Landscape evolution and the karst development in the Ojo Guareña multilevel cave system (Merindad de Sotoscueva, Burgos, Spain)

ABSTRACT The Ojo Guareña karst system (OG) is located in the SE Cantabrian Range in northern Spain (Burgos, Spain). It is a multilevel cave system composed of 6 levels and is one of the longest cavities in the Iberian Peninsula (110 km). The spatial patterns and geomorphological characteristics of OG constitute a first-order record for studying the principal mechanisms of how the karst evolved by reconstruction and analysis of the external landscape. This extended karst system is attributed to the action of the local drainage system driven by Quaternary climatic fluctuations and lithological-structural controls. To contribute to this debate, we performed a detailed geomorphological mapping of this area (1:25,000 scale), differentiating the landforms according to the main geomorphological processes (structural, gravity, fluvial, glacial, weathering and polygenetic) involved. These datasets were used to draw a detailed geomorphological map and give a preliminary interpretation of the local landscape evolution.


Introduction
Large extensive karst systems constitute key records for comprehending regional geomorphic history (Audra and Palmer, 2011).Caves are formed due to the karstification process all over the globe in all climatic domains and are frequently located in highly soluble bedrocks such as limestones, dolomites, evaporites, and marbles, but not uncommonly also in crystalline rocks (Ford and Williams, 1989;Bigot and Audra, 2010).Karstification is a long-term geochemical procedure of infiltration, dissolution and erosion of the bedrock from atmospheric water and carbonic acid through penetrable fissures (e.g.bedding planes, stratification joints, faults) (Ford and Williams, 1989;Palmer, 1991).A basic and important result of karstification in large karstic systems is extensive horizontal conduits (Ford and Ewers, 1978;Bögli, 1980;Palmer, 1987;White, 1988;Ford and Williams, 1989).These features are formed in the saturated zone under phreatic conditions and correspond to a long period of local base level stability.Large horizontal pipes are usually cut by narrow and deep underground canyons during local river incision under vadose conditions (Ford and Williams, 1989;Audra et al., 2006;Audra et al., 2007;Audra & Palmer, 2011).Thus, it seems obvious that these morphologies are closely related with the long-term evolution of the local drainage system and its modification due to climatic fluctuations and regional uplift (Ford and Ewers, 1978;Bögli, 1980;Ford et al., 1981;Palmer, 1987Palmer, , 1991;;White, 1988;Granger et al., 1997Granger et al., , 2001;;Audra et al., 2006;Westaway et al., 2010;Piccini, 2011;Piccini & Iandelli, 2011;Ortega et al., 2013a;Harmand et al., 2017;Zumpano et al., 2019;Pisano et al., 2020).
Moreover, detailed geomorphological mapping of the surroundings of large karst systems permits reconstruction of the local base levels sequence and analysis of how the local drainage evolved.The study and reconstruction of long-term geomorphological systems can contribute to a better understanding of the dominant processes of the landscape evolution (Antoine et al., 2000;Bridgland, 2000;Van den Berg & Van Hoof, 2001;Westaway et al., 2002;Westaway, 2006;Bridgland & Westaway, 2007), driven by combinations of tectonics, climate and eustasy (Bridgland & Westaway, 2007, 2014).Therefore, studying the external-internal base levels and their correlation can offer valuable information about the mechanisms and processes responsible for the landscape evolution.
The OG natural monument, and its setting in the SE segment of the Cantabrian Range (northern Spain, Figure 1), constitutes a natural laboratory for testing this hypothesis.OG is one of the longest caves in Spain (110 km), with 6 horizontal cave levels.OG has been the focal point of diverse research studies and multidisciplinary topics, like geology (Olmo Zamora et al, 1978, Olivé Davó et al., 1978), karst geomorphology (Eraso, 1965(Eraso, , 1986;;Grupo Espeleológico Edelweiss, 1986;Ruiz et al., 2009;Ortega & Martín, 2011;Ortega et al., 2013b), biology (Rodríguez & Giani, 1989;Camacho-Pérez et al., 2010;Rodríguez & Achurra, 2010) and archaeology (Osaba, 1960;Osaba & Uribarri, 1968;Jordá, 1969;Uribarri & Liz 1973;Ortega & Martín, 1986, 2015;Corchón et al., 1998;Gómez-Barrera et al., 2003, 2004;Navazo & Díez, 2005;Ortega et al., 2020Ortega et al., , 2021;;Navazo Ruiz, et al., 2021).Nevertheless, despite the numerous research studies, the geomorphological evolution of the surrounding landscape and its relationship with the formation of the endokarstic system has been little addressed.As a contribution to this debate, we have conducted a detailed geomorphological mapping of the surroundings of OG, defining the external base levels and correlating them with the internal horizontal cave levels.This area is especially appropriate for this kind of study, since the following are welldefined: (i) OG is fairly well studied, mapped and documented; (ii) a staircase of the local fluvial system is preserved; and (iii) the glacial and tectonic activity are well documented.

Regional setting and geological context
OG is located in the SE sector of the Cantabrian Range and the Basque Mountains, which constitute the natural drainage divide between the Cantabrian and Mediterranean basins (Figure 1).The spatial patterns of the landscape of this area are characterized by a succession of wide synclines and narrow anticlines, mainly moulded by the action of the Upper Ebro River network, extensive karst development and Late Pleistocene glacial activity (Eraso, 1965(Eraso, , 1986;;Ortega, 1974;González Pellejero, 1986;González Amuchastegui and Serrano, 1996;2013;Serrano et al., 2009;2013;2015;Ortega & Martín, 2011;Ortega et al., 2013b;).The climate is transitional Atlantic-Mediterranean, and the area constitutes an important fluvial and environmental threshold between the Cantabrian and Ebro basins (Serrano et al., 2015).
This part of the Cantabrian Range is geologically identified as the outcome of a series of Alpine orogeny thrusts, and it marks the geological border between the Ebro and Duero Cenozoic foreland basins (Capote et al., 2002; Figure 1).The lithology of the area is mainly composed of coastal and marine sediments of Triassic and Cretaceous age (Olmo Zamora et al, 1978;Olivé Davó et al., 1978; Figure 2).The Triassic sequence is located in the easternmost extreme of our study area, as part of the Rosio diapir structure, and made up of Keuper facies deposits, composed of clayey evaporites and dolomite breccias (López-Gómez et al., 2002; Figure 2).The Early Cretaceous sedimentary sequence lies in the NW part of the study area, coterminous with the Purbeck sediment facies.The lower-mid part of the Early Cretaceous sequence is composed of deltaic sandstones, the socalled 'Arenas de Utrillas' formation, while the upper part is marine Aptian limestones, marls and sandstones (Figure 2).The Late Cretaceous is found in the SE of the study area and is formed by limestones, dolomites, marls and calco-marl limestones (Figure 2).These deposits are the product of a generalized marine transgression prompted by regional subsidence at that time (Olmo Zamora et al, 1978;Olivé Davó et al., 1978;Martín-Chivelet et al., 2002).No Palaeogene or Neogene deposits were located.Finally, the identified Quaternary deposits are associated with glacial, fluvial, karstic and gravitational processes (Hazera, 1962;Olmo Zamora et al, 1978;Olivé Davó et al., 1978Serrano, 1996;Turú et al., 2007;Serrano et al., 2013).

Materials and methods
This study was carried out following a detailed geomorphological mapping and terrain analysis using GIS techniques, sedimentary description and fieldwork.The present study is focused on the Ojo Guareña multilevel cave system and the correlation with the Guareña River watershed.For the external landscape mapping, we utilized 1:10,000 (MapaCyL10-Castilla y León), 1:25,000 (MTN25), 1:33,000 aerial photos (ITACYL; ftp://ftp.itacyl.es/cartografia/03_FotogramasAereos/), 1:50,000 geological maps (Spanish Geological Survey (IGME)), orthophotos from the National Plan for Aerial Orthophotography (PNOA; National Geographical Institute of Spain (IGN)) and fieldwork.
A 2 m resolution DEM was combined with orthophoto images so as to create digital anaglyphs.The anaglyphs and the aerial photographs were used for 3D visual identification of the terrain landforms.These were digitalized using 1:5,000 topographic maps and lithological background, provided by geological maps, detailed geomorphological mapping, sedimentary characterization, and fieldwork.The legend for the geomorphological map was designed following the geomorphological guide proposed by the Spanish Geological Survey (Martín Serrano et al., 2004).The final map was edited at a scale of 1:25,000 (Main Map).

Geomorphological features
Apart from glacial landforms, already represented in the geomorphological map and deeply described in previous studies (Serrano and Gutiérrez, 2002;Turú et al., 2007;Serrano et al., 2016) (for more details see in supplementary data), we classified the following identified geomorphological features according to their morphogenesis:

Structural landforms
The main morphological features in the study area are forms controlled by bedrock structure, corresponding to regional orographic units of small or medium scale.The northern area (Castro-Valnera area, Fig. 2) is characterized by NW-SE fault scarps, affecting the Lower Cretaceous units, whereas in the south (Trema River area, Fig. 2), a series of imbricate thrusts runs E-W, affecting the Late Cretaceous units and forming the Mesa-Pereda syncline and Retuerta anticline.To the east of these structures, the Salinas de Rosío diapir is located (Olmo Zamora et al., 1978;Olivé Davó et al., 1978).By contrast, the sub-structural forms are the result of differential surface erosion and the Alpine structures which affect the Mesozoic bedrock, and principally consist of a series of WNW-ESE parallel slopes.These features are affected by the action of the local river network, extensive karst development and Late Pleistocene glacial activity (Eraso, 1965(Eraso, , 1986;;Ortega Valcárcel, 1974;González Pellejero, 1986;Gonzalez Amuchastegui and Serrano, 1996;2013;Serrano et al., 2009;2013;2015;Ortega & Martín, 2011;Ortega et al., 2013b).

Colluvial landforms
These kinds of landforms have been identified in the hill slopes and were formed by bedrock weathering and slope processes.In the slopes to the north of Ojo Guareña, we identified two classes of superficial deposits, scree deposits and debris cones (Figure 3).Scree or debris talus deposits are composed by broken angular coarse blocks at the foot of steep rock slopes that has accumulated through periodic rockfall (Figure 3(a)), whereas debris cones are composed of diamicton facies, including rounded clasts and water current geometries, suggesting that runoff and slope wash processes due to snow or ice melting could have played a major role (Figure 3(b,c)).

Fluvial landforms
These are formed by the fluvial action of the Upper Ebro local drainage network of the Guareña, Trema and Trueba rivers.We divided them into two groups.(I) depositional landforms, such as alluvial fans and floodplain deposits; and (II) erosive landforms, such as strath terraces and active channels.The alluvial fans in our study area consist of small cone-shaped masses of sediments a few tens of meters long, composed mainly of coarse, clastic detrital materials built up by high-energy local watercourses (Ulemas, Redondo and Peñanegra rivers, and the Tajo, San Miguel, Cueva, Quintanilla and Entrambosríos streams).They are comprised of rounded boulders, pebbles, sands and a red silty-clay matrix.The thickness ranges from 2-3 m up to 6-7 m, with a massive structure, of reddish color, and composed of 10-15% blocks, 70-75% stones, and fine materials.Strath terraces are formed first through levelling by lateral fluvial erosion, and later by river downcutting through bedrock, which is covered by only a thin layer of sediment about 1-4 m thick.

Karst landforms
The OG is a voluminous karst system and stands out as one of the biggest cave networks in Europe (Grupo Espeleológico Edelweiss 1986; Figure 5).It is a multilevel cave system, with 14 identified caves, interconnected by an extended network of conduits and subhorizontal passages approximately 110 km long (Figures 5 and 7).This karst was formed by the erosive action of the Guareña River and the Villamartín Stream.The Guareña River constitutes a blind valley which ends abruptly in the Ojo Guareña solution sinkhole.Solution sinkholes are produced by lowering of the ground surface due to corrosion of the exposed Table 1.Sequence of the identified terrace, karst and polygenetic levels in our study zone.Serrano (1996) This bedrock (Gutierrez et al., 2014;Parise, 2019).In case of Ojo Guareña sinkholes are due to the lowering of the Guareña river bed by solution around a sinkhole.The current discharge ground waters of OG are downcutting across the entire system composed of highly permeable dolomites, and they reemerge downstream at the surface in karst springs of the Trema River (Saenz, 1933;Eraso, 1965Eraso, , 1986;;Grupo Espeleológico Edelweiss, 1986;Ruiz et al., 2009;Ortega et al., 2013b).This Quaternary groundwater action formed a long system of subhorizontal galleries, divided into 8 levels.Several disconnected cavities have been identified at higher elevations in the vicinity of OG (Figure 2).The highest cavity is 2 km NW of the village of Villamartín and it developed from a relict sinkhole, situated +200-210 m above the local base level.This level is probably the oldest and it is related to the 'El Ventanón' early sinkhole and the drainage flows from the Ñejuelos Cave.Below these caves, a subsequent cave level has been identified and corresponds to the Kaite Cave (845 m a.s.l.), whose horizontal conduit (cave floor) is located about +140 m above the Guareña River (Ortega et al. 2013b).Below the Kaite cavity, six more extended levels have been identified (Figure 6).The 1st level begins in the palaeo-sinkhole of the San Bernabé Cave and forms subhorizontal conduits running N-S, with phreatic and vadose morphologies.Fine-grained sediments are infilling the cave passages of this level.The altitude of the cave roofs, with phreatic morphologies, oscillates between 755 and 765 m a.s.l., located at the slope and +60-70 m above the current Guareña sinkhole, 1 km south of the shrine at San Bernabé Cave (Ortega et al., 2013b).The 2nd level is developed at +50-55 m, while the 3rd level is +40-45 m above the modern Guareña sinkhole.It is remarkable that at level 3, important and extensive sequences of stalagmites were identified, covering the cave passage in the Sala de las Huellas and the lower gallery of the San Bernabé Cave (Figure 6).The 4th is the most extensive level of the Ojo Guareña karst system and consists of long subhorizontal passages situated +20-30 m above the Guareña sinkhole (Figure 6).These passages show relict roofs with phreatic and vadose morphologies such as canyons and keyholes, and important collapses of ceilings.In some areas, this level is totally covered and filled by autochthonous-allochthonous sediments and speleothems (Cacique Gallery), while in other sectors it displays notable episodes of downcutting (Sala de las Pinturas).Finally, two more levels were identified.The 5th level mainly presents phreatic morphologies and is located +10 m above the Guareña sinkhole, while the 6th is the shortest cave level and mostly presents phreatic morphologies (Ortega et al. 2013b).Frequently, during strong flooding, the three lowest levels are completely filled up by groundwater from the Guareña River.The surge enters OG from the Guareña sinkhole, stops at the impermeable Coniaciean calcareous strata and starts filling up levels 5 and 6, sometimes reaching the 4th level.
Summarizing, the principal network of OG exceeds 110 km in length, and it ranges vertically over 200 m (Grupo Espeleológico Edelweiss, 1986;Ortega et al., 2013b).The genesis of the local karst system is closely related with the lithological contrast between the upper layer of dolomites and the impermeable lower layer of clayey limestones, but also with the hydrogeological evolution of the Guareña River and fluvial incision by the Trema River, mainly driven by climatic fluctuations.This gave rise to the eight levels of the local karst system, with the oldest level situated +200-210 m above the Guareña sinkhole, and the most recent level reaching the current local base level (Martín Merino, 1986).

Polygenetic landforms
Erosion surfaces and rounded summits are degraded and reworked residual reliefs.These landforms are old polycyclic depositional and/or erosional polygenetic plains configured under steady-state conditions, modified by climatic, tectonic, lithologic, erosional, terrain morphology, and soil factors (Pérez-González, 1994).These characteristics hamper mapping and dating them (Stokes et al., 2018).These features are located at the highest altitudes of our study area, forming the Montes del Somo and Valnera mountain range.Their poor preservation and the lack of correlative sediments render the sequence classification very difficult.
Erosive pediments (R.E.), colluvial flow pediments, polygenetic erosive slopes (G.E.), and conical hills (Figure 7), are residual landforms associated with degradation and/or the replacement of landforms produced by polygenetic processes, whether these are fluvial and/or structural and/or glacial and/or periglacial, located in the piedmont and slopes of the Redondo Valley.The G.E. are a very gently inclined bedrock surface, formed by the erosive action of the local stream system of the Trema River.The maximum length of the low relief landforms can reach about 100 m.They are located on the northern hillside of the Sotoscueva Valley, between altitudes of 650-750 m a.s.l.

Morphological setting
According to the geomorphological mapping, the main characteristics of the landforms in the study  1).
The local karst system is composed of 8 cave levels: 6 levels of subhorizontal galleries at OG (+60-70 m, +50-55 m, +40-45 m, +20-30 m, +10 m and the current base level of the cave passage), and 2 higher levels disconnected from OG: El Ventanón-Ñejuelos Cave (+200-210 m) and Kaite Cave (+140 m) (Grupo Espeleológico Edelweiss, 1986;Ortega et al., 2013b; Table 1).This morphology indicates a genesis from a water table level with moderate hydraulic gradient, rather similar to the lowest karstic level through which the modern Guareña River floods (Grupo Espeleológico Edelweiss, 1986;Ruiz et al., 2009;Ortega & Martín, 2011;Ortega et al., 2013b).This layout suggests the existence of external base levels which would have controlled the internal water table level, whose action during the Quaternary would have gradually formed OG and the higher cavities.
Collating the geomorphological mapping data with the transverse cave information profile, we can test the connection between the identified local base levels and the Ojo Guareña karst levels.The oldest identified landforms of the surroundings correspond to the highest level (S.E.; see map), composed of an erosive, deformed and degraded planation surface.On the other hand, the lower erosive pediment sequence and the fluvial terrace staircase sequence (see map) make up a complex Quaternary landscape, formed by fluvial downcutting processes, glacial deposition and erosion, which shaped the karst topography.In the present work, we venture a first approach to correlating the external base level with internal karstic levels, based on the landform geometries and their relative heights above the modern rivers (Table 1).The oldest karst level (El Ventanón-Ñejuelos Cave, +200-210 m) is likely associated with the first erosive pediment (R.E.1) identified, while one of the highest erosive pediments (R.E.2) lying +185 m above the modern river is related to the level of the Kaite Cave (+140 m).Next, the 1st karst system level (+60-70 m) matches up with one of the consecutive erosive pediments G.E.1 (+93 m) or G.E.2.(+63 m).On the other hand, the 5 successive lower karst system levels can reasonably be linked with the sequential fluvial terraces at +50-55 m (T1), +40-45 m (T2), +26-28 m (T3), +21-23 m (T4), +11-13 m (T5), and +6-8 m (T6).This correlation could be as follows: the 2nd level with T1, 3rd level with T2, the 4th level with T3 and T4, and the 5th level with T5 and T6.The extensive level 4 (+20-30 m) is characterized by several episodes of downcutting and filling, and it seems to match the two consecutive terraces T3 and T4.In the external area, these two levels of terrace could be differentiated, while in the internal area they are not identified.Finally, the floodplain and fluvial valley floor are associated with the lowest karstic level (Figure 6).The incision of these old base levels has been controlled by the incision of the drainage network, currently represented by the Guareña and Trema rivers (Nela tributaries), belonging to the Upper Ebro basin.Thus, these ancient base levels present altitudes equivalent to the fluvial terraces described for the Trueba River.and 397 ka, respectively.On the other hand, the fluvial terraces at +50-55 m and +40-45 m, which are related with karstic levels 2 and 3, have been calculated at 738 and 684 ka, respectively.Moreover, the base level corresponding to the polygenetic level G.E.1 (+93 m) and the karstic level 1 could be placed at the end of the Early Pleistocene, around 952 ka (Benito-Calvo et al., 2022).Finally, the polygenetic levels R.E.1 (+210 m), R.E.2 (+185 m), correlated respectively with the Kaite and El Ventanón levels, would have developed at chronologies older than 1.2 Ma, age calculated for the Upper Ebro terrace T3 (+166 m) (for more details see in supplementary data).

Conclusions
Using GIS techniques, spatial datasets and field work, we performed a detailed geomorphological mapping at a 1:25,000 scale, which contains wide-ranging information about the evolution of the Quaternary landscape encompassing the Ojo Guareña karst system.The landscape evolution in this area is defined by thirteen base levels, consisting of one erosion surface level, six pediments (+210 mm, +185 m, +93 m, +63 m, +50 m and +40 m) and seven fluvial terraces preserved in the Guareña River and the Trema and Trueba valleys (+2-4 m, +6-8 m, +11-13 m, +21-23 m, +26-28 m, +40-45 m, +50-55 m).The pediments at +50 m and +40 m are probably related to fluvial terraces at +40-45 m and +50-55 m, respectively.These levels are related to the downcutting of the regional hydrological network and its correlation in altitude with the subhorizontal levels of the Ojo Guareña karst system.The Ojo Guareña fluviokarst is traversed and controlled by the Guareña and Trema river base levels.The higher pediments located at +210 m, +185 m and +93 m are probably related to the endokarstic passages of the El Ventanón (+200-210 m) and Kaite (+140 m) levels and level 1 (+60-70 m) of OG, while the lower passages seem to correlate to the six fluvial terraces described in the fluvial valleys.In future work, these base levels will be correlated with the regional geomorphological evolution of the Upper Ebro basin.A chronological study of the base levels and the endokarstic deposits will be undertaken.

Software
To conduct this mapping, we used the software and hardware of the Digital Cartography and 3D Analysis Laboratory at the CENIEH.Specifically, we used the application ArcGIS 10.8 to manage and analyse vectorial and raster datasets corresponding to different thematic layers.We also used the program Erdas Imagine 2011 to create digital anaglyph models for analysing terrain landforms.We used the 10.8.1.ArcGIS and Corel Draw software to create each map and the synthetic map.The legend was made in Corel Draw as vector format and directly imported to ArcGIS.

Figure 1 .
Figure 1.(A) Location of the study area in the central Iberian Peninsula (Spain).The red box indicates the work area.(B) Simplified geological map modified from Alvaro et al. (2001).The grey box indicates the study area.

Figure 3 .
Figure 3. (A) Colluvial deposits on the slopes of the La Churra mountain.(B) Scree deposits and (C) Debris flow deposits.The yellow arrows indicate the slope direction, the dotted red line the limits of the drainage basin and the dotted blue line the local stream.Picture coordinates (ETRS89 UTMH30N): (A) X:442917, Y:4768837; (C) X:446625, Y:4770003.

Figure 7 .
Figure 7. Panoramic view of the erosive surface and pediments in the Guareña Valley.