Article The Evolution and Development of Solution Dolines with Horizontal Growth and the Processes of Their Floors: A Case Study on the Plate-Shaped Dolines of the Bükk Mountains, Aggtelek Karst and Pádis Plateau Márton Veress Department of Geography, Savaria University Centre, Eötvös Lóránd University, 9700 Szombathely, Hungary; [email protected] Received: 3 September 2020; Accepted: 29 September 2020; Published: 8 October 2020 Abstract: This study investigated the evolution and development of plate-shaped dolines (depressions with a large diameter, small depth and plain floor) within the framework of a case study. For the determination of their morphological characteristics, the morphological parameters of 16 dolines were measured and calculated (their average values were compared to the parameter average values of the dolines of other doline types). Based on the data from the vertical electrical sounding measurements, the superficial deposit and the morphology of the bedrock of six dolines were studied. It can be stated that the plate-shaped dolines increased in size by widening. They were formed at sites where the water drainage and material transport capacities of the epikarst of the bedrock ceased on doline floors, while the drainage and material transport took place at the margin of the dolines. Their genetic varieties were plate-shaped dolines with a karren, plate-shaped dolines with a drawdown doline, plate-shaped dolines with a subsidence doline, plate-shaped dolines without a drawdown doline and plate-shaped dolines with a partial doline. Keywords: epikarst; plate-shaped doline; doline morphometry; superficial deposit of doline floor 1. Introduction The aim of this study was to investigate and describe the characteristics, the evolution and the development of plate-shaped dolines of some study areas, the karstic features of their floors and the effects of these dolines on the geomorphic evolution of karsts. The study of dolines of various types is an important area of karst research [1–9]. A variety of solution dolines is the plate-shaped doline, which is a flat-floored depression with a large diameter (80–200 m), according to Hevesi [10,11]. These dolines are widespread in the Bükk Mountains on Aggtelek Karst (Hungary) and on the Pádis Plateau (Apuseni Mountains, Romania), but they also occur in some karst areas of the Alps (for example the Asiago Plateau, Italy) and probably in other karst areas as well. The nature of their slopes changes during their growth. Primarily taking into consideration the lengths of the concave and convex slope sections compared to each other, the dolines may be categorised into four kinds [1]: doline widening at rim and base (the concave slope section is dominant), doline widening at the base (the concave slope section is dominant but the slope becomes steep during its development and the floor becomes plain), doline widening at the rim (the convex slope becomes longer compared to the concave slope, while the slope becomes gentle) and doline deepening but not widening (the length of the convex slope section increases compared to the concave section, while the slope becomes increasingly steeper). Earth 2020, 1, 49–74; doi:10.3390/earth1010005 www.mdpi.com/journal/earth Earth 2020, 1 50 According to their development, solution dolines may be categorised into either point recharge dolines, drawdown dolines or inception dolines [12]. Point recharge dolines develop in valleys inherited by the karst. Surface water inflow takes place in the developing proto caves, ensuring the transportation of both the surface water and the dissolved material, and thus, the deepening of the depression [12–15]. Inception dolines are formed where there is an impermeable bed close to the surface. Where the water gets through this, both the drainage and the transmission of the dissolved material will be concentrated. Therefore, depression formation takes place above this site [12]. Drawdown dolines develop during feedback processes [15,16]. In the epikarst, where drainage is accelerated, passages develop faster along the fractures (in the case of a subsoil karren [17,18]. The faster drainage that also takes place above these sites results in more intensive dissolution, and thus, the deepening of the developing depression will also be faster towards the increasingly more developed passages. Even more water arrives at the deepest point of the depression, which enhances the dissolution here, as well as the increase of secondary porosity in the epikarst. The growth of the secondary porosity of the epikarst makes the flow more intensive, which also contributes to the increase of dissolution at the deeper section of the doline and below it in the epikarst. The outlined development results in the fact that the doline floor (and the bedrock at the floor) will be increasingly deeper towards the center of the doline. On a glaciokarst, different varieties of solution dolines are distinguished: giant solution doline (paleodoline), small-sized (recent) solution doline and a schachtdoline (a depression with a steep slope and flat floor that is small-sized and with a diameter and depth of some meters), but subsidence dolines and buried dolines also occur [5,19–23]. The floor of various solution dolines may be covered not only by soil in large and small thicknesses but by nonkarstic cover too. In this case, covered (concealed) karstic dolines (subsidence dolines) are often formed on the cover [5,14,20,23,24]. 2. Methods Dolines have measurable and calculable parameters that were studied by several researchers [1,25–35]. Bondesan et al. [30] distinguished 65 kinds of dolines. The size, morphology, occurrence, distribution, orientation and pattern of dolines can be described using these parameters (for instance, density, change in density, the closest neighbour). Measurable parameters can be determined using field measurements, maps or aerial photographs. In this study, such parameters were used, created (since plate-shaped dolines have partly different morphological characteristics from other dolines) and calculated, which are methods that are suitable for characterising and describing plate-shaped dolines, as well as comparing dolines belonging to other doline types. Geophysical methods are used to determine the composition of doline cover and to investigate their structure and morphology of their bedrock [24,36–40]. Our methods are described below: On the Aggtelek karst in the Bükk Mountains on the Pádis and Asiago Plateaus, which were identified as plate-shaped dolines during field studies. These dolines were chosen and their topographic maps were created, where the maps were drawn at 1:250 or 1:500 scales and the contour lines were constructed at every 0.5 m or 1.0 m. To determine the morphological characteristics of the plate-shaped dolines, the following parameters were measured and plotted on the topographic maps (Figure1): the longitudinal diameter of the doline (dd), the longitudinal expansion of the plain doline floor (bd), the largest elevation difference of the bottom (bed) and the length of its slope (sl). Both the depth and the slope length were given and compared to the mound next to the doline and also to the margin of the doline. In the case of a doline where there was a karst saddle, the contact between the floor and the slope was given by the contour line that left the doline at the karst saddle. In the lack of a karst saddle, the contour line was determined by the line from where the contour line density is smaller towards the centre of the doline than towards the margin. The degree of the lack of the side slope was determined by the size of the angle at the centre of the floor (α). The centre point was taken at the cross point of the longest and Earth 2020, 1 51 Earth 2020, 1, FOR PEER REVIEW 3 of 26 the shortest floor diameters that were perpendicular to each other. Where the side slopes were absent the longest and the shortest floor diameters that were perpendicular to each other. Where the side at several sites, the angles situated at the centre point were summed (Sα). slopes were absent at several sites, the angles situated at the centre point were summed (Σα). Figure 1. Some morphometric parameters of a plate-shaped doline: (a) plan view and (b) lateral view. Figure 1. Some morphometric parameters of a plate-shaped doline: (a) plan view and (b) lateral Legend:view. Legend: (1) limestone, (1) limestone, (2) sediment (2) sediment of a doline of a dolinefloor, (3) floor, dividing (3) dividing wall surrounding wall surrounding the doline the doline floor, (4)floor, ridge (4) ridgeof the ofdividing the dividing wall (margin wall (margin of the ofdoline) the doline),, (5) inner (5) innerside slope side slopeof theof dividing the dividing wall (side wall slope of the doline), (6) floor of the doline, (7) karst saddle, where bd—the longer diameter of the (side slope of the doline), (6) floor of the doline, (7) karst saddle, where bd—the longer diameter of doline floor, pd—diameter that is perpendicular to the longest diameter, dc—centre of the doline floor, the doline floor, pd—diameter that is perpendicular to the longest diameter, dc—centre of the doline α1 and α2—angles expressing the expansion of the karst saddle, ddf—depth of the doline floor floor, α1 and α2—angles expressing the expansion of the karst saddle, ddf—depth of the doline floor (apparent depth), dfb—depth of the doline (actual depth), Sl—length of the side slope of the doline, (apparent depth), dfb—depth of the doline (actual depth), Sl—length of the side slope of the doline, dd—the diameter of the doline along the longer axis, bed—the largest elevation difference of the floor, dd—the diameter of the doline along the longer axis, bed—the largest elevation difference of the floor, tsd—thickness of the superficial deposit, Vs—apparent shape, As—actual shape, P—plate shapedness tsd—thickness of the superficial deposit, Vs—apparent shape, As—actual shape, P—plate shapedness value of the doline, Be—expansion of doline floor and Se—expansion of the side slope of the doline.