through the analysis of distribution of density and directions of networks of gul- *Corresponding Author: Libor Burian: Department of Physical lies, valleys and faults. Geography and Geoecology, Faculty of Natural Sciences, Comenius University, 842 16, Bratislava IV, Slovakia, E-mail: [email protected] Michal Šujan: Department of Geology and Palaeontology, Faculty 2 State of art of Natural Sciences, Comenius University, 842 16, Bratislava IV, Slovakia Miloš Stankoviansky: Department of Physical Geography and The agent of the gully erosion process is the ephemeral Geoecology, Faculty of Natural Sciences, Comenius University, 842 flow of running water [1]. The main cause of gully forma- 16, Bratislava IV, Slovakia tion is too high overland flow, which may be caused by ei- Jakub Šilhavý: Department of Mathematics, Faculty of Applied Sci- ther climatic conditions or alteration in land use. Accord- ences, University of West Bohemia, 301 00, Plzeň, Czech Republic ing to various criteria, it is possible to distinguish valley Ashie Okai: Institute for Ecosystem Research, Group of Ecosys- floor from valley side gullies, continuous and discontinu- tem Research and Archaeology, Christian-Albrechts Universität, Olshausenstrasse 40, D-24098 Kiel, Germany ous gullies, ephemeral and permanent gullies, linear (par-

© 2017 Libor Burian et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 102 Ë Libor Burian et al. allel) and dendritic gullies (the last ones occurring mostly tioned determining factors (besides topographical ones, in valley heads) see Fig. 1. there are many artificial linear landscape features) were important also in the formation of historical permanent gullies, which could have originally started as ephemeral gullies. The study of permanent gullies in the Myjava Hill Land, Slovakia, revealed that the majority of gullies linked to linear artificial landscape features typical for the pre-collectivization arable land pattern, mostly to access roads, trails and field boundaries. One of the most curi- ous artificial predispositions for gully formation is plough furrow which has been used as an indicator of cadastral Figure 1: Examples of types of gullies according to: a, position b, boundaries. In several cases, gully position can be linked area of cross-section c, presence of accumulation form d, position to pastures in the original land use mosaic [16]. of other gullies. The abovementioned overview would suggest that gullies have been formed only since the beginning of human pressure due to climatic fluctuations and that their spatial The main trigger of the gully erosion process is still un- distribution is controlled by land use pattern and topog- clear. However, it can be assumed that land use changes raphy. However, there are hypotheses that many large gul- and climatic changes are the main triggers [2]. One group lies were formed under forest during the wetter Atlanticum of authors prefers only one of these causes whereas the (7800–5000 BP) [23] or were formed under periglacial con- other group considers both causes as equivalent. More- ditions and were cut into the loess blanket long before over, the influence of land use and climatic changes a protective vegetation cover was established during the is changing in time and space. According to Bučko, Holocene [24], as shown on gullies in the forested area of Mazúrová [3] and Klimaszewski [4], a genesis and evolu- Central Belgium. tion of gullies was associated with extreme rainfall and A different point of view was presented by Knox [25]. sudden snowmelt. The effect of these events was accel- According to his work, at long-time scale, gully develop- erated by a deforestation and agricultural utilization of ment was a complex response to a set of driving factors, a land with deep and soft regolith. An extensive synopsis including tectonic movements. In recent years, some ef- of literature on historical gully erosion is provided in arti- forts have been made to establish a long-term chronology cles by Vanwalleghem et al. [5] and Dotterweich [6, 7]. of gully erosion events for the understanding of both pa- Several stages of evolution of gully networks can be leoenvironmental dynamics and the present-day state of delineated on the basis of land use changes or climatic geomorphic systems under the influence of multiple natu- fluctuations, such as frequently occurring heavy rainfall ral and anthropogenic factors (e.g. [26]). Huang et al. [27] events during the Little Ice Age (LIA) (e.g. [6, 8–12]). There confirmed synchronous down-cutting of gullies and rivers are huge sets of studies suggesting that many permanent over the Loess Plateau, China, which implies that neotec- gullies have been formed since the beginnings of agricul- tonic uplift of the land mass was a major dynamic force ture (e.g. [8, 13–17]). According to Vanwalleghem et al. [5], causing gully incision. According to this study, a gully in- it was confirmed that the majority of gullies studied in Cen- cision occurred when large-scale monsoonal climatic shift tral and north-western Europe were formed between the met with coincided neotectonic uplift; it was dated to ca. 14th and 20th centuriey, with most of them being formed 600 ka, 240 ka, 125 ka and 11.5 ka. in the 20th century, after LIA. However, examples of older In literature coming from the former Czechoslovakia, gullies still exist, e. g. [18, 19]. The oldest gullies in Europe we can find only a handful of works that mention apos- are related to the Neolithic. Schmidtchen and Bork [20] sible relationship between gully formation and tectonics, dated charcoal at the bottom of an infilled gully. Sem- either directly or indirectly. According to Láznička [28], the mel [21] described a Neolithic fossilized gully, though with- creation of a dense network of gullies in the Jihlava River out chronometric dating. valley around Ivančice, Czech Republic, was caused by According to Poesen [22], current ephemeral gullies tectonic movements. He distinguishes younger (historical are formed not only in natural drainage lines (thalwegs and recent) valley side gullies from older (prehistorical) of zero order basins or hollows), but also along linear valley floor gullies which have been formed in glacials and landscape elements (e.g. drill lines, dead furrows, head- interglacials. Harčár [29] states on the basis of research lands, parcel borders, access roads, etc). Most of the men- in the area of NW part of the Low Beskids, Slovakia, that Dependence of Gully Networks on Faults and Lineaments Networks Ë 103 tectonics, together with further geological factors, created primary conditions leading to gully formation. Hrašna and Liščák [30] found out that in the framework of an as- sessment of geodynamic phenomena in the Pokoradzká tabul’a Plateau, Slovakia, the occurrence of gullies is related to the tectonically deteriorated rocks and the orientation of gullies coincides with main tectonic directions in this area. In the area of Hronska pahorkatina Hill Land, several authors point to the relationship between tectonics and some topographical erosion linear features, though not gullies. It is the relationship between tectonics and dell- like landforms formed between the Latest Pleistocene age and Holocene age [31] and the relationship among tectonics and deflation troughs [32]. This paper presents the first integral analysis of the relationship between gully networks and tectonics in the area of Hronska pahorkatina Hill Land. Figure 2: Location of Hronska pahorkatina Hill Land in Slovakia and its division into smaller units.

3 Regional settings version (sensu [44]) and uplift, which was active during the Pleistocene. The Hronská pahorkatina Hill Land lies in the south- Elementary information about geomorphometric set- western part of Slovakia. It is the most extensive part of tings are contained in Fig. 3. the Danube Hill Land, representing the higher part of Danube Lowland and a transition zone between the West- ern Carpathians and the West-Pannonian Basin [33] or border area of Western Carpatians [34]. The location and division of the study area into units of lower hierarchical levels is described in Fig. 2. Basic information about characteristics of the study area is contained in Table 1. From among the five hilly subunits of the Danube Hill Land, Hronská pahorkatina Hill Land together with Ipel’ská pahorkatina Hill Land were uplifted more often and earlier [12] and tilted less [42] during the Quaternary Figure 3: Statistical distribution of morphometric variables calcu- than the remaining three parts. lated for the whole area of Hronská pahorkatina Hill Land. The reso- The bedrock of the area consists of continual Late lution of DEM is 10 m. Z – altitude, m. a. s. l.; GA – slope steepness, aspec Miocene succession represented by lacustrine Ivanka For- degree; t – aspect of the land surface, degree. mation, deltaic Beladice formation and alluvial Volkovce formation [43]. The bedrock in the dominant part of the Two local extremes of altitude can be found in Fig. 3 study area was formed by the Volkovce Formation with the (135 and 160 m. a. s. l.). These two peaks represent flat upper age limit around 6.0–7.0 Ma and consist of major areas in the western, eastern and southern margins of floodplain facies (clays and silts) and minor meandering the study area. The large area of flatlands also influences channel belt facies (sands). In the east, south and south- the statistical distribution of slope steepness. Thus, global east parts of the study area, extensive areas of Neogene for- maximum around zero is caused by the existence of large mations in the bedrock are covered by river terrace gravels areas of fluvial flatlands (see below). Two local maxima blanketed by the loess of variable thickness or locally by in the bar chart of aspect are linked to south-western and eolian sands. Quaternary strata are deposited discordantly north-eastern slopes. on older sequences due to the Pliocene basin margins in- 104 Ë Libor Burian et al.

Table 1: Basic information about natural conditions of Hronska pahorkatina Hill land.

location information Source north average altitude = 205 m. a. s. l., average slope steepness = Morphometry 3.3 degrees ASTER DEM [35] middle average altitude = 167 m. a. s. l., average slope steepness = 2.3 degrees south average altitude = 146 m. a. s. l., average slope steepness = 2.0 degrees north average annual temperature = 8°C, average annual precipita- Climatic conditions tion = 600 mm [36], [37] middle average annual temperature = 9°C, average annual precipitation = 575 mm south average annual temperature = 9°C, average annual precipitation = 525 mm north loamy, clayed-loamy Soil texture middle loamy [38] south sandy-loamy, loamy north sand covered by loess, loess, coluvial deposits Quaternary deposits middle sand covered by loess, loess [39] south sand covered by loess, loess north claystones, sandstones Rock types middle claystones, sandstones [40] south claystones, sandstones north non-irrigated arable land, broad-leaved forest, discontinuous Land cover urban fabric, industrial and commercial units [41] middle non-irrigated arable land, discontinuous urban fabric, broad leaved forests, land pricpally occupied by agriculture south non-irrigated arable land, discontinuous urban fabric, broad leaved forests

Two types of land surface can be distinguished in the just dells with an almost uniform NW-SE direction rep- area. The first category is represented by hilly land sur- resent the youngest (Würm-Holocene) tectonically con- face, which is prevalent. Altitudes for this type of land trolled landforms in the area. surface reach usually 200–320 m. a. s. l. and vertical dis- The second category is represented by gently inclined section is between 31 and 100 m. In comparison with sur- plain land surfaces of river terraces, covered by loess, rounding plains, it represents a system of moderately up- which is also visible in Fig. 3. Altitudes are ranging from lifted and vertically differentiated blocks. In general, the 115 to 200 m. a. s. l. and vertical dissection is up to 30 m. land surface is monotonous with broad, rounded to flat Characteristic landforms for hilly and plain land sur- ridges, bearing traces levelled in the Upper Pliocene [45, faces are marked by deflation troughs with NW-SE direc- 46]. Ridges are elongated mostly in NW-SE, N-S direction, tions [47, 48]. According to [49], their formation was influ- less in NE-SW and W-E direction. They are separated by enced besides deflation by tectonics. widely opened valleys. The shape and direction of valleys is controlled by tectonics and reflects evolution under periglacial conditions. A typical feature of valleys is prin- 4 Methods cipally their distinct slope asymmetry and right-angled texture. Steep slopes are usually free of the Quaternary Directions of networks of valleys, gullies and tectonic cover, while gentle slopes are covered by thick layers of faults for the Hronska pahorkatina Hill Land area are an- loess. Dells, gullies and cart-tracks are often situated on alyzed in this work. For the purpose of this work, a valley steep slopes (e. g. [46]). According to Králiková et al. [31], Dependence of Gully Networks on Faults and Lineaments Networks Ë 105 is defined as a depression form of a linear shape withor model (DEM) using v.surf.rst (tension = 40, smoothing = without a permanent river. A network of gullies was con- 0.1) interpolation algorithm. For the verification of results, structed based on Military topographic maps on a scale of axes of valleys were delineated in two ways: 1: 10 000 [50]. For purposes of advanced analyses, a sim- – Axes of valleys were manually digitalized by opera- ple gully was represented by an axis and systems of gul- tor on the basis of DEM. An example of the process lies (complex gully) were split into sets of simple gullies of valleys axes delineation is shown in Fig. 5. and each simple gully was again represented by an axis as – Axes of valleys were delineated on basis of DEM us- explained in Fig. 4. ing semiautomatic algorithm developed by Šilhavý et al. [52]. Firstly, the DEM is resampled from source raster in a specific resolution. The resolution of the DEM influences the level of detail of the extracted lines. Secondly, the set of variously illuminated and rotated hillshade rasters is computed from the input DEM. Lastly, the set of vector lines is generated from each hillshade using PCI Geomatica software. These sets are processed by hierarchical cluster line algorithm to one final representative set of lineaments which are classified as positive or negative in thefi- nal step of computation. Dataset of faults was adapted from the Geological map of Slovak Republic 1:50 000. Fault lines in these datasets Figure 4: Representations of simple and complex gullies used in consist of three geological maps on a scale of 1:50 000 [53– a b the database for analysis. ,– simple gully , – simple gully repre- 56]. Fault lines were delineated on the basis of datasets sented by axis of gully c, – complex gully split into sets of simple of boreholes and on the basis of land surface, mainly gullies d, – complex gully represented by axes of partial simple gullies. on shape of valleys in the area. Although normal faults were observed only on outcrop rupturing Late Miocene deposits [31], a widely accepted concept of tectonically predisposed asymmetric valleys (e.g. [31, 46, 57]) allows us to expect the activity of these faults during the Pleistocene. However, this approach means that distributions of valleys and faults are partially supported by the same data and their informative capability is similar. Higher density of faults is expected to occur in the highest lying areas, where Late Miocene deposits are exposed. The density of networks was computed using Line density algorithm (search radius 4 000 km2) incorporated into ArcGIS 10.0. One of the crucial questions in case of using networks is sufficient density of these networks. Where is thebor- der between sufficient and insufficient density of allthree types of networks? This border depends not only on value Figure 5: Dataset of axes of valleys used for analyses. of density of network but also on the number of features described by dataset. Therefore, an area with one long A network of valleys was created based on a digital el- fault line can have comparable density with an area with evation model of this area derived from the Topographic several shorter faults. However, it can be assumed that in- map of Slovakia 1: 10 000 [51]. Firstly, isolines of altitude formation value of second dataset is higher. Since there is were digitalized using ArcGIS 10.0. Secondly, dataset of no exact threshold between significant and insignificant isolines with additional information about altitude was networks threshold values were only estimated. imported into GRASS GIS 6.3.2. Lastly, imported dataset Datasets of axes of valleys gullies and faults were di- was used for the interpolation of new digital elevation vided into ten meters long segments (see Fig. 6). This ap- 106 Ë Libor Burian et al. proach has two advantages. First of all, concerning the length of axes, short axes are not as important as long axes; and secondly, the approximate length of axes in each direction can be computed by multiplying the total number of elements by ten. The directions of networks were presented by rose diagrams constructed using Golden Gra- pher 9. Constructed diagrams were bidirectional; hence each axis was computed twice in both directions. It is thus irrelevant, whether the axis of the gully is constructed in the upslope or downslope direction.

Figure 7: Comparison of methods for manual and semiautomatic delineation of axes of valleys in two areas. On the left are the results of manual delineation and on the right, the results of delineation by algorithm.

Figure 6: Example of splitting axes in gullies network. a,– representation of axes in dataset before split b, – representation of dataset after split.

5 Results

Dataset of valleys was delineated in two ways (see Method- ology). In Fig. 7 we can find a comparison of results between manual delineation and semiautomatic delineation of axes of valleys. Results of manual delineation are more precise and were used for further analyses. A representativeness of all three networks was analyzed using their densities in the first stage of research. It could be assumed that only areas with the sufficient density of networks can be taken into account in the next steps of research; however, it is problematic to set the exact value of threshold. The threshold has to be set on the basis of density of network in a combination with a total number of features described by this network (see Meth- ods). Maps of densities of networks are shown in Fig. 8. Density of all three networks is uncomparable because of their different average density. It is obvious that den- Figure 8: Density of valley, gully and fault networks in the Hronska sity of all three networks is varies in space. On one hand, pahorkatina Hill Land. there is a high density of valley network in the whole area, Dependence of Gully Networks on Faults and Lineaments Networks Ë 107 on the other, density of gully and fault networks reaches zero mainly in the southern part of the key site. Density of faults in Hurbanovské terasy, Chrbát, Strekovské terasy and Belianske kopce can be assumed as unrepresentative because we can find only one or two fault lines in every part, which cannot be considered as a sufficient occurrence. This fact has to be taken into account during the next steps of analysis. A most interesting phenomenon is the presence of areas with higher density of all three networks, which are situated in the northern and western part of Bešianska pahorkatina, as well as in Belianske kopce. Basic information about average densities of networks for each part of Hronska pahorkatina Hill Land is given in Table 2. The highest density of all three networks is located in some areas of Belianske kopce Hills, which is situated in the southern margin of the key site. This is one of the pre- viously mentioned areas with higher density of all three networks. The lowest density of all three networks is found in Búčske terasy part, next to Belianske kopce. While interpreting the information about density of networks, it is necessary to remember that parts of Hronska pahorkatina Hill Lands were delimited on the basis of topography and geological attributes of the area and density of networks is not homogenous in the whole area. Average densities Figure 9: of all three networks are distributed logically because we Orientation of axes of valleys in Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by are able to delineate majority of valleys and summarized multiplication of values by 10. lengths of their axes is the longest from all three types of networks. It can be assumed that the total density of faults is underestimated, due to the restricted ways for Bešianska pahorkatina where direction of network of val- their delineation and mapping. All the mentioned infor- leys and gullies is almost equal. In case of faults it is pos- mation about density of networks has to be taken into ac- sible to distinguish two major directions from one minor count for the verification of the representativeness of any direction. The minor direction of faults is perpendicular further experiments. to the major directions of valleys and gullies. In case of The main directions of all three types of networks were Búčske terasy only network of valleys and faults can be analyzed using non-weighted rose diagrams on basis of taken into account because network of gullies is unrepre- split (see Methodology) datasets of all three networks. The sentative. The major directions of both networks coincide. results for all three networks are showed in figures 9, 10 An absence of gullies is conditioned by topography of area and 11. because the slope steepness is not sufficient to create gully It has to be taken into account that there exists one network. There are several possible explanations of these major and several minor directions on rose diagrams. An coincidences (see Discussion). analysis has revealed identical major direction of networks In several parts of Hronska pahorkatina Hill Land, the of gullies and faults only in the subunit of Strekovské major direction of gully network was either perpendicu- terasy. However, the representativeness of fault network lar to the major direction of faults (e.g. Chrbát, Strekovské in this subunit is questionable. In case of subunits Chrbát terasy) or perpendicular to the major direction of valleys and Belianske kopce, the differences between major and (e.g. Strekovské terasy, Chrbát). Perpendicularity of net- minor directions of all three systems are minimal. In case works can be explained by the indirect correlation be- of these two networks the major direction of one network tween network of gullies and network of faults (see Dis- and the first minor direction of another network (e.g. ma- cussion). On basis of the results, we are not able to define jor direction of valleys and faults and minor direction of the general direction of networks for Hronska pahorkatina gullies) coincide. The specific situation occurs in case of Hill Land as one unit. 108 Ë Libor Burian et al.

Table 2: Basic information about density of networks on subunits of Hronska pahorkatina Hill Land.

valleys density/km·km−2/ gullies density/km·km−2/ faults density/km·km−2/ Belianske kopce 1.68 0.12 0.36 Chrbát 1.66 0.08 0.14 Hurbanovské terasy 1.08 0.02 0.16 Hronská tabul’a 1.51 0.01 0.19 Bešianska pahorkatina 1.85 0.11 0.24 Búčske terasy 0.97 0.03 0.14 Strekovské terasy 2.00 0.02 0.16 average 1.54 0.06 0.20

Figure 11: Orientation of axes of faults on Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by multiplying values by 10.

Figure 10: Orientation of axes of gullies on Hronska pahorkatina Hill Land. Absolute length of axes in meters can be computed by out any coincidence or to be unrepresentative (e.g. multiplying values by 10. Hronská tabul’a). – No coincidence of networks. This case is typical mainly for the areas without sufficient density of all In fact, three main situations can occur: three networks (e.g. Hrušovské terasy). – A coincidence of all three networks, namely a coincidence of network of valleys, faults and gullies (e.g. Belianske kopce). This case is connected with the ar- 6 Discussion eas with the highest densities of all three networks. – A coincidence of two networks (e.g. Bešianska pa- Higher preciseness of manual delineation of valleys axes horkatina). A main direction of the last network is caused by an ability of the operator to change the hi- can be situated perpendicularly (e.g. Chrbát), with- Dependence of Gully Networks on Faults and Lineaments Networks Ë 109 erarchy level of observation during the process of delin- The direction of slope lines of these slopes is perpendicu- eation. The semiautomatic method, however, has this hi- lar to direction of fault lines. Slopes are also characteristic erarchy level defined by hard during computation of algo- by high slope values. A linearity of slopes is changed to a rithm. The ability of the operator to change the hierarchy set of convex spurs and shallow concave valleys. A trans- level of the analysis does not mean that operator changed formation from linear slope is mainly caused by a set of ex- the scale of the base map. He only has the ability to recog- ogenic processes. Thalwegs of these valleys generally cre- nize several hierarchical levels at the same time. ate linear network. The presence of valleys increases val- While interpreting the results, it has to be taken into ues of a catchment area. The combination of high values of account that endogenous processes can explain only a mi- catchment area with high values of slope steepness creates nor set of gullies, the majority is still explainable by com- conditions suitable for gully formation. These gullies are in bination of erosion processes and suitable predisposition. a fact situated on topographic predispositions (see Fig. 13). It cannot be assumed that the direct relationship be- The less often case is a formation of dendritic fault network tween tectonic processes and gully networks is often. Not at the end of the fault valley. The frequency of linear gullies every gully or gully system is conditioned by a fault be- is usually incomparably higher than the frequency of den- low it. These cases are rare, however, possible. It can be dritic gullies because of the presence of waste areas with assumed that under certain conditions, the presence of slopes and relatively small area where valleys end. It can a strike-slip fault line can accelerate the interception of be assumed that gully network in this scenario is perpen- ground water. This, in combination with the disruption of dicular to network of faults and valleys. material due to fault movement, produces conditions that are favourable for the development of gully (see Fig. 12). In fact, the strike-slip fault line only increases the erodi- bility of the material and decreases its persistence. It can be assumed that such a case is rare and is probable under laboratory conditions. In this case, there is no coincidence between gully, fault and valleys networks on larger scales. Moreover, no neotectonic strike-slip faults were ever ob- Figure 13: served in the area. Example of development of gully on linear slope which is determined by the presence of fault line. a, initial block disrupted by fault b, presence of valley conditioned by fault with developed parallel gully network.

The second alternative is rarer. The main driver is tectonic regime of river valley mostly without presence of a fault line. The sedimentary cascade is disrupted by tectonic up-lift or down-lift. The disruption causes the development of river terraces (erosion or accumulation). The presence of gullies on these terraces is conditioned by Figure 12: An example of development of gully on a strike-slip fault line. a, presence of fault line b, disrupted material due to move- low resistivity of terrace material against erosion. Thus ment of blocks and acceleration of interception of ground water c, not all river terraces have to be covered by dense network development of gully on attenuated material. of gullies. A good example of this indirect dependence is the situation in the Chinese loess plateau around Yellow river [27]. Gully networks were formed on river terraces It can be assumed that an indirect dependence be- which had been produced by episodic incision of the Yel- tween tectonic processes and gully networks is more com- low river (see Fig. 14). Such scenario is highly improbable mon in the landscape. In this case, mentioned tectonic in the study area due to insufficient thickness of erodible processes are just influencing the land surface, in terms material on these terraces. In this case, it can be assumed of changing its shape and creating conditions, which are that there is no dependence between direction of faults suitable for formation of gullies. There are mainly two al- and gullies or valleys. However, it can be assumed, that ternatives to such a situation. direction of axes of valleys and gullies is perpendicular. It was proven in many cases that the presence of fault During our research we are not considering this scenario. lines conditions the development of steep linear slopes. 110 Ë Libor Burian et al.

section of land surface, which is connected with higher ratio of erosion. Exposure of older - Late Miocene deposits instead of Quaternary accumulations is the result of erosion processes. These strata of the Volkovce Formation exhibit much higher rupture by faults compared to Quaternary deposits by means of outcrop study [31]. The observed distribution allows us to assume that higher density of gul- Figure 14: Example of development of gully on slopes produced on lies occur in areas consisting of older sediments with risers of fluvial terraces. a,initial incision of river caused by tectonic uplift b, developed system of river terraces with system of gullies much higher density of fault rupture. It may also be developed on risers of fluvial terraces. connected directly to higher position of the areas causing higher intensity of erosion, without a direct relationship to fault orientation and rupture. An interpretation of dependence of gully network on – Study of Quaternary river terraces morphometry [58] network of valleys is comparably complex. Under specific documented that these fluvial accumulations show conditions a difference between direction of gully and val- no apparent tectonic displacement in the study area ley systems can be used for analysis of type of triggers with an exception of the Búčske terasy area, where of gully formation. Coincidence of direction between net- a normal fault pattern oriented in the NW-SE di- works could signalize, that gullies are strictly topographi- rection was observed and with offsets at 10–15 m cally conditioned. Thus a direction of thalweg (valley) co- scale. These faults are most probably continuous to incides with a direction of gully. This presumption works the Chrbát subunit and predisposed valleys, where just in case of shallow valleys because in case of deep val- a perpendicular orientation of gullies appears. This leys there are permanent flows. However, in case of deep could be therefore an example of gullies caused by incised valleys a situation often occurs that gullies are lo- differential vertical tectonic movements of blocky cated on valley slopes. Thus the direction of thalweg (val- patterned area, as described in the study by Huang ley) is perpendicular to a direction of gully The other differ- et al. [27]. ences between networks increase possibility that gully formation is triggered by set of different drivers, than only by Mentioned arguments are only indirect thus the presented topography (e.g. artificial structures, footpaths etc.). This scenario is only the most probable, not confirmed. How- relation will appear only in case of datasets with sufficient ever, it is highly improbable that the majority of gullies is accuracy (grid spacing, inaccuracy of elevation etc.) situated on faults and there is no evidence (direct or in- Since the existence of general direction for the three direct) that there is an increased spatial variability of up- networks in Hronska pahorkatina Hill Land cannot be as- lift in the Hronska pahorkatina Hill Land area and its sur- sumed, the area at the level of lower hierarchical units has rounding. to be analyzed [33]. It has to be taken into account that bor- The theory of formation of valley networks by eolian ders of these geomorphologic units have been delineated processes in the Panonian basin has a long tradition be- only on the basis of geomorphological and geological at- ginning in early 20th century [59, 60]. Recently, it was tributes. On the basis of achieved results, we are able to demonstrated that an extensive valley-ridge system in the suppose that mainly indirect dependence exist between region was formed by deflation and eolian erosion [61– gully networks and tectonic processes. The most probable 63]. Therefore eolian processes have to be taken into ac- scenario in this area is a development of linear gully net- count while analyzing land surface in the southern part works on fault slopes, which are covered by loess with dif- of the Hronska pahorkatina Hill Land. According to the ferent thickness. This scenario is proven by: mentioned theory of valleys and ridges (yardangs) formation, the area of the Panonian basin is characterized by – The perpendicular direction of faults (valleys) net- prevailing winds which blow from north-north-west direc- works and networks of gullies in some geomorpho- tion. This theory is in discordance with the commonly ac- logical units in Hronska pahorkatina Hill Land. Not cepted theory of development of valleys in the area of the only same direction of major directions, however, Danubian basin on the basis of tectonic predispositions. identical direction of major and minor directions of The theory about eolian formation of land surface was networks. proven in the area of Hungary mainly by geophysical mea- – The higher density of faults in all three networks surements, structural analysis of the area and orientation in several areas could be explained by higher dis- Dependence of Gully Networks on Faults and Lineaments Networks Ë 111 of ventifacts located in the northern and western parts of rection of each segment of network against the closest ele- Hungary. It can be assumed that similar situation can be ment, only major directions of both networks. Thus the set described also in southern part of Hronska Pahorkatina of gullies can be parallel to the set of valleys of zero order Hill Land. This assumption can be proven by: and perpendicular to the set of larger valleys at the same time. The methodology is mostly restricted by the fact that – Similar directions of valley networks in Chrbát, we do not take into account artificial predispositions as Hurbanovské terasy, Strekovské terasy and Búčske one of limiting factors. On the basis of works (e.g. [16]) we terasy with supposed directions of prevailing winds. are assume that linear predispositions in the area of Cen- – Similar directions of valley networks in the southern tral Europe are crucial drivers for gully formation. part of Hronska pahorkatina Hill Land and yardangs All presented experiments were done as best as pos- situated in the north-western part of Hungary. sible on the basis of best available information, however, – Asymmetry of distribution of loess deposits mainly the level of uncertainty in the case of network of faults is on the north-western side of valleys, which is ex- high. pected to be leeward. This could be an alternative explanation for valley asymmetry described by Krá- liková et al. [31]. Only sufficiently extensive dataset of ventifact from the 7 Conclusion area of Hronska pahorkatina Hill Land can be considered as direct evidence which can be used for verification or The main goal of the proposed work was to find the possi- falsification of this assumption. However, there is nooc- ble relation between networks of valleys, gullies and faults currence of sufficiently resistant rocks in the study area, in the area of Hronska pahorkatina Hill land. This hypoth- which may prevent ventifacts from being eroded. esis was tested using methodology of analysis of directions Several limitations and restrictions of the used of networks. The coincidence of all three networks was methodology were indentified during the experiments. rare. It was proved only in a part of Belianske kopce. More The quality of presented results is decreased by any often scenario was the coincidence of network of valleys of these limitations; however, using information and and gullies. Directions of network of faults were different datasets currently available, we are not able to achieve or perpendicular. In many areas a density and number of better results. The majority of the limitations is connected features were insufficient for the other types of analyses with the availability of dataset of faults for the studied (e.g. gullies on Hronská tabul’a, faults on Búčske terasy). area. Limitations are: One of the most surprising facts is that of perpendicular direction of gullies and valleys. There are two possible ex- – Map of fault lines was mainly in the northern part planations for this fact. The first one is that gullies are sit- of the study area derived from the shape of valleys. uated on slopes of large valleys, which are almost perpen- Therefore, it can be assumed that the information dicular against direction of the main thalweg. Presence of from dataset of axes of valleys and dataset of fault these valleys can be conditioned by fault network in many lines are similar. cases. The second explanation is that gully formation was – As stated before, faults observed on outcrops rup- driven by another set of factors (e.g. foot paths or borders ture only Late Miocene deposits were probably ac- of fields). The most probable scenario is the development tive mainly during the Pliocene. Due to moderate to- of linear gully networks on fault slopes, which are covered pography, there is a lack of outcrops and their in- by loess with different thickness. This fact was proven by formative value may be overestimated. It should be results of analysis of networks in combination with geolog- considered that tectonic rupture is much higher in ical findings. All interpretations have to take into account pre-Quaternary sedimentary units and distribution a high level of uncertainty. of fault rupture is dependent on the stratigraphic position (and age) of the unit forming the immediate Acknowledgement: This work was supported by the substrate. Slovak Research and Development Agency under the In this text we are present only one possible explanation contract No. APVV-0625-11, APVV-0099-11, APVV-0315-12 of relationships between all three networks. However due and by Grant from Comenius University UK/451/2015, to used methodology, we have to take into account several UK/503/2015. restrictions and consequently several other scenarios. The main restriction lies in the fact that we do not analyze di- 112 Ë Libor Burian et al.

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