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Geomorphological characteristics and slope processes associated with different basins: Mallorca (Western Mediterranean)
Geomorphology 52 (2003) 253 – 267 www.elsevier.com/locate/geomorph
Geomorphological characteristics and slope processes associated with different basins: Mallorca (Western Mediterranean) Bernadı´ Gelabert *, Joan J. Forno´s, Lluı´s Go´mez-Pujol Departament de Cie`ncies de la Terra, Universitat de les Illes Balears, Crta. Valldemossa, Km 7.5. 07071 Palma de Mallorca, Balearic Islands, Illes Balears, Spain Received 3 December 2001; received in revised form 5 August 2002; accepted 15 August 2002
Abstract We compare the different geomorphological processes which occurred in Pliocene – Quaternary times on two very similar slopes in Mallorca, one located at the Tramuntana Range and the other in the Llevant Ranges. Both slopes have the same geological structure, the same stratigraphic and lithological levels, and the same altitude and orientation. The different slope processes are due to the relationship between the accommodation space and the sedimentation rate in the adjacent basins: In the Valencia Trough (located N of the Tramuntana Range), the sedimentation rate has not been sufficient to fill the accommodation space, whereas in the Alcudia Basin (adjacent to the Llevant Ranges), the sedimentation rate has been sufficient to fill the accommodation space. This difference has resulted in major landslides on the Tramuntana slope, whereas the Llevant slope is characterized mainly by alluvial fans and debris screes. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Slope processes; Accommodation space; Normal faults; Landslides; Alluvial fans; Mallorca
1. Introduction One way of assessing interactions between tectonics, climate and surficial processes is to study the geomorphological characteristics of alluvial piedmonts of both active and inactive mountain fronts (Bull and McFadden, 1977; Bull, 1991). A sound knowledge of the relationships between tectonics, climate and surficial processes is crucial for an accurate interpretation of the geological record. Numerous links and feedbacks between tectonics, climate and surficial processes relating to long-term landscape evolution have been estab-
* Corresponding author. E-mail address: [email protected] (B. Gelabert).
lished (Beaumont et al., 1992; Small and Anderson, 1998; Brandon et al., 1998). However, the relative importance of one of these factors with respect to the two others continues to be debated (e.g. Ritter et al., 1995; Riebe et al., 2001). In this paper, we present an example of the different types of landscape and landforms resulting from the interaction between tectonics and surficial processes, regardless of the climate. Given the similar climatic characteristics of the two slopes studied, we sought to assess the interactions between tectonics and surficial processes, and to account for the differences in the slope processes despite the similarities existing between the slopes. The purpose of this paper is to compare the different processes occurring simultaneously on similar
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(02)00260-X
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slopes in Mallorca. We compare the dynamics of two slopes, one located in the N of the Tramuntana Range, the other in the NE of Mallorca, in the Llevant Ranges. Both slopes have the same altitude, geological structure, stratigraphy and lithology, climatic conditions, vegetation and orientation. Despite these similarities, the slope of the Tramuntana Range displays major landslides and debris flows whereas the Llevant Ranges slope exhibits debris sheets and alluvial fans.
2. Study area The island of Mallorca covers an area of 3640 km2 and is located in the middle of the Western Mediterranean. It has a typical Mediterranean climate with hot dry summers and mild wet winters. The mean annual temperature is approximately 17 jC, with mean winter and summer values of 10 and 25 jC, respectively; the mean annual precipitation is about 500 mm and is mostly concentrated in autumn (Guijarro, 1986). The vegetation is typically Mediterranean with two clear community types: holm oaks, Cyclamini-Quercetum ilicis, with boreal characteristics abundant at the lowest altitudes and macchia and garrigue bushes, Oleo-Ceratonion, Hypericion balearici, RosmarinoEricion mainly in the drier lowlands (Bolo`s, 1996). The Balearic Islands constitute the emerged part of the Balearic Promontory (Fig. 1), which extends from Cabo de la Nao (SE of the Iberian Peninsula) as far as the Liguro-Provencßal Basin, in a NE direction for 440 km. The Balearic Promontory corresponds to the northeast prolongation of the Neogene Betic System in southern Spain and is bounded by three basins: the Valencia Trough to the NW, the North African Basin to the S and the Liguro-Provencßal Basin to the NE. The Balearic Promontory consists of Paleozoic to Middle Miocene materials deformed in a thrust and fold system during the Late Oligocene – Middle Miocene (Fallot, 1922; Darder, 1925; Rangheard, 1972; Bourrouilh, 1983; Sabat et al., 1988; Gelabert et al., 1992). The recent ESCI seismic reflection profile (location in Fig. 1) shows the crustal structure of the Balearic Promontory (Sa`bat et al., 1995). From the ESCI profile (Fig. 2) and the geometry and acoustic properties of the upper reflectors, Sa`bat et al. (1995)
deduced the existence of three minor sub-basins with Middle Miocene to Quaternary sediments below a water layer of 0.5 or 1 s. (375 or 750 m), bounded by two basement highs in the NW and SE. This has many similarities with observations made on land: the overall structure of Mallorca consists of a set of horsts and grabens (Fallot, 1922; Ramos-Guerrero et al., 1989) which are bounded by Upper Miocene– Quaternary listric normal faults (Gelabert, 1998). The horsts are segments of a Lower Miocene thrust and folded belt that constitutes the Ranges, which are composed mainly of carbonate deposits varying in age from Carboniferous to Middle Miocene. Three main ranges can be distinguished from SE to NW (Fig. 1): the Llevant Ranges, the Central Ranges and the Tramuntana Range. The grabens constitute the basins and are filled with Upper Miocene –Quaternary sediments. The island displays three main morphotectonic divisions on account of its geological structure: the Tramuntana Range, the Central Zones and the Llevant Ranges. The Tramuntana Range forms the northwest side of the island and comprises several tectonic zones (Fallot, 1922; Alvaro and Del Olmo, 1984; Gelabert, 1998), mainly built of Mesozoic sediments, cut by longitudinal valleys, karstic canyons and poljes, and with a spectacular cliffed coast. The Central Zone is composed of subsiding basins, and the Llevant Ranges in the southeast constitute a series of low mountains and hills mainly made up of Jurassic limestones and dolomites. Focusing our attention again on the upper part of the ESCI seismic reflection profile, the base of the Pliocene –Quaternary package produces a reflection of great intensity and lateral continuity (Fig. 2) and the reflectors corresponding to both the Pliocene– Quaternary and the Middle – Upper Miocene sediments thicken at the center of the sub-basins. In southeasternmost sub-basin (which onland is equivalent to the basin adjacent to the Llevant Ranges), the upper reflector, which corresponds to the sea bottom, remains horizontal, whereas in the northwesternmost basin (which onland is equivalent to the basin adjacent to the Tramuntana Range), the upper reflector has a gentle slope to the NW, towards the center of the Valencia Trough. According to Sa`bat et al. (1995), normal faults dipping either NW or SE, with small displacements, deform the Middle and Late Miocene sediments, and compressive structures are unconform-
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Fig. 1. Topographic map of Mallorca, showing the main streams and location of the slopes studied and the seismic reflection profiles. 255
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Fig. 2. The part of the ESCI seismic reflection profile corresponding to the Balearic Promontory. Reflection identification according to Sa`bat et al. (1995).
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ably overlain by Upper Miocene and younger sediments.
3. Geomorphological processes on the slope in the Llevant Ranges 3.1. Description of the area The slope studied is located along the southeastern coast of Alcudia Bay, in the Llevant Ranges, in the northeastern peninsula of Mallorca (Fig. 1). In this area, the boundary between the Llevant Ranges and Alcudia Bay is a NE – SW striking, NW dipping normal fault, the hanging wall corresponding to the Alcudia Bay block. The southeastern flank of Alcudia Bay is characterized by a piedmont zone made up of Quaternary alluvial sediments that occur as an array of coalescent alluvial-fan complexes and coastal dunes (Figs. 3 and 4), which provide evidence for fluvial activity over a
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period spanning the last 140 ka (Rose et al., 1999, from optically stimulated luminescence—OSL-age estimation). Different catchment sections have been described by Rodrı´guez-Perea (1998), Rose and Meng (1999) and Rose et al. (1999) southward and by Go´mez-Pujol (1999) northward. Both catchments reflect interactions between beach and dune deposits with different fan deposits (debris flows, sheetfloods, lags, etc.) or red/brown soils. The geometric relations between sedimentary bodies and thickness are variable and are closely related to the basement or old dune topography. Sections of the coast show bioclastic eolianite at the base into which a shore platform was cut during the Last Interglacial (OIS 5e) (Rose and Meng, 1999). From this level, the section changes to more continental (fluvial) sediments, mainly sheetflood- and lag deposits, which are interbedded at least with one yellowish red paleosol. Henceforth, lateral variations are more important, and the sections can range from beach deposits to bioclastic sands reworked by fluvial processes with some clasts and
Fig. 3. Geomorphological map of the slope in the Llevant Ranges. The main geomorphological features are the alluvial fans and the talus scree.
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Fig. 4. Photography and cross-section showing the alluvial-fan complexes on the slope in the Llevant Ranges. Location of cross-section in Fig. 3.
some erosive channels or eolianite levels, and can appear completely overlain by fanglomerates. The top section usually finishes with a new fan deposit episode. This synthesis section summarizes all the features of the coastal cliffs from the Sa Canova alluvial fan to the Es Calo´ alluvial fan, with the exception of the Es Barracar alluvial fan and surroundings. At this location, the exposures are made up of alluvial and colluvial sediments and eolianite outcrops can be identified only at the base of the cliff. Alluvial fan geometric features and the inland section are described at the Es Calo´ alluvial fan; the fan evolution was established by mapping fan surfaces, correlating them on the basis of soil development and the
interactions with paleoshoreline features (Fig. 5). Three main accretion phases with their respective dissection phases interbedded with two dune formations were characterized. Similar evolution and phases have been described for coastal fans in the southeastern Iberian Peninsula by Harvey et al. (1998) and Segura (1990). At the present time, fluvial activity takes the form of storm-generated flood events capable of transporting gravels with individual clasts attaining boulder size. During cooler episodes of the Quaternary, these river catchments extended much further northwestward across what is now Alcudia Bay (Van Andel, 1989).
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Fig. 5. Es Calo´ alluvial-fan (location in Fig. 3), with seven stratigraphic sections and two cross-sections.
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Quaternary unit fades abruptly into a less reflective and prograding unit of possible Pliocene age.
3.2. Alcudia basin In order to study the relationship between the Alcudia Basin and the Llevant Ranges slope, we describe the geometrical properties of the Alcudia Basin and present a seismic reflection profile shot at Alcudia Bay. The Alcudia Basin has a very gentle slope of 0.3% dipping NE and water depths less than 50 m. This flat bathymetry coincides with the horizontal position of the upper reflector at the southeasternmost sub-basin deduced from interpretation of the ESCI seismic reflection profile (Fig. 2). The seismic profile of the Alcudia Basin was shot during the oceanographic cruise ‘‘Geocarbal-85’’. It has a SE – NW orientation, perpendicular to the slope studied (location in Fig. 1). This is a high-resolution sparker seismic reflection profile from the NE end of Mallorca (Acosta et al., 1986). Given that no well data are available for the region, reflector identification is based on acoustic similarities to published profiles (Alla et al., 1971; Montadert et al., 1978) from the Western Mediterranean. In the seismic profile, the uppermost reflector, corresponding to the sea bottom, is horizontal; the upper reflectors of great intensity and lateral continuity correspond to undisturbed Quaternary sediments (Fig. 6). Along this profile, the
4. Geomorphological processes on the slope in the Tramuntana Range 4.1. Description of the area The slope of the Tramuntana Range is steeper and narrower than the slope in the Llevant Ranges despite the similarity in altitude (600 versus 500 m), orientation (NE –SW, both), geological structure and stratigraphic and lithological sequences forming the mountain slope. The slope corresponding to the Tramuntana Range (Fig. 7) consists of major sea cliffs (300 m or even more) with some rockfalls at the base. The most outstanding geomorphological features of this area are the major landslides shown in Fig. 8. The surface affected by these land movements is between 1 and 2 km2. The landslides are rotational with several break surfaces. We present cross-sections (Fig. 9) of two landslides (Ba`litx and Sa Costera) which are visible in Fig. 7. The Ba`litx landslide is a multiple rotational landslide affecting the Liassic (Lower Jurassic) limestones and the marls and dolomites of the Upper Triassic. According to Mateos (2000), the surface affected by this
Fig. 6. NW – SE Geocarbal-85-B seismic reflection profile, SE of the Alcudia Bay.
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Fig. 7. Photograph showing a major landslide on the slope in the Tramuntana Range. The back scar and the lateral flanks are almost vertical, forming a 100 m high cliff.
Fig. 8. Geomorphological map of the slope in the Tramuntana Range. The main geomorphological features are the major landslides and sea cliffs.
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Fig. 9. Cross-sections of the main landslides mapped in Fig. 8.
landslide covers an area of 1.2 km2, the depth of the basal break surface is 230 m below sea level and the volume of mobilized material is over 200 106 m3. The value of the imaginary axis of this rotation is 750 m and the displacement of the mobilized mass 150 m. The back scar and the lateral flanks of this landslide (Figs. 7 and 9) are very steep, forming almost vertical walls that are approximately 100 m high. The Sa Costera landslide is also a multiple rotational landslide affecting the Lower Jurassic limestones and the Upper Triassic marls and dolomites. The surface affected covers an area of 3.1 km2, the depth of the main break surface is 150 m below sea level, the volume of mobilized material is 300 106 m3 and the minimum displacement of the mobilized mass is 125 m. The radius of the axis rotation is 700 m. The back scar of this landslide also forms vertical
walls 100 m high. The dimensions of these landslides are better reflected in Fig. 7, where we present an aerial photograph of both, Ba`litx in the foreground and Sa Costera, in the background. Drainage basins on the Tramuntana slope are much larger than those on the slope in the Llevant Ranges: The Na Mora and Sa Coma stream drainage basins occupy areas of 26 and 25 km2, respectively. Both streams are incised in the Jurassic limestones and their drainage basins are characterized by the predominance of karstic processes and morphologies. These episodic streams form submerged prograding sedimentary bodies at their mouths. The upper slopes of the catchments are composed of hard, joined Lower Jurassic limestones, whereas the lower slopes consist of lithified Quaternary gravels and partially lithified soils.
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4.2. The Valencia Trough adjacent to the Tramuntana Range We describe the main characteristics of the part of the Valencia Trough adjacent to the Tramuntana Range and present a seismic reflection profile of this area in order to relate onshore and offshore processes. In the littoral zone between Valldemossa and Lluc (location in Fig. 1), offshore areas with considerable accumulation of loose sediment are related to onshore areas in which major landslides have been mapped (Mateos, 2000). The sea bottom has a steep gradient near the coast (gradient of 17.5%) and is constituted by the accumulation of big boulders due to rockfalls.
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Offshore (at some distance from the coast), the bottom gradient is lower and coarse, medium and fine sands outcrop. The Valsis-821 seismic profile was shot in November 1988 over 150 km from Barcelona to the northern coast of Mallorca (location in Fig. 1). The southeasternmost part of this seismic profile shows the offshore continuation of the onshore structure of the Tramuntana Range (Fig. 10). For the seismic interpretation, the different seismic boundaries observed along the profile have been interpreted from well data (Lanaja, 1987) and published data (Fontbote´ et al., 1990; Roca and Desegaulx, 1992; Torres et al., 1993). The interpretation of
Fig. 10. Valsis-821 seismic reflection profile, showing aggradation and propagation of reflectors in Plio-Quaternary sediments at the SE border.
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the southeasternmost part of the Valsis-821 seismic profile (Fig. 10) consists of two pre-Pliocene extensional faults, dipping NW, covered by unconformably overlying Plio-Quaternary sediments. At the SE border (i.e. NW of the Tramuntana Coast), aggradation and progradation of reflectors corresponding to PlioQuaternary sediments are visible.
5. Discussion The slopes of the Tramuntana and Llevant Ranges are located at mountain fronts bounded by NE –SW striking, NW dipping, pre-Pliocene normal faults, which separate the Ranges from the adjacent basins (located on the hanging wall). Both slopes have
similar characteristics as regards altitude, structure, lithological succession and orientation despite showing different slope processes and morphologies. Other differences remain in the adjacent basins: (1) the geomorphological context in which the two littoral range-fronts are located is not the same: the Tramuntana front is located in an open-sea context (adjacent to the Valencia Trough), and the Llevant front in the lateral margin of Alcudia Bay (a semi-confined context); (2) at Alcudia Bay, the source of sediment is not exclusively from the slope studied but also is from the streams in central Mallorca that discharge into the Bay (Fig. 1). By contrast, in the Valencia Trough (offshore Tramuntana), the sediment discharge proceeds solely from the Tramuntana streams (with drainage areas that are smaller than those in Central Mallorca).
Fig. 11. (A) Sketch of the geomorphological processes on the slope in the Llevant Ranges, with a sedimentation rate sufficient to fill the created accommodation space. Alluvial fan and debris scree occur onshore. Offshore, sea grass sedimentation with major terrigenous influx predominates. (B) Schematic cross-section of the slope in the Llevant Ranges.
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The geomorphological context (open-sea context versus semi-confined context) exerts an influence on the accommodation space: the bigger and deeper the basin, the greater the amount of sediments that can be accommodated. It is obvious that the Valencia Trough is a much bigger and deeper than the Alcudia Basin and therefore has a larger accommodation space. On the other hand, the different sources of sediment and the different drainage areas for the streams feeding the two basins exert an influence on the sedimentation rate in both basins. Given the similar climatic conditions, the sedimentary load carried by the streams entering the Alcudia Basin is larger than that of the streams entering the Valencia Trough from the Tra-
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muntana because of the larger drainage basin. Therefore, the relationship between the accommodation space versus the sedimentation rate in the adjacent basins is fundamental in understanding the slope processes of the two littoral range-fronts. If the sedimentation rate is sufficient to fill the accommodation space (Fig. 11), as suggested for Alcudia Bay, the basin shallows and the bathymetry flattens. Given that Alcudia Bay emerged during the lower sea levels of the Quaternary, small lateral inputs from the slope studied cannot be accommodated, giving rise to the formation of alluvial fans (Figs. 3, 4 and 5). The flat bathymetry of Alcudia Bay is due to the small size of the basin (approximately 50 km2) and
Fig. 12. (A) Sketch of the geomorphological processes on the slope in the Tramuntana Range, with a sedimentation rate insufficient to fill the created accommodation space. Major landslides occur onshore. Offshore, prevalence of red algae sedimentation with minor terrigenous influx. (B) Schematic cross-section of the slope in the Tramuntana Range.
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to the considerable terrigenous discharge from the streams entering Alcudia Bay from Central Mallorca, with drainage areas exceeding 450 km 2. If the sedimentation rate (Fig. 12) is insufficient to fill the accommodation space, as in the Valencia Trough, the sediments discharged from the Tramuntana streams accumulate as submarine bodies displaying aggradation and progradation (as in the seismic profile of Fig. 10), whereas material coming from major landslides accumulates at the sea bottom, at the foot of the slope. Major landslides occur on the slope owing to the unstable slope gradient. Break surfaces of major landslides continue below sea level, and probably reach the sea bottom. The ESCI seismic reflection profile (Fig. 2) reflects the main characteristics of the two basins studied. The upper reflectors are horizontal in the basin adjacent to the Llevant Ranges whereas they dip some degrees in the part of the Valencia Trough adjacent to Tramuntana Range, suggesting that the sedimentation rate was sufficiently high in the former basin to fill the accommodation space. In the latter basin, the sedimentation rate was insufficient to fill the accommodation space and thus sediments only cover the basin floor, with a final dip resembling the initial dip of the basin floor. The final morphology of the basins, which depends on the accommodation rate/sedimentation ratio, influences not only the processes occurring on lateral slopes but also the recent sediment formation in the Alcudia Basin and the Valencia Trough. Although the ‘‘in situ’’ biogenic sedimentation is similar, red algae sedimentation prevails in the NW basin because of the higher gradient of the bathymetric profile near the coast, whereas the more gentle bathymetric profile allows the extension of seagrass meadows in the shallow waters at Alcudia Bay.
6. Conclusions In the piedmonts of the NW flanks of the Tramuntana and Llevant Ranges, the relationship between the accommodation space and the sedimentation rate in the adjacent Valencia Trough and the Alcudia Basin governs the surficial processes that modify the local landscape. The slope in the Llevant Ranges, characterized by the alluvial fans and debris scree, is associated with
the Alcudia Basin, which has a higher accommodation/sedimentation ratio than the Valencia Trough. This Trough is related to the Tramunatana slope, which is affected by major landslides. We provide an example of interaction between a local tectonic phenomenon and a short-term, regionalscale landscape development. A clear definition of the roles of tectonics and the geomorphological context is necessary for a fuller understanding of the past and current behavior of landscapes.
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Geomorphological characteristics and slope processes associated with different basins: Mallorca (Western Mediterranean) Bernadı´ Gelabert *, Joan J. Forno´s, Lluı´s Go´mez-Pujol Departament de Cie`ncies de la Terra, Universitat de les Illes Balears, Crta. Valldemossa, Km 7.5. 07071 Palma de Mallorca, Balearic Islands, Illes Balears, Spain Received 3 December 2001; received in revised form 5 August 2002; accepted 15 August 2002
Abstract We compare the different geomorphological processes which occurred in Pliocene – Quaternary times on two very similar slopes in Mallorca, one located at the Tramuntana Range and the other in the Llevant Ranges. Both slopes have the same geological structure, the same stratigraphic and lithological levels, and the same altitude and orientation. The different slope processes are due to the relationship between the accommodation space and the sedimentation rate in the adjacent basins: In the Valencia Trough (located N of the Tramuntana Range), the sedimentation rate has not been sufficient to fill the accommodation space, whereas in the Alcudia Basin (adjacent to the Llevant Ranges), the sedimentation rate has been sufficient to fill the accommodation space. This difference has resulted in major landslides on the Tramuntana slope, whereas the Llevant slope is characterized mainly by alluvial fans and debris screes. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Slope processes; Accommodation space; Normal faults; Landslides; Alluvial fans; Mallorca
1. Introduction One way of assessing interactions between tectonics, climate and surficial processes is to study the geomorphological characteristics of alluvial piedmonts of both active and inactive mountain fronts (Bull and McFadden, 1977; Bull, 1991). A sound knowledge of the relationships between tectonics, climate and surficial processes is crucial for an accurate interpretation of the geological record. Numerous links and feedbacks between tectonics, climate and surficial processes relating to long-term landscape evolution have been estab-
* Corresponding author. E-mail address: [email protected] (B. Gelabert).
lished (Beaumont et al., 1992; Small and Anderson, 1998; Brandon et al., 1998). However, the relative importance of one of these factors with respect to the two others continues to be debated (e.g. Ritter et al., 1995; Riebe et al., 2001). In this paper, we present an example of the different types of landscape and landforms resulting from the interaction between tectonics and surficial processes, regardless of the climate. Given the similar climatic characteristics of the two slopes studied, we sought to assess the interactions between tectonics and surficial processes, and to account for the differences in the slope processes despite the similarities existing between the slopes. The purpose of this paper is to compare the different processes occurring simultaneously on similar
0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(02)00260-X
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slopes in Mallorca. We compare the dynamics of two slopes, one located in the N of the Tramuntana Range, the other in the NE of Mallorca, in the Llevant Ranges. Both slopes have the same altitude, geological structure, stratigraphy and lithology, climatic conditions, vegetation and orientation. Despite these similarities, the slope of the Tramuntana Range displays major landslides and debris flows whereas the Llevant Ranges slope exhibits debris sheets and alluvial fans.
2. Study area The island of Mallorca covers an area of 3640 km2 and is located in the middle of the Western Mediterranean. It has a typical Mediterranean climate with hot dry summers and mild wet winters. The mean annual temperature is approximately 17 jC, with mean winter and summer values of 10 and 25 jC, respectively; the mean annual precipitation is about 500 mm and is mostly concentrated in autumn (Guijarro, 1986). The vegetation is typically Mediterranean with two clear community types: holm oaks, Cyclamini-Quercetum ilicis, with boreal characteristics abundant at the lowest altitudes and macchia and garrigue bushes, Oleo-Ceratonion, Hypericion balearici, RosmarinoEricion mainly in the drier lowlands (Bolo`s, 1996). The Balearic Islands constitute the emerged part of the Balearic Promontory (Fig. 1), which extends from Cabo de la Nao (SE of the Iberian Peninsula) as far as the Liguro-Provencßal Basin, in a NE direction for 440 km. The Balearic Promontory corresponds to the northeast prolongation of the Neogene Betic System in southern Spain and is bounded by three basins: the Valencia Trough to the NW, the North African Basin to the S and the Liguro-Provencßal Basin to the NE. The Balearic Promontory consists of Paleozoic to Middle Miocene materials deformed in a thrust and fold system during the Late Oligocene – Middle Miocene (Fallot, 1922; Darder, 1925; Rangheard, 1972; Bourrouilh, 1983; Sabat et al., 1988; Gelabert et al., 1992). The recent ESCI seismic reflection profile (location in Fig. 1) shows the crustal structure of the Balearic Promontory (Sa`bat et al., 1995). From the ESCI profile (Fig. 2) and the geometry and acoustic properties of the upper reflectors, Sa`bat et al. (1995)
deduced the existence of three minor sub-basins with Middle Miocene to Quaternary sediments below a water layer of 0.5 or 1 s. (375 or 750 m), bounded by two basement highs in the NW and SE. This has many similarities with observations made on land: the overall structure of Mallorca consists of a set of horsts and grabens (Fallot, 1922; Ramos-Guerrero et al., 1989) which are bounded by Upper Miocene– Quaternary listric normal faults (Gelabert, 1998). The horsts are segments of a Lower Miocene thrust and folded belt that constitutes the Ranges, which are composed mainly of carbonate deposits varying in age from Carboniferous to Middle Miocene. Three main ranges can be distinguished from SE to NW (Fig. 1): the Llevant Ranges, the Central Ranges and the Tramuntana Range. The grabens constitute the basins and are filled with Upper Miocene –Quaternary sediments. The island displays three main morphotectonic divisions on account of its geological structure: the Tramuntana Range, the Central Zones and the Llevant Ranges. The Tramuntana Range forms the northwest side of the island and comprises several tectonic zones (Fallot, 1922; Alvaro and Del Olmo, 1984; Gelabert, 1998), mainly built of Mesozoic sediments, cut by longitudinal valleys, karstic canyons and poljes, and with a spectacular cliffed coast. The Central Zone is composed of subsiding basins, and the Llevant Ranges in the southeast constitute a series of low mountains and hills mainly made up of Jurassic limestones and dolomites. Focusing our attention again on the upper part of the ESCI seismic reflection profile, the base of the Pliocene –Quaternary package produces a reflection of great intensity and lateral continuity (Fig. 2) and the reflectors corresponding to both the Pliocene– Quaternary and the Middle – Upper Miocene sediments thicken at the center of the sub-basins. In southeasternmost sub-basin (which onland is equivalent to the basin adjacent to the Llevant Ranges), the upper reflector, which corresponds to the sea bottom, remains horizontal, whereas in the northwesternmost basin (which onland is equivalent to the basin adjacent to the Tramuntana Range), the upper reflector has a gentle slope to the NW, towards the center of the Valencia Trough. According to Sa`bat et al. (1995), normal faults dipping either NW or SE, with small displacements, deform the Middle and Late Miocene sediments, and compressive structures are unconform-
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Fig. 1. Topographic map of Mallorca, showing the main streams and location of the slopes studied and the seismic reflection profiles. 255
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Fig. 2. The part of the ESCI seismic reflection profile corresponding to the Balearic Promontory. Reflection identification according to Sa`bat et al. (1995).
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ably overlain by Upper Miocene and younger sediments.
3. Geomorphological processes on the slope in the Llevant Ranges 3.1. Description of the area The slope studied is located along the southeastern coast of Alcudia Bay, in the Llevant Ranges, in the northeastern peninsula of Mallorca (Fig. 1). In this area, the boundary between the Llevant Ranges and Alcudia Bay is a NE – SW striking, NW dipping normal fault, the hanging wall corresponding to the Alcudia Bay block. The southeastern flank of Alcudia Bay is characterized by a piedmont zone made up of Quaternary alluvial sediments that occur as an array of coalescent alluvial-fan complexes and coastal dunes (Figs. 3 and 4), which provide evidence for fluvial activity over a
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period spanning the last 140 ka (Rose et al., 1999, from optically stimulated luminescence—OSL-age estimation). Different catchment sections have been described by Rodrı´guez-Perea (1998), Rose and Meng (1999) and Rose et al. (1999) southward and by Go´mez-Pujol (1999) northward. Both catchments reflect interactions between beach and dune deposits with different fan deposits (debris flows, sheetfloods, lags, etc.) or red/brown soils. The geometric relations between sedimentary bodies and thickness are variable and are closely related to the basement or old dune topography. Sections of the coast show bioclastic eolianite at the base into which a shore platform was cut during the Last Interglacial (OIS 5e) (Rose and Meng, 1999). From this level, the section changes to more continental (fluvial) sediments, mainly sheetflood- and lag deposits, which are interbedded at least with one yellowish red paleosol. Henceforth, lateral variations are more important, and the sections can range from beach deposits to bioclastic sands reworked by fluvial processes with some clasts and
Fig. 3. Geomorphological map of the slope in the Llevant Ranges. The main geomorphological features are the alluvial fans and the talus scree.
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Fig. 4. Photography and cross-section showing the alluvial-fan complexes on the slope in the Llevant Ranges. Location of cross-section in Fig. 3.
some erosive channels or eolianite levels, and can appear completely overlain by fanglomerates. The top section usually finishes with a new fan deposit episode. This synthesis section summarizes all the features of the coastal cliffs from the Sa Canova alluvial fan to the Es Calo´ alluvial fan, with the exception of the Es Barracar alluvial fan and surroundings. At this location, the exposures are made up of alluvial and colluvial sediments and eolianite outcrops can be identified only at the base of the cliff. Alluvial fan geometric features and the inland section are described at the Es Calo´ alluvial fan; the fan evolution was established by mapping fan surfaces, correlating them on the basis of soil development and the
interactions with paleoshoreline features (Fig. 5). Three main accretion phases with their respective dissection phases interbedded with two dune formations were characterized. Similar evolution and phases have been described for coastal fans in the southeastern Iberian Peninsula by Harvey et al. (1998) and Segura (1990). At the present time, fluvial activity takes the form of storm-generated flood events capable of transporting gravels with individual clasts attaining boulder size. During cooler episodes of the Quaternary, these river catchments extended much further northwestward across what is now Alcudia Bay (Van Andel, 1989).
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Fig. 5. Es Calo´ alluvial-fan (location in Fig. 3), with seven stratigraphic sections and two cross-sections.
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Quaternary unit fades abruptly into a less reflective and prograding unit of possible Pliocene age.
3.2. Alcudia basin In order to study the relationship between the Alcudia Basin and the Llevant Ranges slope, we describe the geometrical properties of the Alcudia Basin and present a seismic reflection profile shot at Alcudia Bay. The Alcudia Basin has a very gentle slope of 0.3% dipping NE and water depths less than 50 m. This flat bathymetry coincides with the horizontal position of the upper reflector at the southeasternmost sub-basin deduced from interpretation of the ESCI seismic reflection profile (Fig. 2). The seismic profile of the Alcudia Basin was shot during the oceanographic cruise ‘‘Geocarbal-85’’. It has a SE – NW orientation, perpendicular to the slope studied (location in Fig. 1). This is a high-resolution sparker seismic reflection profile from the NE end of Mallorca (Acosta et al., 1986). Given that no well data are available for the region, reflector identification is based on acoustic similarities to published profiles (Alla et al., 1971; Montadert et al., 1978) from the Western Mediterranean. In the seismic profile, the uppermost reflector, corresponding to the sea bottom, is horizontal; the upper reflectors of great intensity and lateral continuity correspond to undisturbed Quaternary sediments (Fig. 6). Along this profile, the
4. Geomorphological processes on the slope in the Tramuntana Range 4.1. Description of the area The slope of the Tramuntana Range is steeper and narrower than the slope in the Llevant Ranges despite the similarity in altitude (600 versus 500 m), orientation (NE –SW, both), geological structure and stratigraphic and lithological sequences forming the mountain slope. The slope corresponding to the Tramuntana Range (Fig. 7) consists of major sea cliffs (300 m or even more) with some rockfalls at the base. The most outstanding geomorphological features of this area are the major landslides shown in Fig. 8. The surface affected by these land movements is between 1 and 2 km2. The landslides are rotational with several break surfaces. We present cross-sections (Fig. 9) of two landslides (Ba`litx and Sa Costera) which are visible in Fig. 7. The Ba`litx landslide is a multiple rotational landslide affecting the Liassic (Lower Jurassic) limestones and the marls and dolomites of the Upper Triassic. According to Mateos (2000), the surface affected by this
Fig. 6. NW – SE Geocarbal-85-B seismic reflection profile, SE of the Alcudia Bay.
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Fig. 7. Photograph showing a major landslide on the slope in the Tramuntana Range. The back scar and the lateral flanks are almost vertical, forming a 100 m high cliff.
Fig. 8. Geomorphological map of the slope in the Tramuntana Range. The main geomorphological features are the major landslides and sea cliffs.
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Fig. 9. Cross-sections of the main landslides mapped in Fig. 8.
landslide covers an area of 1.2 km2, the depth of the basal break surface is 230 m below sea level and the volume of mobilized material is over 200 106 m3. The value of the imaginary axis of this rotation is 750 m and the displacement of the mobilized mass 150 m. The back scar and the lateral flanks of this landslide (Figs. 7 and 9) are very steep, forming almost vertical walls that are approximately 100 m high. The Sa Costera landslide is also a multiple rotational landslide affecting the Lower Jurassic limestones and the Upper Triassic marls and dolomites. The surface affected covers an area of 3.1 km2, the depth of the main break surface is 150 m below sea level, the volume of mobilized material is 300 106 m3 and the minimum displacement of the mobilized mass is 125 m. The radius of the axis rotation is 700 m. The back scar of this landslide also forms vertical
walls 100 m high. The dimensions of these landslides are better reflected in Fig. 7, where we present an aerial photograph of both, Ba`litx in the foreground and Sa Costera, in the background. Drainage basins on the Tramuntana slope are much larger than those on the slope in the Llevant Ranges: The Na Mora and Sa Coma stream drainage basins occupy areas of 26 and 25 km2, respectively. Both streams are incised in the Jurassic limestones and their drainage basins are characterized by the predominance of karstic processes and morphologies. These episodic streams form submerged prograding sedimentary bodies at their mouths. The upper slopes of the catchments are composed of hard, joined Lower Jurassic limestones, whereas the lower slopes consist of lithified Quaternary gravels and partially lithified soils.
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4.2. The Valencia Trough adjacent to the Tramuntana Range We describe the main characteristics of the part of the Valencia Trough adjacent to the Tramuntana Range and present a seismic reflection profile of this area in order to relate onshore and offshore processes. In the littoral zone between Valldemossa and Lluc (location in Fig. 1), offshore areas with considerable accumulation of loose sediment are related to onshore areas in which major landslides have been mapped (Mateos, 2000). The sea bottom has a steep gradient near the coast (gradient of 17.5%) and is constituted by the accumulation of big boulders due to rockfalls.
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Offshore (at some distance from the coast), the bottom gradient is lower and coarse, medium and fine sands outcrop. The Valsis-821 seismic profile was shot in November 1988 over 150 km from Barcelona to the northern coast of Mallorca (location in Fig. 1). The southeasternmost part of this seismic profile shows the offshore continuation of the onshore structure of the Tramuntana Range (Fig. 10). For the seismic interpretation, the different seismic boundaries observed along the profile have been interpreted from well data (Lanaja, 1987) and published data (Fontbote´ et al., 1990; Roca and Desegaulx, 1992; Torres et al., 1993). The interpretation of
Fig. 10. Valsis-821 seismic reflection profile, showing aggradation and propagation of reflectors in Plio-Quaternary sediments at the SE border.
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the southeasternmost part of the Valsis-821 seismic profile (Fig. 10) consists of two pre-Pliocene extensional faults, dipping NW, covered by unconformably overlying Plio-Quaternary sediments. At the SE border (i.e. NW of the Tramuntana Coast), aggradation and progradation of reflectors corresponding to PlioQuaternary sediments are visible.
5. Discussion The slopes of the Tramuntana and Llevant Ranges are located at mountain fronts bounded by NE –SW striking, NW dipping, pre-Pliocene normal faults, which separate the Ranges from the adjacent basins (located on the hanging wall). Both slopes have
similar characteristics as regards altitude, structure, lithological succession and orientation despite showing different slope processes and morphologies. Other differences remain in the adjacent basins: (1) the geomorphological context in which the two littoral range-fronts are located is not the same: the Tramuntana front is located in an open-sea context (adjacent to the Valencia Trough), and the Llevant front in the lateral margin of Alcudia Bay (a semi-confined context); (2) at Alcudia Bay, the source of sediment is not exclusively from the slope studied but also is from the streams in central Mallorca that discharge into the Bay (Fig. 1). By contrast, in the Valencia Trough (offshore Tramuntana), the sediment discharge proceeds solely from the Tramuntana streams (with drainage areas that are smaller than those in Central Mallorca).
Fig. 11. (A) Sketch of the geomorphological processes on the slope in the Llevant Ranges, with a sedimentation rate sufficient to fill the created accommodation space. Alluvial fan and debris scree occur onshore. Offshore, sea grass sedimentation with major terrigenous influx predominates. (B) Schematic cross-section of the slope in the Llevant Ranges.
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The geomorphological context (open-sea context versus semi-confined context) exerts an influence on the accommodation space: the bigger and deeper the basin, the greater the amount of sediments that can be accommodated. It is obvious that the Valencia Trough is a much bigger and deeper than the Alcudia Basin and therefore has a larger accommodation space. On the other hand, the different sources of sediment and the different drainage areas for the streams feeding the two basins exert an influence on the sedimentation rate in both basins. Given the similar climatic conditions, the sedimentary load carried by the streams entering the Alcudia Basin is larger than that of the streams entering the Valencia Trough from the Tra-
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muntana because of the larger drainage basin. Therefore, the relationship between the accommodation space versus the sedimentation rate in the adjacent basins is fundamental in understanding the slope processes of the two littoral range-fronts. If the sedimentation rate is sufficient to fill the accommodation space (Fig. 11), as suggested for Alcudia Bay, the basin shallows and the bathymetry flattens. Given that Alcudia Bay emerged during the lower sea levels of the Quaternary, small lateral inputs from the slope studied cannot be accommodated, giving rise to the formation of alluvial fans (Figs. 3, 4 and 5). The flat bathymetry of Alcudia Bay is due to the small size of the basin (approximately 50 km2) and
Fig. 12. (A) Sketch of the geomorphological processes on the slope in the Tramuntana Range, with a sedimentation rate insufficient to fill the created accommodation space. Major landslides occur onshore. Offshore, prevalence of red algae sedimentation with minor terrigenous influx. (B) Schematic cross-section of the slope in the Tramuntana Range.
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to the considerable terrigenous discharge from the streams entering Alcudia Bay from Central Mallorca, with drainage areas exceeding 450 km 2. If the sedimentation rate (Fig. 12) is insufficient to fill the accommodation space, as in the Valencia Trough, the sediments discharged from the Tramuntana streams accumulate as submarine bodies displaying aggradation and progradation (as in the seismic profile of Fig. 10), whereas material coming from major landslides accumulates at the sea bottom, at the foot of the slope. Major landslides occur on the slope owing to the unstable slope gradient. Break surfaces of major landslides continue below sea level, and probably reach the sea bottom. The ESCI seismic reflection profile (Fig. 2) reflects the main characteristics of the two basins studied. The upper reflectors are horizontal in the basin adjacent to the Llevant Ranges whereas they dip some degrees in the part of the Valencia Trough adjacent to Tramuntana Range, suggesting that the sedimentation rate was sufficiently high in the former basin to fill the accommodation space. In the latter basin, the sedimentation rate was insufficient to fill the accommodation space and thus sediments only cover the basin floor, with a final dip resembling the initial dip of the basin floor. The final morphology of the basins, which depends on the accommodation rate/sedimentation ratio, influences not only the processes occurring on lateral slopes but also the recent sediment formation in the Alcudia Basin and the Valencia Trough. Although the ‘‘in situ’’ biogenic sedimentation is similar, red algae sedimentation prevails in the NW basin because of the higher gradient of the bathymetric profile near the coast, whereas the more gentle bathymetric profile allows the extension of seagrass meadows in the shallow waters at Alcudia Bay.
6. Conclusions In the piedmonts of the NW flanks of the Tramuntana and Llevant Ranges, the relationship between the accommodation space and the sedimentation rate in the adjacent Valencia Trough and the Alcudia Basin governs the surficial processes that modify the local landscape. The slope in the Llevant Ranges, characterized by the alluvial fans and debris scree, is associated with
the Alcudia Basin, which has a higher accommodation/sedimentation ratio than the Valencia Trough. This Trough is related to the Tramunatana slope, which is affected by major landslides. We provide an example of interaction between a local tectonic phenomenon and a short-term, regionalscale landscape development. A clear definition of the roles of tectonics and the geomorphological context is necessary for a fuller understanding of the past and current behavior of landscapes.
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