DEM based morphotectonical analysis of the Kisgyőr Basin (Bükk Mts – Hungary)

Acknowledgement: The described article/presentation/study was carried out as part of the EFOP-3.6.1-16-201600011 "Younger and Renewing University – Innovative Knowledge City – institutional development of the University of Miskolc aiming at intelligent specialisation" project implemented in the framework of the Szechenyi 2020 program. The realisation of this project is supported by the European Union, co-financed by the European Social Fund


Introduction
The Kisgyőr Basin is located in the south-eastern foreland of the Bükk Mountains. During our field investigations, we found that in the surroundings of this basin, many valleys are asymmetrical in nature, and stream captures, wind gaps were also identified in the study area. In previous researches (Less et al., 2005), the development of the basin was explained by fluvial erosion directed by differences in rock resistance to the erosion, although the latter geological mapping campaigns indicated faults in the area (Radócz, 1989;Lukács et al., 2010). Based on our field surveys, we assumed that the valley asymmetry and the presence of such landforms cannot be explained simply by differences in rock quality characterised by diverse petrological properties, and we hypothesised that structural geological processes also played a role in the surface development of the area.
The aim of our work was to prove this hypothesis. For this purpose, we performed swath profile analysis on a digital elevation model. We also analysed the elevation of the hilltops covered by the rocks of the Tar Dacite Tuff Formation, which rock type has a decisive role in the landscape evolution of the area. Borehole data was also examined to investigate tectonic movements.

Location
Based on the official Hungarian landscape classification, the research area (291 km 2 ) is situated in the south-eastern part of the Bükk Region, microregion group of the North-Hungarian Mountain Range region (Csorba et al., 2018). The north-western part of the area (65 km 2 ) belongs to the mountainous microregion of South-Eastern Bükk (Hevesi, 2003), the south-eastern hills (226 km 2 ) are located on the eastern part of the Bükkalja foothills microregion (Csorba et al., 2018). The boundary of the microregions can be drawn along the fault lines between the Mesozoic and Cenozoic rock formations (Dobos, 2000). Two submontane basins can be found at this boundary with different geological features and geomorphological landscapes, the Kács Basin and the Kisgyőr Basin (Hevesi, 2003;Pecsmány, 2017). In our research, we focused mainly on the Kisgyőr Basin and its southern surroundings (Fig. 1).
The swath method horizontally expands the cross-section line into a rectangular swath. Based on their distance from the baseline of the swath, the pixels in the swath are grouped, and their statistical parameters (minimum, mean, maximum, quantiles) are calculated (Telbisz et al., , 2013. In our study, two swath profiles were made at all locations. One profile was made to analyse the terrain; the other was made for the remnant surfaces delineated by the method of Telbisz (2009). (The method of Telbisz (2009) is based on the determination of local maximum values of the digital terrain model.) Both profiles, since they were made in the same swath, were plotted on one diagram. In the case of the terrain, the minimum, maximum, and mean values were plotted by lines, while the remnant surfaces were visualised by dots (maximum values) (Fig. 3).

Result of swath profile analysis
In the Southern-Bükk area, the maximum elevation values of the delineated remnant surfaces show that the karstic peneplain gradually steps off towards the basin (Fig. 4). Based on the position of the remnant surfaces on the northern edge of the Kisgyőr Basin, the fault(zone) that was already explored during structural geological research can be designated (Less et al., 2005;Németh, 2005). The remnants of the upper pediment framing the northern edge of the basin can be "connected" by a straight line to other upper pediment segments located to the south of the basin, with the same slope angle (Fig. 5).
In the southern part of the basin at the tilted escarpment, another level can be designated, which cannot be paralleled with the remnant surfaces of the northern side with the same slope angle. The southern edge of the basin is bordered by an asymmetrically tilted cuesta. This is confirmed by field observations and the swath profile analysis (Fig.4, Fig 5). Due to the tilted position of the cuesta, we assume that the southern edge of the basin was also appointed by a fault. The remnant surfaces inside the basin may also have tilted along faults (Fig.  5), as they can be connected by a straight line. The faults can be the synthetic faults of the main fault tilting the cuesta. These are inclined almost in the same direction as the main fault in the line of the cuesta.
The fault on the southern edge of the basin also causes the asymmetric nature of the Tarizsa Valley (Fig. 6). Less et al. (2005) assumed a fault north of this valley. However, based on the morphology of this area, the fault might be in the valley. It cannot be ruled out that these are two different faults; this could only be verified by geophysical measurements.
Based on field trips, the valleys south of the basin were perceived as asymmetric. To prove this, we prepared a SSW-NNE (Fig. 7) and a WSW-ENE (Fig. 8) swath profile.
In the case of the Sály Creek, the asymmetrical character is caused by a fault, which has already been confirmed by seismic cross-sections (Pecsmány & Vágó, 2020). Figures 7 and 8 show that the Csincse Creek, leaving the basin, runs in an asymmetric valley. On the profile "D", the asymmetrical nature is less visible in the maximum elevation values. This may also be due to the orientation of the swath profile section. Based on the average values and the position of the remnant surfaces, we can conclude a significant fault displacement in the case of Csincse and Geszt Creeks.  The Kulcsárvölgy Creek runs in a valley performed by a structural element in the Kisgyőr Basin (Németh, 2005;Less et al., 2005). In the basin, probably a fault causes the asymmetrical nature of the valley (Fig. 7). Since this asymmetrical shape is retained, leaving the basin, we conclude that the structural element continues towards the SE direction. The stream flows in the tuff strip and to the south of it in a structurally preformed valley (Fig.  8).
Based on field surveys and the analysis of the digital elevation model, we identified stream captures in the research area (Fig. 3). Swath profile analysis was performed in the abandoned valleys of these stream captures. As a result of this examination, morphological levels were identified in two wind gaps, which were identified as rock terraces (Fig. 9).
In the abandoned valley of the Tatár-árok (Szentlászló Valley) (Fig. 9, profile "I") a morphological level can be identified, at the same height on both valley sides (Fig. 9, profile "I"). This abandoned valley is structurally preformed (Németh, 2005). Previously prepared geomorphological maps indicate several sinkholes in the valley (Hevesi & Záhorszki, 2000). It is probable that the Tatár-árok could originally run in the NW-SE direction towards the Kulcsárvölgy Creek. It probably belonged to the catchment area of the Kulcsárvölgy stream until the diversion. The cause of this capture was the lowering of the Sajó Valley and the rising of the Bükk Mountains, which caused intense headward erosion.
In the abandoned valley of the Csincse Creek (Fig. 9, profile "II"), a morphological level can be observed on the side of the Meleg Hill. On the side of Halomvár Hill, this level was destroyed by erosion. The stream capture was caused by a fault tilting the cuestas. In our previous research (Vágó, 2012), we observed a breakpoint on the longitudinal profile of Csincse Creek at the location of the stream capture; it also proves its existence.
In both cases, the relative height of the geomorphologic levels is 40-50 meters above the valley bottom. Based on the terrace morphological studies carried out in Bükkalja (Pinczés, 1957;Krolopp et al., 1989), these can be identified as remnants of the oldest terrace level (terrace level No. V), (Fig. 9).
The rectangular change of direction of the Csókás Valley in the northern part of the study area (Fig. 3) can also be interpreted as a stream capture. During our field trips, we identified several sinkholes in and around the wind gap, which show the former flow direction. Both the recent and former valleys of the Csókás Creek are structurally preformed (Fig. 2). The formation of the stream capture was probably caused by the downfaulting of the current valley of the Csókás Creek and the backward erosion of a side valley.

Analysis of height conditions of Tar Dacite Tuff Formation by histogram.
The histogram is the distribution of the elevation values of the digital elevation model. Based on the height distribution of the DEM, conclusions can be drawn regarding erosion surface development. By examining the elevation of each rock types, we can infer not only the erosional surface development but also structural processes in the case of rocks with the same properties (Zámolyi et al., 2010), since the different elevation of areas with the same lithological features may indicate structural movements.
We successfully applied the DEM based histogram analysis in our studies for the examination of Bükkalja's pediment levels (Vágó & Hegedűs, 2011) and its basins (Pecsmány, 2017).
In this investigation, the DEM based histogram analysis was performed only for the area of the middlevolcanic complex (Tar Dacite Tuff Formation) because this formation is located in the NE-SW direction in one strip, and due to its petrological features (most of these rocks are welded ignimbrites) it is more resistant to erosion, and based on geological maps (Gyalog & Síkhegyi, 2005) it covers the hilltops of the interfluves (Fig.  10). Analysing the histogram of the rock types, 6 elevation classes can be assigned. Several breakpoints can be observed on the histogram showing the elevation of locations covered by this lithological unit. Based on these breakpoints, we reclassified the DEM. It can be clearly seen (Fig. 10) that the ignimbrite patches on both sides of the main valleys are located at different elevations above sea level. As it is the same rock type, the asymmetric nature cannot be explained by denudation processes. This analysis also confirms our finding mentioned in the previous chapter that the main valleys running in NE -SW direction are structurally preformed.
The valley of Nyék Creek is asymmetrical in the examined section. Unfortunately, we did not have enough borehole data in the upper and middle sections of the catchment to be able to examine the creek along a section. According to the data, in the M-7 borehole deepened to the north of the catchment, the lower rhyolite tuff was crossed at ~63 m, while in the borehole Nya-1, which was deepened within the catchment, it was crossed at ~ -381 m. There is no direct evidence for the structural nature of the stream at this upper section. However, on the basis of morphology, its preformation cannot be ruled out.
In the boreholes deepened on the valley bottom of the southern section of the Kulcsárvölgy Creek and on the top of its eastern interfluve, the position of the Pannonian sediments reveals significant structural movements. The floor level of the Nagyalföld Variegated Clay Formation is located at ~63 m in the Em-71 borehole deepened on the western interfluve, while it is at ~51 m in the Em-17 borehole on the valley bottom. In the Em-70 borehole deepened on the eastern interfluve of the creek, this level is located at ~112 m. These differences indicate the downfaulting of the valley and the uplift of its eastern interfluve, which confirms the structural preformation of the valley of Kulcsárvölgy Creek. The uplift of this eastern interfluve hill is also proved by the floor level of the Bükkalja Lignit Formation. This level was reached at ~93 m in the Em-70 borehole, but in the H-5 borehole deepened in the Csincse Valley, it was found at ~ -24 m. It is likely that this movement forced the Kulcsárvölgy and Nyék Creeks to merge. Based on these, we proved the preformation of the lower section of the Nyék Creek.
The position of the Pannonian sediments refutes the structural preformation of the section of the Csincse Creek south of the tuff strip. However, the valley asymmetry here points to a special situation. In the H-5 borehole deepened in the valley of Csincse Creek, the foot of the Nagyalföld Variegated Clay Formation is located higher than in the adjacent H-2 and Bg-2 boreholes. It is likely that the fault zone running in the Csincse Valley may have been active for a long period of time. We assume that before the deposition of the Nagyalföld Variegated Clay Formation, a narrowing lateral displacement took place in this area. As a result of this movement, the core of the zone emerged (Fig. 11 B). Presumably, the H-5 borehole was deepened into this upthrow block, which may explain the difference in elevation of floor level. After this uplift, a sedimentation phase took place again, which resulted in the formation of the Nagyalföld Variegated Clay Formation. Following the sedimentation, the fault zone reactivated, but probably with different kinematics. This structural element may have caused the preformation of the valley.
The valley of the Geszt Creek is also asymmetrical. In the Bg-4 borehole deepened on the western interfluve hill of the stream, the floor of the Nagyalföld Variegated Clay Formation is located much higher than in the Bg-2 borehole deepened at the valley floor. This significant difference cannot be explained by purely denudation. It was probably caused by tectonic movements; however, the proof of this hypothesis requires further investigations.  (Twiss & Moores, 1992).

Conclusion
Based on swath profile analysis performed in the research area, the Kisgyőr Basin can be interpreted as a structural basin. Its northern edge is bounded by a fault zone, separating the Southern-Bükk area from the Bükkalja. The southern edge of the basin is a structurally tilted escarpment (cuesta). The fault which caused this tilting can also be followed in the Tarizsa Valley, and we assume that this fault can be traced along the northern edge of the central-volcanic complex. The valley sections of Csincse, Kulcsárvölgy and Geszt Creeks leaving the volcanic strip are structurally preformed. It was proven by the existence of asymmetric valleys identified by the swath profile analysis and by the different elevations of areas covered by the welded ignimbrites of Tar Dacite Tuff Formation used as an indicator of tectonic movements.
We proved the existence of 4 hypothesised stream captures in the abandoned valleys of the Csincse Creek, the Tatar-árok, Geszt Creek and Csókás Valley. We also managed to identify a morphological level (rock terrace) along the Csincse Creek and Tatar-árok by swath profile analysis. The formation of these landforms can be explained by structural processes.
Based on our examinations, it can be concluded that the structural development of the region strongly influenced the formation of the landforms of the research area.