Detecting the Occurrence and Explaining the Origin of Lithologic Discontinuities in Low-Mountain Soils: An Example from the Carpathians, Southern Poland

Abstract

This study investigates the internal structure and lithologic variability of slope deposits in a small catchment in the Polish Outer Carpathians using pedological methods supported by geochemical analyses and Electrical Resistivity Tomography (ERT). It addresses the occurrence of lithologic discontinuities in the soils of flysch-dominated mountain areas. Diagnostic criteria from the WRB system—based on particle-size distribution and the content and lithology of coarse fragments—were applied to identify lithologic discontinuities, complemented by computation of sand and silt separates on a clay-free basis. Geochemical analyses and ERT were then used to assess their likely origin. Three major vertical sections were distinguished, separated by discontinuities: an uppermost unit consisting of aeolian material mixed with solifluctional deposits; a middle unit dominated by solifluctional materials; and a lowermost unit composed of colluvial deposits. The study confirms the utility of ERT in detecting subsurface differentiation of stratified slope sediments and provides a model for interpreting pedosedimentary sequences in Carpathian low-mountain environments.

1. Introduction

Vertical differentiation of soil properties, broadly called “layering”, may result not only from the action of soil-forming processes, leading to the formation of pedogenic horizons (i.e., layers formed by the action of soil-forming processes), but also from the heterogeneity of soil parent material, usually described as stratification. This heterogeneity is related to a variety of past and present geologic and geomorphic processes, as extensively reviewed by Phillips & Lorz [1]. A crucial aspect is the recognition of the occurrence of lithologic discontinuities, i.e., significant changes in soil properties that represent differences in lithology within a soil profile. These changes are particularly visible in the particle size distribution or mineral composition, e.g., [2,3]. It has often been emphasized, however, that a lithologic discontinuity is one of just a few diagnostic characteristics lacking a clear, quantitative definition and is thus, to some degree, subjective, as it is difficult to construct a universal definition recognizing all conditions reflecting a change in lithology or age [4].

The latest edition of the international soil classification system, the World Reference Base (WRB) [5], attempts to tackle the issue and includes as many as 11 diagnostic criteria for distinguishing significant differences in parent material within a soil profile, termed as lithic discontinuities. These criteria, albeit not completely quantitative, include differences in particle-size distribution and selected fraction ratios, differences in the content, lithology, and shape of coarse fragments, abrupt differences in soil color, marked differences in the size and shape of resistant minerals in microscopic analyses, differences in the TiO₂/ZrO₂ ratios of the sand fraction, and differences in cation exchange capacity (CEC) per kg clay. In our study, we sought to employ the applicable WRB diagnostic criteria, complemented with computation of soil separates on a clay-free basis, recommended by the Soil Taxonomy [4], to test the occurrence of lithologic discontinuities in soil profiles on a selected, Central European, low-mountain slope. In this region, slopes are mantled by layers of cover materials of complex genesis, presently often termed “cover beds” [6]. The state of research on this phenomenon was recently comprehensively summarized in a book edited by Kleber & Terhorst [7]. The downslope displacement of cover bed material may exceed 100 m [8].

The presence of lithologic discontinuities has been described worldwide in numerous studies, usually combining field observations with various laboratory analyses. A comprehensive review of the results and methodology of those studies can be found in a recent paper on lithologic indicators of discontinuities in carbonate-rich soils of the Polish Carpathians by Kowalska et al. [9]. Lithologic discontinuities in mountain soils of the Carpathians were earlier investigated by Kacprzak & Derkowski [10] and Kacprzak et al. [11], including XRD analyses of mineral composition. Waroszewski et al. [12,13] described the role of lithologic discontinuities in the formation of soils in selected areas of the nearby Sudetes range. The contribution of aeolian admixture to soil parent material in mountain landscapes has been attracting increasing interest, as exemplified by the recent publications of Martignier et al. [14], Waroszewski et al. [15,16], Yang et al. [17], D’Amico et al. [18], Styllas et al. [19] or Zhang et al. [20]. To verify the presence of aeolian material, we used geochemical analysis of principal oxide and selected trace element concentrations.

In our research, to better understand the controls of discontinuities in soil parent material, we employed the two-dimensional electrical resistivity tomography (2D ERT) method. This technique integrates traditional geo-electrical survey methods—sounding and profiling—and is among the most widely used geophysical techniques for imaging shallow subsurface geological structures [21]. Its rapid development and widespread application in Earth sciences research have been made possible by improvements in measuring equipment and the availability of user-friendly software, which have significantly reduced both survey and data processing times [22]. The growing popularity of ERT results not only from the method’s versatility in geological investigations but also from the relatively straightforward interpretation of the resulting images. Although the origins of ERT, like those of several other geophysical techniques, are linked to mineral resource exploration [23], it is now commonly used in a variety of environmental studies, including investigations of mountain slopes. It is particularly well-suited for detecting discontinuities in geological structures. The method’s range of applications includes imaging geological conditions related to landslide formation [24,25] as well as other types of mass movements [26,27]. A noteworthy advancement in recent decades is the application of high-resolution ERT surveys, which allow for the imaging of even relatively small-scale subsurface features, e.g., [28]. For this reason, ERT has also found applications in soil science [29] and biopedological studies [30,31], providing insights not only into the influence of bedrock on the development of weathering covers but also into the internal heterogeneity of such weathering deposits and the formation of soil horizons, e.g., [32]. Survey sections conducted on slopes enable a broader understanding of the diversity of slope processes and complement data collected from boreholes and test pits.

2. Materials and Methods

2.1. Study Area

The study area (Figure 1) lies in the outer part of the Carpathian Mountains, composed of flysch formations, i.e., sedimentary rocks of Late Jurassic to Early Miocene age, deposited by turbidites in a deep marine basin and typically consisting of rhythmically interbedded shales, mudstones, and sandstones, showing considerable lithologic diversity. Summit and shoulder parts of the slopes are usually formed on sandstone-dominated layers, whereas foot- and toeslopes develop predominantly on shales.

The top and upper part of the slope of Mt Zamkowa at Lanckorona are built of Lower Cretaceous deposits of the middle Lgota Formation (Figure 2), which consists of thin- and medium-bedded siliceous sandstones interbedded with slightly calcareous shales. The upper division of the Lgota Formation, known as the Mikuszowice Chert Member, comprises medium- to thick-bedded sandstones with bluish cherts, intercalated with gray and black shales. Further downslope, a complex of non-calcareous shales is exposed. Green and black shales represent the Barnasiówka Radiolarian Shale Formation (BRSF), followed by Upper Cretaceous green and red shales of the Variegated Shales [33,34,35].

The landscape can be classified as low mountains, with the prominent landforms consisting of elongated ridges orientated generally east–west, corresponding to the prevalent strike of the flysch layers. Most slopes are complex, displaying a typical convex–straight–concave profile. Frequent slope breaks are associated with the lithologic diversity of the flysch, resulting in slope angles ranging from less than 5° to more than 25°.

Most slopes in the Carpathians are mantled by relatively thin solifluctional and wash deposits, typically about 1 m thick, except at the lowermost and flattest toeslopes adjacent to river terraces [36]. In the foreland and river valleys, Upper Pleistocene loess deposits—dated from approximately 60,000 to less than 14,000 years BP—are present, reaching a maximum thickness exceeding 10 m [37].

The mean annual temperature in the study area ranges from 6 to 8 °C, with a mean annual precipitation of approximately 900 mm [38]. The investigated slope is covered by a lower montane Abieti-Piceetum forest, dominated by fir (Abies alba) and spruce (Picea abies); this species composition is likely a consequence of long-term anthropogenic influence, particularly the selective logging of beech (Fagus sylvatica) due to its economic value [39].

2.2. Soil Study

Soil pits were dug by hand to a depth controlled by feasibility and safety. Profiles 2 and 3 were located at the scarps of small valleys, which allowed a deeper insight into slope deposits—in Profile 3, we were able to expose slope material to a depth of 600 cm. The profiles were described according to the Guidelines for Soil Description [40] and classified following the WRB system [5]. The colors of soil material were determined in the field using the Munsell Soil Color Chart. The content of coarse fractions (>2 mm) was evaluated by volume in the field. Approximately 2 kg of soil material was taken from each soil horizon. Soil material collected from all soil horizons was air-dried, gently crushed, and sieved (2 mm mesh).

Particle size distribution was determined using wet sieving for the 1–2 mm fraction and the hydrometer method after the removal of soil organic matter using hydrogen peroxide and sample dispersion with hexametaphosphate–bicarbonate [41,42]. Soil pH was measured potentiometrically in distilled water and 1M KCl solution with a soil/solution ratio of 1:2.5 [42]. Organic carbon content was determined using a modified Tyurin method [43].

2.3. Geochemical Analysis

A geochemical analysis of the fine earth material was conducted using Fusion ICP-OES Whole Rock Analysis (ACTLABS, Ancaster, ON, Canada). Samples were prepared and analyzed in a batch system. Each batch contained a method reagent blank, certified reference material, and 17% replicates. Samples were mixed with a flux of lithium metaborate and lithium tetraborate and fused in an induction furnace. The molten melt was immediately poured into a solution of 5% nitric acid containing an internal standard and mixed continuously until completely dissolved (~30 min). The samples were run for major oxides and selected trace elements on a combination simultaneous/sequential Thermo Jarrell-Ash ENVIRO II ICP. Calibration was performed using 7 prepared USGS- and Canmet-certified reference materials.

2.4. Geomorphometry

For the analysis of the topographic context, we utilized a digital terrain model (DTM) derived from airborne laser scanning (ALS), available through the national geospatial data platform (geoportal.gov.pl). This model was generated from a point cloud with a minimum density of 4 points per square meter and has a spatial resolution of 1 × 1 m and a vertical accuracy of ≤0.15 m [44]. The terrain was analyzed using hillshade maps in SAGA GIS software [45]. We additionally generated slope, convergence index, and topographic wetness index models, which are commonly applied in terrain analysis of landslide-prone areas [46,47].

2.5. ERT Analysis

The geological context was investigated using 2D electrical resistivity tomography (ERT). This method is based on measuring the subsurface electrical resistivity using multiple four-electrode configurations [48]. These configurations are created by arranging electrodes at equal intervals along a predefined survey line. Each measurement involves two current electrodes (C₁, C₂) and two potential electrodes (P₁, P₂). The result, expressed as apparent resistivity, is then subjected to inversion modeling to derive true resistivity values, which are interpretable in geological terms.

We used the ARES I device (GF Instruments, Brno, Czechia) with a cable setup allowing for the simultaneous connection of 48 electrodes. Measurements were conducted using the Wenner–Schlumberger array, a versatile configuration suitable for detecting both horizontal and vertical subsurface structures [49]. The surveys were designed to provide a general overview of the geological structure via a more extended profile and a more detailed image of two selected zones. We made the longer section using the “roll-along” method, a technique that allows the extension of the survey line beyond the initial length of the electrode cables. The location of the geophysical measurements was a compromise between thorough verification of sites investigated by other methods and field accessibility. The first profile was 315 m long, with 5 m electrode spacing. The following two profiles were each 47 m in length, with a finer electrode spacing of 1 m.

The acquired resistivity data were processed using the Res2DInv (v. 3.5) software (Geotomo, Malaysia). We applied a smooth (L2 norm) inversion with a topographic correction. The topographic data used were read from the DTM mentioned in Section 2.4. The resulting resistivity models were presented using the fifth iteration and a unified color scale. These models were utilized for geological interpretation in conjunction with existing geological data and our field observations.

3. Results

3.1. Profile Morphology

Three soil profiles (Figure 3) were chosen for this study based on the outcome of previous field research by Kacprzak et al. [11]. They were situated in the lower section of Mt. Zamkowa slope (Figure 1), underlain by the Variegated Shales formation (Figure 2). The profiles were described (

Table 1. Morphology of the investigated profiles (partly based on [11], with changes).

Depth [cm]	Horizon	Moist Munsell Color	Structure 1	Consistence 2	Roots 3	Coarse Fragments [% vol]	Boundary
Profile 1—Epidystric Eutric Episkeletic Cambisol (Episiltic, Endoloamic, Humic)
0–5	Ah	7.5YR1.7/1	sb	fr	++	20	clear
5–27	AB	10YR4/4	sb	vf	++	50	gradual
27–45	Bw1	10YR5/4	sb	vf	++	50	gradual
45–65	Bw2	7.5YR5/4	sb	vf	+	15	clear
65–80	2BC	5YR5/4 + 10YR7-6/2	sb	vf	+	20	gradual
80–115	2C	5YR5/4 + 10YR7-6/2	m	exf	+	40
Profile 2—Epidystric Eutric Katoskeletic Cambisol (Anosiltic, Ochric)
0–5	Ah	10YR2/1	sb	fr	+++	10	abrupt
5–25	Bw1	10YR6/4	sb	f	++	10	clear
25–60	Bw2	10YR6/4	sb	vf	+	50	gradual
60–85	BCg	10YR6/6 + 2.5YR7/3 + 10YR6/8	sb	vf	+	60	clear
85–120	2C1	5YR4/3 + 10YR7/4 + 7.5YR6/2	m	exf		70	abrupt
120–135	2C2	10YR6/6 + 7.5YR6/8 + 2.5Y6/3	m	exf		70	abrupt
135–160	2C3	5YR4/4	m	exf		70	clear
160–200	2C4	7.5YR5/4	m	exf		70
Profile 3—Albic Endostagnic Luvisol (Pantosiltic, Cutanic, Humic)
0–4	Ah	10YR1.7/1	sph	fr	+++	0	abrupt
4–8	A	10YR5/3	sb	fr	+++	0	clear
8–22	AE	10YR6/4	sb	f	++	5	gradual
22–50	E	10YR6/4 + 10YR5/4	sb	f	++	5	gradual
50–70	Btg1	10YR6/4 + 7.5YR5/6	sb	vf	++	5	abrupt
70–85	Btg2	7.5YR5/4 + 2.5Y7/2	ang	vf	++	10	abrupt
85–100	BCg	10YR6/6 + 7.5YR5/6 + 2.5Y7/2	ang	f	++	25	abrupt
100–110	2BC	5YR4/4 + 10YR7/2	ang	vf	+	45	abrupt
110–160	2C1	2.5Y6/4	m	vf	+	60	clear
160–195	2C2	5YR4/4 + 2.5Y6/3	m	exf		40	clear
195–225	3C	5YR4/3-4	m	exf		30	clear
225–290		5YR4/4 + 5Y5-6/2	m			25	abrupt
290–300		7.5YR3/4 + 5Y5-4/1	m			40	clear
300–350		7.5YR3/3 + 7.5Y4/6 + 5Y6-7/2	m			25	abrupt
350–360		7.5YR4/6 + 5YR4/4 + 5Y5-4/1	m			50	abrupt
360–395		5YR4/4 + 5Y5-4/1	m			25	clear
395–460		5YR4/4 + 5Y5-4/1	m			30	gradual
460–515		7.5YR4/6	m			20	clear
515–530		5Y6/2 + 5YR4/6 + 7.5YR6/8	m			30	abrupt
530–540		2.5Y6/3	m			30	abrupt
545–570		5YR4/4 + 5Y5-6/2	m			20	gradual
570–580		5YR4/4	m			30	clear
580–600		5YR3/4 + 5Y5/2	m			40

¹ Structure: sph—spheroidal, sb—subangular blocky, ang—angular blocky, m—massive; ² consistence: fr—friable, f—firm, vf—very firm, exf—extremely firm; ³ roots: +++ abundant, ++ many, + few.

and

Table 2. Soil reaction and content of organic carbon in the investigated profiles (partly based on [11], with changes).

Depth [cm]	Horizon	pH H2O	pH KCl	Corg [%]
Profile 1—Epidystric Eutric Episkeletic Cambisol (Episiltic, Endoloamic, Humic)
0–5	Ah	3.74	3.13	10.40
5–27	AB	4.40	3.53	0.73
27–45	Bw1	4.93	3.74	0.36
45–65	Bw2	5.13	3.80
65–80	2BC	4.86	3.37
80–115	2C	4.78	3.28
Profile 2—Epidystric Eutric Katoskeletic Cambisol (Anosiltic, Ochric)
0–5	Ah	3.83	3.03	8.12
5–25	Bw1	4.49	3.61	0.32
25–60	Bw2	4.72	3.65
60–85	BCg	4.98	3.54
85–120	2C1	5.08	3.46
120–135	2C2	5.02	3.54
135–160	2C3	5.20	3.44
160–200	2C4	5.26	3.53
Profile 3—Albic Endostagnic Luvisol (Pantosiltic, Cutanic, Humic)
0–4	Ah	3.66	2.94	12.93
4–8	A	3.98	3.06	1.68
8–22	AE	4.54	3.88	0.51
22–50	E	4.59	3.78
50–70	Btg1	4.77	3.47
70–85	Btg2	4.81	3.39	0.44
85–100	BCg	4.90	3.39
100–110	2BC	4.99	3.31
110–160	2C1	4.14	3.43
160–195	2C2	5.82	3.83
195–225	3C	6.21	4.07

) following the Guidelines for Soil Description [40] and classified according to the World Reference Base [5]. Designations of soil horizons were given to a depth of 2 m. The presence of the argic horizon in Profile 3 was identified based on the occurrence of clay coatings covering ≥ 15% of the surfaces of soil aggregates.

Soil colors dominating in layers other than humus horizons display a significant degree of diversity (Table 1), from reddish 5YR3-4/3-4, through grayish 2.5Y6-7/2-3 to brownish 7.5YR3-6/3-8 and 10YR4-6/3-4. The 10YR colors prevail in the upper parts of the profiles, while the middle and lowermost sections show a greater diversity, with most layers characterized by the occurrence of more than one color. This can be partly attributed to the varied colors and thin beds of variegated shales (hence their name), but probably also to the action of past slope processes, e.g., solifluction, creating discontinuities. One of the diagnostic criteria to detect lithic discontinuities included in the WRB is the occurrence of abrupt differences in color not resulting from soil formation. This criterion appears to be fulfilled with the abrupt changes in color at a depth of 120 and 135 cm in Profile 2 and at a depth of 100 and 110 cm in Profile 3.

3.2. Particle Size Distribution and Ratios

Particle size distribution shows significant variety but also certain regularities in all three investigated profiles (

Table 3. Particle size (in mm diameter) distribution (in %) within the fine earth fraction and texture in the investigated profiles.

Depth [cm]	Horizon	2.0–1.0	1.0–0.1	0.01–0.05	0.05–0.02	0.02–0.006	0.006–0.002	<0.002	Texture 1
0–5	Ah	n.d.	n.d	n.d	n.d	n.d	n.d	n.d
5–27	AB	1	8	4	23	24	19	21	SiL
27–45	Bw1	2	7	5	21	23	17	25	SiL
45–65	Bw2	2	8	5	19	21	17	28	SiCL
65–80	2BC	3	12	4	8	13	22	38	SiCL
80–115	2C	2	14	2	5	16	23	38	SiCL
0–5	Ah	n.d.	n.d	n.d	n.d	n.d	n.d	n.d
5–25	Bw1	1	5	6	23	25	17	23	SiL
25–60	Bw2	4	9	7	21	21	16	22	SiL
60–85	BCg	1	8	7	21	23	14	26	SiL
85–120	2C1	4	23	5	7	12	16	33	CL
120–135	2C2	2	10	7	30	21	11	19	SiL
135–160	2C3	4	16	5	9	14	16	36	CL
160–200	2C4	6	22	7	9	14	14	28	CL
0–4	Ah	n.d.	n.d	n.d	n.d	n.d	n.d	n.d
4–8	A	1	5	10	32	26	12	14	SiL
8–22	AE	1	5	8	34	26	14	12	SiL
22–50	E	1	8	8	33	24	11	15	SiL
50–70	Btg1	0	5	8	34	24	11	18	SiL
70–85	Btg2	2	14	8	26	19	12	19	SiL
85–100	BCg	2	9	8	33	22	9	17	SiL
100–110	2BC	4	29	4	11	10	15	27	L
110–160	2C1	5	25	5	7	10	15	33	CL
160–195	2C2	8	21	5	6	10	16	34	CL
195–225	3C	5	25	4	7	10	16	33	CL
225–290		6	29	6	8	8	13	30	CL
290–300		5	26	5	7	4	26	27	CL
300–350		8	32	4	6	12	16	22	L
350–360		9	26	5	6	12	19	23	L
360–395		7	21	6	6	13	22	25	L
395–460		7	28	5	6	14	17	23	L
460–515		11	28	4	4	12	19	22	L
515–530		10	30	5	5	13	16	21	L
530–540		8	18	6	10	14	17	27	CL
545–570		13	45	3	6	8	12	13	SL
570–580		16	24	5	4	14	18	19	L
580–600		9	30	4	7	13	17	20	L

¹ Texture: SiL—silt loam, SiCL—silty clay loam, CL—clay loam, L—loam, SL—sandy loam.

). The uppermost sections, displaying an increased content of silt and a lower content of clay, are silt loams. The thickness of those sections increases downslope from 45 cm in Profile 1 to 100 cm in Profile 3. In Profile 1, silt loams overlie material of a texture of silty clay loam, and in Profile 2, silt loams overlie clay loams, interbedding with them in the middle section of the profile. In Profile 3, the silt loam cover lies upon clay loams c. 2 m thick, while the lowermost part of the profile, exposing weathered flysch, generally has a texture of loam.

According to the WRB, an abrupt textural difference is a very sharp increase in clay content within a limited (≤2 cm) depth range. This sharp increase in the underlying layer is understood as at least twice as much clay if the overlying layer has <20% clay or ≥20% (absolute) more clay if the overlying layer has ≥20% clay. These requirements are not met in the investigated profiles, even though abrupt horizon boundaries are common (6 out of 20 cases) and the content of clay varies significantly between 12 and 38% within the first 2 meters of profiles. In Profile 1, the largest difference in clay content is 10 percentage points (increase by 36%) between the 45–65 cm and the 65–80 layers with a clear (2–5 cm) boundary. In Profile 2, the largest difference in clay content is 17 percentage points (increase by as much as 89%) between the 120–135 cm and 135–160 cm layers. As the boundary is abrupt, this falls just short of meeting the requirements for a lithic discontinuity, which would be met with 38% clay content in the 135–160 layer. In Profile 3, the largest difference in clay content is 10 percentage points (increase by 59%) between the 85–100 cm and the 100–110 cm layers, with an abrupt boundary. These significant relative increases in the content of clay do not appear to result from pedogenic processes. In Profile 2, it occurs in the weakly changed part typified by a massive structure with no distinct soil aggregates. In Profile 3, it occurs below the Bt horizons, and the amount of clay coatings on the surface of soil aggregates is actually lower than in the overlying horizons. That indicates a rather arbitrary character of the requirements for an abrupt textural change used in the WRB.

An important diagnostic criterion offered in the WRB to detect a lithic discontinuity is based on differences in the absolute contents of fine earth fractions other than clay and their ratios. As the layers in the investigated profiles contained ≥10% sand and ≥10% silt, we were able to use the variant requiring a difference of ≥25% in the ratio sand to silt and a difference of five percentage points in the content of sand and/or silt between the overlying and underlying layers. This criterion was fulfilled in all investigated profiles (

Table 4. WRB 2022 diagnostic criteria for lithic discontinuity in the investigated profiles. Values indicating fulfillment of the criteria are presented in bold.

Depth [cm]	Horizon	Sand (2–0.05 mm) Content 1	Silt (0.05–0.002 mm) Content 1	Sand Content Difference 2	Silt Content Difference 2	Sand/Silt Ratio	Sand/Silt Ratio Difference 3
0–5	Ah
5–27	AB	13	66	−1	5	0.20	−14%
27–45	Bw1	14	61	−1	4	0.23	−13%
45–65	Bw2	15	57	−4	14	0.26	40%
65–80	2BC	19	43	1	−1	0.44	8%
80–115	2C	18	44			0.41
0–5	Ah
5–25	Bw1	12	65	−8	7	0.18	−46%
25–60	Bw2	20	58	4	0	0.34	25%
60–85	BCg	16	58	−16	23	0.28	−70%
85–120	2C1	32	35	13	−27	0.91	198%
120–135	2C2	19	62	−6	23	0.31	−52%
135–160	2C3	25	39	−10	2	0.64	−32%
160–200	2C4	35	37			0.95
0–4	Ah
4–8	A	16	70	2	−4	0.23	21%
8–22	AE	14	74	−3	6	0.19	−24%
22–50	E	17	68	4	−1	0.25	33%
50–70	Btg1	13	69	−11	12	0.19	−55%
70–85	Btg2	24	57	5	−7	0.42	42%
85–100	BCg	19	64	−18	28	0.30	−71%
100–110	2BC	37	36	2	4	1.03	−6%
110–160	2C1	35	32	1	0	1.09	3%
160–195	2C2	34	32	0	−1	1.06	3%
195–225	3C	34	33	−7	4	1.03	−27%
225–290		41	29	5	-8	1.41	45%
290–300		36	37	−8	3	0.97	−25%
300–350		44	34	4	−3	1.29	20%
350–360		40	37	6	−4	1.08	30%
360–395		34	41	−6	4	0.83	−23%
395–460		40	37	−3	2	1.08	−12%
460–515		43	35	−2	1	1.23	−7%
515–530		45	34	13	−7	1.32	70%
530–540		32	41	−29	15	0.78	−67%
545–570		61	26	16	−10	2.35	88%
570–580		44	36	2	−1	1.25	8%
580–600		43	37			1.16

¹ absolute value as percentage, ² absolute value of content compared with the underlying horizon, ³ difference in sand/silt ratio compared with the underlying horizon.

), suggesting the presence of one lithic discontinuity in Profile 1 (at a depth of 65 cm), five discontinuities in Profile 2 (at a depth of 25, 85, 120, 135, 160 cm), and three discontinuities in Profile 3 (at a depth of 70, 85, 100 cm). If we extend this analysis beyond the 2 m limit of “soil” to the whole profile of exposed slope materials, we observe seven further discontinuities within Profile 3, which corresponds well with the nature of flysch formations.

Computation of sand and silt separates on a clay-free basis, even though not used in the WRB, is widely perceived as a very useful manipulation in assessing lithologic changes cf. [3,4]. In the USDA Soil Taxonomy system, an analysis of particle size distribution on a clay-free basis is recommended to detect lithologic discontinuities, although no fixed values are given [4]. For comparison and discussion, we computed the content of sand and silt fractions on a clay-free basis (

Table 5. Particle size (in mm diameter) distribution (in %) within the fine earth fraction calculated on a clay-free basis. Content values differing by 25% or more versus the underlying horizon are indicated in bold.

Depth [cm]	Horizon	2.0–1.0	1.0–0.1	0.01–0.05	0.05–0.02	0.02–0.006	0.006–0.002
0–5	Ah	n.d.	n.d	n.d	n.d	n.d	n.d
5–27	AB	1.3	10.1	5.1	29.1	30.4	24.1
27–45	Bw1	2.7	9.3	6.7	28.0	30.7	22.7
45–65	Bw2	2.8	11.1	6.9	26.4	29.2	23.6
65–80	2BC	4.8	19.4	6.5	12.9	21.0	35.5
80–115	2C	3.2	22.6	3.2	8.1	25.8	37.1
0–5	Ah	n.d.	n.d	n.d	n.d	n.d	n.d
5–25	Bw1	1.3	6.5	7.8	29.9	32.5	22.1
25–60	Bw2	5.1	11.5	9.0	26.9	26.9	20.5
60–85	BCg	1.4	10.8	9.5	28.4	31.1	18.9
85–120	2C1	6.0	34.3	7.5	10.4	17.9	23.9
120–135	2C2	2.5	12.3	8.6	37.0	25.9	13.6
135–160	2C3	6.3	25.0	7.8	14.1	21.9	25.0
160–200	2C4	8.3	30.6	9.7	12.5	19.4	19.4
0–4	Ah	n.d.	n.d	n.d	n.d	n.d	n.d
4–8	A	1.2	5.8	11.6	37.2	30.2	14.0
8–22	AE	1.1	5.7	9.1	38.6	29.5	15.9
22–50	E	1.2	9.4	9.4	38.8	28.2	12.9
50–70	Btg1	0.0	6.1	9.8	41.5	29.3	13.4
70–85	Btg2	2.5	17.3	9.9	32.1	23.5	14.8
85–100	BCg	2.4	10.8	9.6	39.8	26.5	10.8
100–110	2BC	5.5	39.7	5.5	15.1	13.7	20.5
110–160	2C1	7.5	37.3	7.5	10.4	14.9	22.4
160–195	2C2	12.1	31.8	7.6	9.1	15.2	24.2
195–225	3C	7.5	37.3	6.0	10.4	14.9	23.9

) and, matching the WRB criteria, interpreted differences ≥ 25% as suggesting the presence of lithologic discontinuities. As this manipulation is designed to eliminate a possible influence of soil-forming processes, we performed it only for the layers within the first 2 m from the surface.

In the investigated profiles, particle size distribution, when calculated on a clay-free basis, shows differences ≥ 25% for particular separates between nearly all layers. Differences in the content of one or two separates, especially when the contents are low, should be treated with caution. But when such significant differences are observed for three of more fractions out of the analyzed six, this may be interpreted as an occurrence of lithologic discontinuity. This applies to a depth of 65 cm in Profile 1; 85, 120 and 135 cm in Profile 2; and 70 and 100 cm in Profile 3. Most notably, all profiles have an increased content of the coarse silt (0.05–0.02 mm) fraction in their top parts. It exceeds 25% to a depth of 65 cm or 85 cm in Profile 1 and 2, respectively, and 30% to a depth of 100 cm in Profile 3. In all three profiles, the amount of coarse silt in the lower part of the section showing the increase in its content is at least twice as much as in the layer directly underlying it. This may be interpreted as a very clear indication of a lithologic discontinuity.

3.3. Coarse Fragments

Several diagnostic criteria to detect lithic discontinuities according to the WRB refer to the amount and properties of coarse fragments. The results of our analysis of the amount and lithology of coarse fragments in the investigated profiles are included in

Table 6. Coarse fragment amount and lithology in the investigated profiles. Amount values larger by 10 percentage points or more versus the underlying horizon are indicated in bold.

Depth [cm]	Coarse Fragments [% vol]	Cherts	Lgota Sandstones	Variegated Shales
0–5	20	90	10	0
5–27	50	90	10	0
27–45	50	90	10	0
45–65	15	80	10	10
65–80	20	80	0	20
80–115	40	90	5	5
0–5	10	20	80	0
5–25	10	20	80	0
25–60	50	70	30	0
60–85	60	70	30	0
85–120	70	65	30	5
120–135	70	25	70	5
135–160	70	25	50	25
160–200	70	75	25	0
0–4	0	0	0	0
4–8	0	0	0	0
8–22	5	50	50	0
22–50	5	50	50	0
50–70	5	90	10	0
70–85	10	40	60	0
85–100	25	70	30	0
100–110	45	75	20	5
110–160	60	95	5	0
160–195	40	95	5	0
195–225	30	90	0	10
225–290	25	95	0	5
290–300	40	95	0	5
300–350	25	80	0	20
350–360	50	85	0	15
360–395	25	70	0	30
395–460	30	95	0	5
460–515	20	65	0	35
515–530	30	95	0	5
530–540	30	85	0	15
545–570	20	80	0	20
570–580	30	90	0	10
580–600	40	65	5	30

. It needs to be said that lithologic analysis of rock fragments on a slope underlain by flysch rocks poses considerable difficulties, as flysch itself is a complex combination of varied shales, mudstones, cherts, and sandstones, showing significant diversity even within individual formations. The study area, as described in detail in Section 2.1, is built of relatively thin- to medium-bedded flysch, which adds to the difficulty. We were able to distinguish three main lithologies of coarse fragments, namely, (1) cherts, including very highly siliceous fine-grained rocks; (2) gray-colored and not highly siliceous sandstones characteristic of the Middle Lgota Formation; and (3) red or green shales. The lithologic composition of coarse fragments was markedly dominated by cherts, being the most resistant to weathering in the study area. Since cherts occur both in the Lgota Formation, building the middle and upper sections of slopes, and within the Variegated Shales formation underlying the lower section, they could not be used as indicators of material provenance. We inferred material provenance based on the presence and proportion of contents of sandstones characteristic of the Lgota Formation and red or green shales typical of the Variegated Shales formation.

The upper parts of the profiles were devoid of any fragments of variegated shales to a depth of 45, 85, and 100 cm in profiles 1 to 3, respectively. The content of Lgota sandstones in the coarse fragments, on the other hand, was up to 80%. The lower parts of soil profiles, to a depth of 115, 200 and 195 cm in Profiles 1 to 3, respectively, consist of several layers with varying proportions of the Lgota sandstones and variegated shales. Fragments of the Lgota sandstones were not observed only in the lowermost section of Profile 3, below a depth of 195 cm.

There were no significant differences in the shape and degree of weathering of coarse fragments observed, as they were generally angular, often tabular, typically with weathering rinds several mm thick. So, these diagnostic criteria included in the WRB were not applicable. According to the WRB, if an overlaying layer contains more coarse fragments by ≥10 percentage points than the underlying layer, it may suggest a lithic discontinuity. This condition was met in Profile 3 in the 110–160 and 160–195 cm layers and, in a very distinct way, in Profile 1, where the content of coarse fragments was the highest (50%) between 5 and 45 cm, markedly exceeding the content of rock fragments (15–20%) in the underlying layers, implying the presence of a lithic discontinuity at a depth of 45 cm.

3.4. Geochemical Composition

Geochemical analyses were conducted for selected layers representing the main depth sections of the investigated profiles. Chemical composition (

Table 7. Chemical composition of material in selected horizons of the investigated profiles.

Depth [cm]	SiO2 [%]	Al2O3 [%]	Fe2O3 [%]	MnO [%]	MgO [%]	CaO [%]	Na2O [%]	K2O [%]	TiO2 [%]	P2O5 [%]	LOI [%]	SiO2/ R2O3
Profile 1
27–45	79.42	8.30	2.54	0.316	0.62	0.25	0.6	1.92	0.768	0.09	5.33	6.84
80–115	65.81	15.38	5.07	0.04	1.35	0.29	0.24	2.77	0.648	0.05	8.79	3.12
Profile 2
25–60	81.28	7.99	2.2	0.117	0.48	0.17	0.54	1.84	0.736	0.04	3.85	7.44
85–120	67.58	13.99	5.08	0.281	1.32	0.35	0.28	2.58	0.638	0.08	8.58	3.43
135–160	67.03	13.82	4.92	0.164	1.27	0.41	0.42	2.6	0.635	0.09	8.02	3.46
Profile 3
40–60	83.07	7.76	2.09	0.093	0.48	0.25	0.71	2.08	0.759	0.04	3.48	7.83
100–110	66.46	13.43	4.5	0.296	1.42	0.31	0.4	2.51	0.595	0.06	8.74	3.59
110–160	69.18	12.92	3.81	0.225	1.59	0.47	0.23	2.32	0.563	0.07	9.26	4.00
195–225	65.57	14.22	3.6	0.763	1.79	0.79	0.4	2.91	0.575	0.08	9.4	3.56
290–300	65.2	13.81	4.85	0.299	1.53	0.73	0.41	2.69	0.579	0.09	8.65	3.39
350–360	64.4	14.24	4.59	1.931	1.55	0.74	0.26	2.77	0.573	0.1	9.53	3.32
360–395	63.78	13.63	6.41	0.14	1.42	0.66	0.35	2.61	0.559	0.18	9.21	3.10
545–570	65.46	13.55	6.41	0.172	1.49	0.66	0.27	2.69	0.541	0.19	9.05	3.19

) shows a clear difference between the samples representing the uppermost sections of all three profiles and all other samples. The samples from the uppermost parts contain distinctly more SiO₂ (79.42–83.07% vs. 63.78–67.81%) and less Al₂O₃ (7.76–8.30% vs. 12.92–15.38%). The loss on ignition (LOI) is distinctly lower (3.48–5.33% vs. 8.02–9.53%) and the SiO₂/R₂O₃ ratio higher (6.84–7.83% vs. 3.10–4.00%). This suggests an increased proportion of quartz versus aluminosilicates and corresponds well with the results of mineralogical studies conducted in the soils of this area [11].

The concentration of selected trace elements (

Table 8. Trace elements concentrations in selected layers of the investigated profiles.

Depth [cm]	Ba ppm	Sr ppm	Y ppm	Sc ppm	Zr ppm	Be ppm	V ppm
Profile 1
27–45	383	68	23	8	344	2	67
80–115	294	73	19	14	131	2	119
Profile 2
25–60	354	70	22	7	394	<1	52
85–120	334	84	22	13	158	2	118
135–160	323	61	24	13	155	2	114
Profile 3
40–60	386	68	25	6	541	1	46
100–110	327	70	27	13	175	2	104
110–160	301	66	26	12	113	2	84
195–225	420	58	25	14	104	2	97
290–300	320	56	23	13	128	2	108
350–360	859	82	29	14	113	2	110
360–395	260	51	25	13	115	2	101
545–570	343	84	25	13	112	2	104

) shows a significant difference between the samples representing the uppermost sections of all three profiles and all other samples. The samples from the uppermost parts contain distinctly more Zr (344–541 ppm vs. 104–175 ppm) and clearly less Sc (6–8 ppm vs. 12–14 ppm) and V (46–67 ppm vs. 84–119 ppm). In the deep (290–600 cm) section of slope deposits exposed in Profile 3, the observed changes in the concentrations of other trace elements, Ba and Sr in particular, appear to result not from pedogenetic transformations or external admixtures but the diversity of thin-bedded flysch rocks.

The geochemical ratios computed based on the concentrations of oxides and trace elements will be presented and discussed in Section 4.

3.5. Geomorphometric Analysis

The topography of the investigated slope (Figure 4) is well represented by the derived slope, convergence index (CI), and topographic wetness index (TWI) models. The slope is situated directly below the main summit of Mt Zamkowa and is bordered to the east and west by deeply incised V-shaped valleys. It is characterized by steep inclinations, locally exceeding 30 degrees, which gradually decrease from mid-slope downward. In the upper part of the slope, a lithologically controlled break in slope is evident, clearly visible as a convex edge on the CI map. Below this level, the slope morphology transitions from divergent to convergent, with additional erosional incisions at the base, as illustrated by the TWI map.

3.6. ERT Results

The interpretation of geophysical imaging (Figure 5) was based on field observations. We did not perform any special drilling to determine the stratigraphy, relying on the analysis of geological maps and data from nearby boreholes provided by the Polish Geological Institute—National Research Institute [50] and the exposure of slope deposits in pits dug on this slope, most notably in Profile 3. The extended ERT profile revealed significant variability in the geoelectrical properties of the bedrock. The interpretation indicates substantial gravitational displacement of the entire upper part of the slope, reaching depths of 15–20 m and more. Recurring sequences of resistivity evidence this contrast with the corresponding, deeper lithologic units. The displacement of rock material is challenging to quantify, but for each of the visible blocks with the highest resistivity, it is 40–80 m. The displacement should likely be interpreted as resulting from translational landsliding. Within the bedrock, considerable lithologic heterogeneity is apparent, along with the probable presence of a tectonic fault zone, as indicated by a bedrock of reduced electrical resistivity. The slope’s morphology significantly influences the moisture content of the slope deposits. This is clearly shown in Section 2 and Section 3, where tests were conducted at a higher resolution. Near-surface layers in some locations exhibit the highest resistivity values, corresponding to concentrations of coarse rock debris. Based on the inversion models, it is possible to identify heterogeneity in the near-surface colluvial deposits.

4. Discussion

As presented in Section 3.2, the uppermost parts of the investigated profiles have a texture of silt loam, while the underlying parts are predominantly clay loams or, in the lowermost part of Profile 3, loams. The analysis of particle size distribution on a clay-free basis reveals a particularly distinct increase in the content of the coarse silt fraction in the uppermost sections, having a texture of silt loam. This, along with the thickness of the silt loam layer increasing downslope may suggest an aeolian provenance of the material. It must be noted, however, that some soil horizons within this layer contain up to 50% of coarse fragments (Table 1, Figure 3). This suggests an aeolian enrichment, in this case within the Central European continental subdomain [51], of periglacial slope deposits, as often described in Central European mountains, e.g., [6,12,13]. Alternation between SiL and CL below 85 cm in Profile 2 suggests polygenetic sedimentation, with interbedded aeolian silt and more clayey solifluctional layers.

The integrated geochemical dataset demonstrates compelling evidence for aeolian contributions in several horizons (

Table 9. Geochemical ratios in selected horizons of the investigated profiles.

Depth [cm]	Zr/Y	Zr/Sc	Zr/TiO2	Zr/Al2O3	Ti/Si	Ti/Al	V/Zr	V/TiO2	V/Al2O3
Profile 1
27–45	14.96	43.00	447.92	41.45	0.0097	0.0925	0.1948	87.24	8.07
80–115	6.89	9.36	202.16	8.52	0.0098	0.0421	0.9084	183.64	7.74
Profile 2
25–60	17.91	56.29	535.33	49.31	0.0091	0.0921	0.132	70.65	6.51
85–120	7.18	12.15	247.65	11.29	0.0094	0.0456	0.7468	184.95	8.43
135–160	6.46	11.92	244.09	11.22	0.0095	0.0459	0.7355	179.53	8.25
Profile 3
40–60	21.64	90.17	712.78	69.72	0.0091	0.0978	0.085	60.61	5.93
100–110	6.48	13.46	294.12	13.03	0.009	0.0443	0.5943	174.79	7.74
110–160	4.35	9.42	200.71	8.75	0.0081	0.0436	0.7434	149.20	6.50
195–225	4.16	7.43	180.87	7.31	0.0088	0.0404	0.9327	168.70	6.82
290–300	5.57	9.85	221.07	9.27	0.0089	0.0419	0.8438	186.53	7.82
350–360	3.90	8.07	197.21	7.94	0.0089	0.0402	0.9735	191.97	7.72
360–395	4.60	8.85	205.72	8.44	0.0088	0.0410	0.8783	180.68	7.41
545–570	4.48	8.62	207.02	8.27	0.0083	0.0399	0.9286	192.24	7.68

). Notably, samples from horizons such as 40–60 cm in Profile 3, 30–60 cm in Profile 2, and 27–45 cm in Profile 1 exhibit strong zircon enrichment, with elevated Zr/Y (≥15), Zr/Sc (>40), Zr/TiO₂ (>400), and Zr/Al₂O₃ (>40) ratios. These ratios align with established signatures of wind-transported silt, wherein durable zircon concentrates through physical sorting [52,53]. The obtained values correspond well with the findings presented in recent studies from similar areas in Europe [15,16,54,55], which supports the concept of aeolian material admixture dominating the properties of the upper parts of the investigated profiles and, thus, constituting a lithologic discontinuity.

In comparison, titanium-based proxies (Ti/Si and Ti/Al) offer mixed support. Lower values of Ti/Si (<0.02) and Ti/Al (<0.08) in some samples are consistent with quartz-dominated loessic inputs. However, elevated Ti ratios in other zircon-enriched horizons suggest incomplete winnowing of Fe-Ti oxides or contributions from local rocks. This decoupling mirrors findings by Kowalska et al. [9], who reported significantly varying TiO₂ concentrations in Carpathian soils, and, thus, could not unambiguously use them to indicate lithologic discontinuities.

The analysis of the content and lithology of coarse fractions (Table 6) provided important evidence of the provenance of material and the occurrence of lithologic discontinuities. Even though the investigated profiles were located on the section of slope underlain by variegated shales, the upper parts of the profiles were devoid of any fragments of these shales. The content of Lgota sandstones, building the slopes of Mt. Zamkowa above the section hosting the investigated profiles, on the other hand, was up to 80%. The lower parts of soil profiles, to a depth of 115, 200, and 195 cm in Profiles 1, 2, and 3, respectively, consist of several layers with varying proportions of the Lgota sandstones and variegated shales. This phenomenon, along with the position of clasts generally parallel to the inclination of slope and discordant with the general dip of strata suggests that the investigated profiles developed within allochthonous slope material, probably of Pleistocene, solifluctional genesis, not within regolith in situ. This corresponds well with the general concept of development of slope covers in Central Europe, e.g., [6,7,56,57], as well as with the findings of earlier research in the Carpathians [10,11] and Sudetes [12,13,16].

Only the lowermost section of Profile 3 between a depth of 195 and 600 cm appears to represent weathered variegated shales entirely. It does not mean, however, that they are flysch layers weathered in situ. As we observed, the orientation of layer boundaries in this section is generally parallel to the slope surface and almost perpendicular to the dip of the beds of variegated shales visible in the stream channel undercutting the scarp in which Profile 3 was exposed. This can probably be explained with the results of geomorphometric analysis and geophysical imaging using ERT. The lowermost, weakly inclined, convergent slope section appears to have been transformed by mass movements of a character of translational landslides with the thickness of colluvial material increasing downslope from c. 2 to more than 10 m. It is challenging to infer the precise age of these colluvial processes. In Profile 3, however, the colluvial material is overlain by alleged solifluctional deposits with aeolian admixture, interpreted to have originated at the end of the Pleistocene, e.g., [6,7,16]. Landsliding should be perceived as the third, along with solifluction and aeolian deposition, process shaping the diversity of properties of soil parent material and creating lithologic discontinuities in the investigated area.

Table 10. Lithologic discontinuities in the investigated profiles indicated by various diagnostic criteria.

Depth [cm]	Sand/Silt Ratio Difference	Sand Content Difference	Silt Content Difference	Coarse Fragments Lithology	Volume of Coarse Fragments	Abrupt Color Difference	Clay-Free PSD Difference
0–5
5–27
27–45				+	+
45–65	+		+	+			+
65–80				+			(+)
80–115
0–5
5–25	+	+	+
25–60
60–85	+	+	+	+			+
85–120	+	+	+	+		+	+
120–135	+	+	+	+		+	+
135–160	+		+	+
160–200
0–4
4–8
8–22
22–50
50–70	+	+	+				+
70–85	+	+	+
85–100	+	+	+	+		+	+
100–110				+		+	(+)
110–160				+	+
160–195				+	+
195–225	+	+
225–290	+	+	+			+
290–300	+	+			+
300–350						+
350–360	+	+			+	+
360–395
395–460					+
460–515
515–530	+	+	+			+
530–540	+	+	+		+	+
545–570	+	+	+
570–580
580–600

+ discontinuity unambiguously indicated, (+) discontinuity ambiguously indicated.

summarizes the results of the application of various diagnostic criteria to detect lithologic discontinuities in the investigated profiles of soils and slope deposits. The number of detected discontinuities is very high, and they occur in all profiles. This is due to the interplay of a number of processes—aeolian accumulation, solifluction and also landsliding, as well as the lithologic diversity of the thin-bedded flysch rock building the investigated slope. In this complex image, it is possible to distinguish three major vertical zones, separated by discontinuities, within slope deposits as soil parent materials—the uppermost one, consisting of aeolian material mixed with solifluctional deposits; the middle one, dominated by solifluctional materials with numerous minor discontinuities; and the lowermost one, consisting of colluvial deposits.

5. Conclusions

1. Lithologic discontinuities play an important role in the development of soils in low mountains of the Carpathians.

2. The range of diagnostic criteria offered by the WRB allows detection of lithologic discontinuities with a high degree of certainty, particularly when supported by computation of sand and silt separates on a clay-free basis.

3. Geochemical and geophysical analyses can substantially aid in explaining the nature and origin of detected discontinuities.

4. Geochemical ratios derived from oxide and trace element concentrations indicated the contribution of aeolian admixture to soil parent material in the upper parts of slope deposit profiles.

5. Geophysical analysis (2D ERT) allowed us to identify a major discontinuity between solifluctional and colluvial deposits.

6. Two-dimensional ERT may help detect past landslide-related ground movements, accounting for the occurrence of some discontinuities within slope deposits, once their traces are no longer evident in terrain morphology.