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

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

Theodoros Karampaglidis, Alfonso Benito-Calvo, Ana Isabel Ortega- Martínez, Miquel Ángel Martín-Merino & Laura Sánchez-Romero

To cite this article: Theodoros Karampaglidis, Alfonso Benito-Calvo, Ana Isabel Ortega-Martínez, Miquel Ángel Martín-Merino & Laura Sánchez-Romero (2022): Landscape evolution and the karst development in the Ojo Guareña multilevel cave system (Merindad de Sotoscueva, Burgos, Spain), Journal of Maps, DOI: 10.1080/17445647.2022.2128907

To link to this article: https://doi.org/10.1080/17445647.2022.2128907

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SCIENCE

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

Theodoros Karampaglidis ^a,b, Alfonso Benito-Calvo^c, Ana Isabel Ortega-Martínez^c,d, Miquel Ángel Martín- Merino^eand Laura Sánchez-Romero^f

aDepartment of Archaeology, the Hebrew University of Jerusalem, Jerusalem, Israel;^bDepartment of Geological Engineering and Mining, Faculty of Environmental Sciences and Biochemistry, University of Castilla-La Mancha, Toledo, Spain;^cCentro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain;^dFundación Atapuerca, Ibeas de Juarros (Burgos), Spain;^eGrupo Espeleológico Edelweiss (GEE), Diputación Provincial de Burgos, Burgos, Spain;^fHuman Evolution Research Center, University of California, Berkeley, CA, USA

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 afirst-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.

ARTICLE HISTORY Received 4 March 2022 Revised 4 August 2022 Accepted 20 September 2022

KEYWORDS

Geomorphological map;

landscape evolution; Ojo Guareña caves; karst;

Quaternary; GIS

1. 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 geo- chemical procedure of infiltration, dissolution and erosion of the bedrock from atmospheric water and carbonic acid through penetrablefissures (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 under- ground 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, 1987, 1991; White, 1988; Granger et al., 1997, 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 combi- nations 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.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrest- ricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CONTACT Theodoros Karampaglidis [email protected]

Supplemental map for this article is available online athttps://doi.org/10.1080/17445647.2022.2128907.

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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, 1965,1986;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., 2020, 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 well- defined: (i) OG is fairly well studied, mapped and documented; (ii) a staircase of the localfluvial system is preserved; and (iii) the glacial and tectonic activity are well documented.

2. 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 Med- iterranean 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 Pleisto- cene glacial activity (Eraso, 1965,1986;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 so- called ‘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 gener- alized marine transgression prompted by regional sub- sidence 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., 1978 Serrano, 1996;Turú et al., 2007; Serrano et al., 2013).

3. Materials and methods

This study was carried out following a detailed geomorphological mapping and terrain analysis using GIS techniques, sedimentary description and ﬁeld- work. The present study is focused on the Ojo Guar- eña 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 (Span- ish 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 ortho- photo 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, andfieldwork. 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).

Whereas for the mapping of the Ojo Guareña karstic system we used GIS tools and CAD packages (Ortega et al., 2013a), combining previous

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geomorphological, speleogical and archaeological maps with ﬁeld observations (Grupo Espeleológico Edelweiss, 1986; Ortega & Martín, 1986; Ortega &

Martín, 2011; Ortega et al., 2013b, 2020, 2021) (for more details see in supplementary data).

4. 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:

4.1. 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, 1965,1986;Ortega Valcárcel, 1974;González Pellejero, 1986;Gonzalez Amuchas- tegui and Serrano, 1996; 2013; Serrano et al., 2009;

2013; 2015; Ortega & Martín, 2011; Ortega et al., 2013b).

4.2. 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 bro- ken 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)).

4.3. 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 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 fromAlvaro et al. (2001). The grey box indicates the study area.

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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, andfine 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.

We identiﬁed 3 strath terraces in the Guareña Val- ley (+2–4 m, +11–13 m and +21–23 m above the modern river), up to 7 levels in the Upper Trema Valley, located at +2–4 m, +6–8 m, +11–13 m, +21–23 m, +26–28 m, +40–45 m and +50–55 m, between the villages of Quisicedo and Redondo, and up to six levels in

the Trueba Valley (+2–4 m, +6–8 m, +11–13 m, +21–

23 m, +26–28 m and +50–55 m) (Figure 4), close to the locality of Espinosa de los Monteros. The settled sediments are formed of rounded boulders, cobbles, and a sparse light red sandy-silt–clay matrix of polymictic composition, whose thickness varies from 1 m to 4 m, with a massive structure. According to Serrano (1996), at the frontal moraine complex (760 m a.s.l.), located in Espinosa de los Monteros (Serrano, 1996;

Serrano and Gutiérrez, 2002;Turú et al., 2007;Serrano et al., 2011), a system of four stepped strath terraces in the Trueba Valley (TI, T-II, T-III, TIV) was identiﬁed.

These forms are well-preserved, corresponding to fluvial-glacial deposits of heterometric and polygenetic rounded blocks, boulders and gravels in a sandy silt matrix. In this area, we used the cartography ofSerrano (1996)as base map. During our work, two further levels were identified, one 2.3 km SE of Figure 2.Geological map of the study area, modified fromOlmo Zamora et al. (1978)andOlivé Davó et al. (1978). Legend: (1) Upper Triassic; Clay, marl and gypsum, (2) Lower Cretaceous; Sandstone, clay-sandstone, marl-sandstone, and carbonatic sandstone, (3) Lower Cretaceous; Limestone, clay-limestome, marl-limestone and limestone with foraminiferans, (4) Upper Cretaceous;

Sandstone, clay-sandstone, marl-sandstone, and carbonatic sandstone, (5) Upper Cretaceous; Limestone, clay-limestome, marl- limestone and limestone with foraminiferans, (6) Quaternary, Boulders, cobbles, pebbles, sand, and clay, (7) Anticline, (8) Syncline, (9) Fault, (10) Diapiric escarpment, (11) Karst spring, and (12) Caves. The purple areas show the local villages.

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Espinosa de los Monteros, an intermediate level at +21–23 m situated between T-II and T-III; and a second level at +6–8 m, located 1.1 km SE of Espinosa

de los Monteros, between T-III and T-IV. Thus, the combined terrace sequence identiﬁed in this work is made up of seven terraces, situated at +2–4 m (T7), Figure 3.(A) Colluvial deposits on the slopes of the La Churra mountain. (B) Scree deposits and (C) Debrisﬂow 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.

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+6–8 m (T6), +11–13 m (T5), +21–23 m (T4), +26–28 m (T3), +40–45 m (T2) and +50–55 m (T1) (Table 1).

Further, there are a few strath terraces that do not pre- serve their alluvial cover. In this case, the terraces are poorly preserved, and they are located in the Trueba Valley, around 2 km SE of Espinosa de los Monteros (+26–28 m), while in the Trema Valley one level has been identiﬁed (+6–8 m), about 1.5 km SW of the settlement of Quintanilla del Rebollar. Finally, the ﬂoodplain and active channels are located at the current local base levels, and composed of highly per- meable sands, silts and clays, dark in color and containing organic matter.

4.4. 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 5and7). This karst was formed by the erosive action of the Guareña River and the Villamar- tín 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 Figure 4.Strath terraces found in the valley depressions of the localities of Espinosa de los Monteros and Quintanilla Sotoscueva.

Picture coordinates (ETRS89 UTMH30N): X:448965, Y:4766611 (south of Quintanilla de Rebollar).

Table 1.Sequence of the identiﬁed terrace, karst and polygenetic levels in our study zone.

Serrano (1996) This work Ortega et al. (2013a)

Fluvial levels Polygenetic levels Karst levels

Trueba River Guareña Stream Trema River Trueba River OG Other local karst levels

S.E.

R.E.1 (+210 m) Ñejuelos-El Ventanon (+200–210 m)

R.E.2 (+185 m) Kaite (+140 m)

G.E.1 (+93 m) 1 (+60–70 m) G.E.2 (+63 m)

T-I (+50–55 m) T1 (+50–55 m)* T1 (+50–55 m)* G.E.3 (+50 m) 2 (+50–55 m) T2 (+40–45 m)* G.E.4 (+40 m) 3 (+40–45 m) T3 (+26–28 m)* T3 (+26–28 m)* 4 (+20–30 m) T-II (+21–23 m) T4 (+21–23 m)* T4 (+21–23 m)* T4 (+21–23 m)*

T-III (+11–13 m) T5 (+11–13 m)* T5 (+11–13 m)* T5 (+11–13 m)* 5 (+10 m) T6 (+6–8 m)* T6 (+6–8 m)*

T-IV (+2–4 m) T7 (+2–4 m)* T7 (+2–4 m)* T7 (+2–4 m)* 6 (Actual) T8 (Actual) T8 (Actual) T8 (Actual)

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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 per- meable dolomites, and they reemerge downstream at the surface in karst springs of the Trema River (Saenz, 1933;Eraso, 1965,1986;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 Villa- martín 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 sub- sequent 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 andfilled by autochthonous-allochthonous sediments and speleothems (Cacique Gallery), while in other sec- tors 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 completelyﬁlled up by groundwater from the Guareña River. The surge enters OG from

the Guareña sinkhole, stops at the impermeable Con- iaciean 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 hydrogeo- logical 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).

4.5. 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 correla- tive sediments render the sequence classification very difficult.

Erosive pediments (R.E.), colluvialﬂow 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 perigla- cial, 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 maxi- mum 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.

In our work area, a wide sequence of polygenetic erosive landforms was found, and they are inset on the Guareña River, R.E.1 (+210 m), R.E.2 (+185 m), G.E.1 (+93 m), G.E.2 (+63 m), G.E.3 (+50 m) and G.E.4 (+40 m).

5. Morphological setting

According to the geomorphological mapping, the main characteristics of the landforms in the study

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Figure 5.Ojo Guareña karstic landforms. (A) The Villamartín Stream going under Sima Dolencias (Dédalo Oeste Sector, fourth level, at its base); (B) Sala Berta is the largest conduit of the OG system (Dulla Sector, second and third levels); (C) Galería Estella (Este-Huesos Sector, fourth level): (D) Galería Principal, Gours de la Vía Seca (Central Sector, fourth level); (E) Cueva Palomera, the main entrance to Ojo Guareña (Central Sector, descends along an incline to the fourth level); (F) The waters of the Guareña and Trema rivers and Villamartín Stream reemerge, though only duringflashflooding, via La Torcona (Resurgencia Sector,fifth level).

Photos: M. A. Martín and F. Pino / G. E. Edelweiss.

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area are associated with structural, karstic,ﬂuvial and glacial processes. Among the morphologies described, a sequence of base levels has been deﬁned, made up of at least one erosion surface, 6 pediments lying +210 m, +185, +93 m, +63 m, +50 m, and +40 m above the Guareña River, and 7 terrace levels for the Guareña, Trema and Trueba rivers. The Guareña Valley pre- serves 3 terrace levels (+2–4 m, +11–13 m, +21–23 m), while in the Trueba Valley six terraces were mapped (+2–4 m, +6–8 m, +11–13 m, +21–23 m, +26–28 m, +50–55 m). Between these two, the

Upper Trema Valley contains the complete ﬂuvial sequence, consisting of seven terraces located at +2–

4 m, +6–8 m, +11–13 m, +21–23 m, +26–28 m, +40–45 m and +50–55 m above the modern river (Table 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 Figure 7.Panoramic view of the erosive surface and pediments in the Guareña Valley.

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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 Riverﬂoods (Grupo Espe- leológico 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 afirst 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 thefirst 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 valleyfloor 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.

The lack of numerical data, which is the objective of planned future work, makes the present study afirst preliminary chronological estimation of this mor- pho-sequence. Following the model by Benito-Calvo et al. (2022) of the continuous age-incision history for the Upper Ebro valley using such chronologies and statistical methods would putatively situate the study area fluvial terraces at +26–28 m and +21–23 m (correlated with the karstic level 4) around 545 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 Figure 6.Projected profile (N-S) of the six OG karst levels (Topography: G. E. Edelweiss, 1986. Modified byOrtega et al. [2013b]).

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Ma, age calculated for the Upper Ebro terrace T3 (+166 m) (for more details see in supplementary data).

6. Conclusions

Using GIS techniques, spatial datasets andﬁeld 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 thir- teen base levels, consisting of one erosion surface level, six pediments (+210 mm, +185 m, +93 m, +63 m, +50 m and +40 m) and sevenfluvial 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 hydrolo- gical 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 sixﬂuvial 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 vec- torial and raster datasets corresponding to diﬀerent thematic layers. We also used the program Erdas Ima- gine 2011 to create digital anaglyph models for analys- ing terrain landforms. We used the 10.8.1. ArcGIS and Corel Draw software to create each map and the syn- thetic map. The legend was made in Corel Draw as vector format and directly imported to ArcGIS.

Acknowledgements

The Environment Ministry of Castilla y León, the Culture Ministry of Castilla y León and the Provincial Government of Burgos furnished administrative support. Finally, special thanks to all the members of the Grupo Espeleológico Edel- weiss for their continuous work and support during the studies in the Ojo Guareña karst. AIOM has been supported by a postdoctoral grant from the Fundación Atapuerca.

Disclosure statement

No potential conﬂict of interest was reported by the author(s).

Funding

This work wasﬁnanced by the Fundación Villalar (Region of Castilla y León, Spain), as part of the research project:

‘Evolution of the landscape and evolution of the Ojo Guar- eña Karst System (Merindad de Sotoscueva, Burgos)’.

Data availability statement

The data supporting the ﬁndings of this study are freely available from: (1) National Center for Geographic Infor- mation (CNIG) at [http://centrodedescargas.cnig.es/

CentroDescargas/locale?request_locale = en#]; and (2) Spanish Geological Survey (IGME) at [http://info.igme.es/

cartograﬁadigital/geologica/Magna50.aspx].

ORCID

Theodoros Karampaglidis http://orcid.org/0000-0001- 5626-4548

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