Journal of the Geological Society, London, Vol. 156, 1999, pp. 761–769. Printed in Great Britain.
Climatic and tectonic controls on fluvial incision and aggradation in the Spanish Pyrenees
STUART J. JONES1,3, LYNNE E. FROSTICK2 & TIMOTHY R. ASTIN1 1PRIS, University of Reading, Whiteknights, PO Box 227, Reading RG6 6AB, UK 2Research Institute for Environmental Science and Management, Geography Department, University of Hull, Hull HU6 7RX, UK 3Present address: School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK ([email protected])
Abstract: The influences of tectonic and climatic changes on upland river systems are investigated using data from Plio-Pleistocene terraces of the Rio Cinca river in the southern Pyrenees. This river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the Ebro Basin. Detailed field mapping, combined with long-profile and palaeohydraulic reconstructions, yields a comprehensive picture of changes in palaeoriver character during the Plio-Pleistocene. As the area is over 150 km from the basin outlet in the Mediterranean Sea, changes in base level are unlikely to have influenced terrace development. Although tectonic activity has exerted a strong control on the position of the river, the main period of thrust propagation pre-dates the terraces and activity has waned from the Pliocene through to the present. It is concluded that the main control on incision in this area is climate, through its influence on sediment supply. Rivers which are starved of sediment by climate change will have the power to incise, whereas aggradational phases are linked to periods of increased sediment flux.
Keywords: Plio-Pleistocene, fluvial features, terraces, tectonics, climate.
Climatic, tectonic and base level changes are all reflected in the of the town of Barbastro (Fig. 1). The main river of the area, geomorphology of river systems. However, the nature of a the Rio Cinca, displays transverse drainage being orientated rivers response to changes in the various extrinsic and intrinsic approximately north–south, orthogonal to the dominant trend controls on morphology is complex and has been the subject of of the thrust front. It is a major fluvial system draining an area considerable debate over the past decade (Schumm 1993; of 220 km2. Its source is in the Pyrenees, to the west of the Westcott 1993; Miall 1996). Much of the focus has been on the village of Bielsa, and its confluence with the Rio Ebro occurs role of base level change in driving cycles of river aggradation near Fraga in the Ebro Basin (Fig. 1). The Pliocene to recent and incision. In a series of conceptual models, Posamentier history of this river is punctuated by well defined cycles of (1988) and Posamentier & Vail (1988) proposed a model incision and aggradation. These cycles have been examined in outlining fluvial aggradation and incision in response to base detail and related to climatic, tectonic and other, linked level change. They assume that all rivers have an equilibrium geomorphic changes, within the catchment. profile which moves basinwards and topographically down- wards as base level falls and landwards and upwards as base level rises causing cycles of incision and aggradation. But rivers rarely achieve equilibrium and upstream propagation of Regional geology base level effects is difficult in an inherently downstream The Pyrenees are a nearly linear mountain belt some 200 km propagating system. wide and stretch for 450 km along the border between France In recent years it has become evident that factors other than and Spain (Fig. 1). They resulted from a phase of Late base level are significant in triggering incision and aggradation. Cretaceous to Miocene convergence and limited underthrust- Climate change is an obvious factor which has been cited to ing of the Iberian plate beneath the European plate (Roest & explain many changes in river systems during the Plio- Srivastava 1991; Mun˜oz 1992). Within the Iberian plate, Pleistocene (Blum & Valastro 1989, 1994; Blum et al. 1994) and southward verging thrust systems dominate, comprising a tectonic uplift is an important factor in actively deforming areas central Axial Zone of uplifted Palaeozoic basement rocks (e.g. Seeber & Gornitz 1983; Sloss 1991; Burbank et al. 1996). and flanked to the south by a fold-thrust belt formed from Despite this, little attention has been paid to ways in which the Mesozoic to Cenozoic sedimentary cover (Puigdefa`bregas relative importance of all of these factors might be reflected in a et al. 1992). river system. This study focuses on establishing the processes The onset of thrust deformation in the southern Pyrenees and factors controlling incision and aggradation in an upland was strongly diachronous from east to west and a series of reach of a river system in the central Spanish Pyrenees. basins formed in concert with thrust sheet development during Palaeocene to early Eocene times, ahead of the southerly translating thrust sheets. These basins have been collectively Study area referred to as the South Pyrenean Basin (Puigdefa`bregas 1975; The study area is situated on the southern flank of the Fig. 1) and this region began to compartmentalize during Pyrenean foreland fold-thrust belt and is located in the vicinity the early Eocene. This partitioning was aided by the initial
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Fig. 1. Regional geological map of the Spanish Pyrenees and Ebro foreland basin. Study area is in the vicinity of the town of Barbastro along the Rio Cinca valley. For detailed geological map of the study area see Fig. 2.
development of the Mediano and Boltan˜a oblique ramp anti- clines in the early Eocene. These oblique ramp folds have long protracted histories (Ca`mara & Klimowitz 1985; De Boer et al. 1991), that have significantly influenced the drainage pattern of the south central Pyrenees (Bentham et al. 1992; Jones 1997). The initial development of a proto-Rio Cinca was during the late Oligocene when the first gravel-bed river deposits devel- oped in the Naval to El Grado area (Fig. 2). The drainage network was structurally constrained to the north by the Boltan˜a and Mediano oblique ramp anticlines, by smaller scale N–S-orientated anticlines at the eastern limit of the External Sierras, the thrust fronts along the western margin of the South Central Unit (e.g. Sierra Marginales) and possibly even by a reactivated Hercynian basement transverse fault (Martinez- Pen˜a et al. 1992; Figs 1 & 2). This regional structural trend maintained the position of the proto-Rio Cinca during the Oligo-Miocene in the Naval-El Grado area with some lateral migration of the main channel belt. However, from late Miocene the Rio Cinca became fixed in its present day position and began to incise. In order to understand the allocyclic controls on river behaviour during the last 6 Ma of Pyrenean evolution, a detailed study of the Plio-Pleistocene fluvial deposits of the Rio Cinca has been undertaken.
Regional geomorphology The drainage network along the southern flank of the Spanish Pyrenees is dominated by transverse rivers draining the Axial Zone, transporting coarse sediment to the Ebro foreland basin to the south (Fig. 1). These rivers (e.g. Alcanadre, Esera, Noguera Ribagorc¸ana and Segre) are very well spaced with a regular distance of 20–30 km between each of them (drainage density). This equates to a spacing ratio of 2.0 according to Hovius (1996) and is similar to many other mountain belts. As a consequence it illustrates that at the present day, after tectonic uplift has ceased, the regularity is still maintained, reflecting the distribution of energy in river systems as they Fig. 2. Detailed geological map of the study area around the village tend towards the most probable state. of El Grado along the Rio Cinca valley. It shows the main The Rio Cinca has only been studied in detail here, but the structural features, distribution of the Oligo-Miocene conglomerates incised form equally applies to the other transverse rivers along (Campodarbe Group and Uncastillo Formation) and the the southern flank of the Pyrenees. The incision of the Rio Plio-Pleistocene gravel terraces along the Rio Cinca.
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Cinca has created a number of paired terraces that constitute former levels of the valley floor or flood plain. Many of these terraces can be traced for several hundreds of metres on both sides of the Rio Cinca valley. The consideration of the heights and internal composition of the terraces has contributed significantly to the understanding of the geological history of the fluvial system and the climatic evolution of the area.
Methodology The fluvial terraces and valley fill deposits of the Rio Cinca have been the subject of geomorphological, sedimentological and palaeo- hydraulic analyses. Lithological and sedimentological data were collected at the decimetre scale from the terrace gravel deposits. Palaeocurent directions were collected from both uni- and bi-directional flow indicators, such as planar cross-bedding, large clast imbrications and pebble clusters. Clast populations were Fig. 3. Three-dimensional topographic contour model of the Rio systematically recorded at all terrace levels by clast counting (Jones Cinca valley illustrating the incised form of the Rio Cinca and the 1997). paired fluvial terraces. The location of the Moscarazos thrust (MT) Detailed field mapping of the palaeoterraces and collection of data from the present Rio Cinca and comparable rivers in the south-central is also shown which influenced the local river profile. Pyrenees was undertaken. Land and aerial photography was used to correlate the paired terraces across the Rio Cinca valley and help with stratified and trough cross-bedded conglomerates. Set heights the regional mapping of the study area. Analysis of the present day are 200–400 cm, and the sets are arranged in 1–2 m scale drainage network of the Rio Cinca and associated rivers was under- stacked units. taken using 1:50 000 scale topographic maps and combined with the The gravel sheets that dominate the fluvial terraces are clast lithologies found in the modern rivers. Longitudinal profiles interpreted as being deposited from widespread, weakly along the middle to upper reaches of the Rio Cinca have been channelized flows carrying coarse bedload. Low-angle, inclined constructed for each of the terrace levels and for the present day profile normal-graded stratification is interpreted as lateral and of the river. frontal accretion surfaces of in-channel and marginal-channel gravel bars. Many of the horizontally stratified sandstones associated with the accretionary surfaces were deposited Results and discussion as supra-bar sands. Transverse bars occur, identified by The Rio Cinca is a long-lived river system that has supplied cross-bedded conglomerates and commonly pass laterally into coarse sediment from the high Pyrenees to the Ebro foreland channel fill complexes. These channel fills have deeply scoured basin since at least Oligo-Miocene times. Evidence of the bases with horizontally stratified boulder lags and trough Pliocene to recent history of the system is contained in a suite cross-bedded conglomerates and coarse pebbly sands. Such of terraces which flank the present-day river valley. In the area channels are interpreted as bar top or seasonal channels only between Abizanda and Barbastro, a series of four Plio- active during flood events. The stratigraphic range from Pleistocene terraces have been identified and analysed. Relative Oligocene to Recent and abundance of conglomerates provides ages have been inferred from the positions of the terraces, evidence for the longevity of the Rio Cinca as a major topographically high terraces are presumed older than those gravel-bed river system supplying coarse gravelly sediment to closer to the present valley floor. the Ebro foreland basin.
Terrace stratigraphy Longitudinal profiles of the Rio Cinca and its terraces The terraces are well exposed as small quarries, road side Detailed mapping combined with air photograph recognition exposures and along the banks of the Rio Cinca (Fig. 3). They of the fluvial terraces has shown that many are paired at are all characterized by cobble-boulder conglomerates, are similar topographic heights above the present Rio Cinca and usually clast-supported, and occur in erosionally based, broad have comparable lithofacies associations. These data have channelized units (over 50–300 m wide; Table 1). Cobble been used as the basis for correlating terraces both across the conglomerates with diffuse stratification are the dominant Rio Cinca valley and in a downstream direction. Numerous lithology, often with imbricated clasts and with abundant workers have criticised the interpretation of the relative timing pebble clusters. The diffuse texture is present throughout all of terrace formation based on geographic, stratigraphic rela- the terraces, but it is particularly common in level 3 terraces. tions and landscape position alone (Blum & Valastro 1989, Stratified conglomerates which fine upwards are also common, 1994; Toomey et al. 1993). However, it is the only option in and these deposits often have boulder sized basal lags. Lateral areas where radiocarbon dating is not possible. In this study accretion conglomerates are well preserved in all terraces, but the probability of misinterpretation was considerably reduced in level 4 terraces, accretionary units can exceed 4 m thick. by carefully reconstructing the terrace positions across the Many of these units grade in grain size from cobble clasts to river valley and relating them to an absolute datum rather than coarse-grained sands, and palaeocurrent data indicate that just the river bed. they formed at frontal or oblique margins of gravel bars. The longitudinal profiles of four terraces have been recon- Cross-stratified conglomerates and coarse-grained sandstones structed in the middle to upper reaches of the proto-Rio Cinca occur as minor components of the sequences. They are usually (Fig. 4). These show relatively smooth profiles which tend to associated with channel-fill complexes of cobble-sized, crudely be convex-up, with only minor perturbations from the general
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Table 1. Descriptions of the four terraces along the middle reaches of the Rio Cinca valley
Mean Terrace Tectonic palaeocurrent level Lithology dip direction Description of facies
T4 Polymict, clast supported conglomerates (Gcm, Gp, 3)SE SSE Gravel sheets with imbrication and pebble clusters. Gt). Coarse sandstones lenses and laterally Gravel bars with accretionary units and supra-bar truncated beds. Clast size range 10–48 cm sands. Sands usually stratified and cross-bedded T3 Polymict, clast supported conglomerate (Gcm, Gt, 4)ESE SSW Well developed conglomerates, laterally extensive Gp). Very well rouned clasts. Medium to coarse and often with convex tops. Imbrication and pebble sandstone beds (Sh, Sp, St). Clast size range clusters common. Small channels with complex fills 6–53 cm of gravels and cross-stratified coarse sands T2 Polymict, clast supported conglomerate, with 3)S SSW Greater coarse sand content, with well rounded numerous rip-up clasts of coarse sandstone (Gcm, boulders, usually polymict. Gravel sheets typically Gp, Gt). Medium to coarse sandstone beds (Sh, St, stratified with imbrication. Lateral and frontal bar Sp). Clast size range 12–39 cm accretionary conglomerates T1 Clast and matrix supported conglomerates (Gcm, 2–5)SE S Sheet conglomerates, gravel bars and channel fill Gmg, Gp, Gt). Abundant sandstone lenses and complexes common. Stratified pebbly sands with pebbly sands. Clast size range 5–36 cm deep erosional scours and lag fills of pebble to cobble size clasts
Each of the terraces have been gently tilted indicating some late stage thrust movement. Palaeocurrent directions are all to the south and are likely to have been controlled by N-S orientated folds and the overall gradient of the south central Pyrenees. Imbrication, pebble clusters and planar cross-stratification were used to determine palaeocurrent directions. The description of the conglomerates uses the fluvial lithofacies scheme of Miall (1978, 1996).
trend. According to many researchers, especially Seidl et al. Factors controlling fluvial incision along the southern (1994), Seidl & Dietrich (1992) and Gardner (1983), all such flank of the Pyrenees perturbations are knickpoints. Knickpoints are generally con- sidered to be non-equilibrium landforms created by successive The processes by which rivers incise into bedrock are poorly drops in base level which increase downstream channel understood. Work by Seidl & Dietrich (1992) suggests that gradient causing incision. However, the results of both incision rates might be related to drainage basin area and experimental and field studies have shown that upstream channel gradient. Other studies have demonstrated the impor- propagation of knickpoints is limited and most reported tance of sediment transport mechanisms (Begin et al. 1981), knickpoints are confined to the lower reaches of rivers (Miller although the majority of studies have concentrated on lowland 1991; Seidl & Dietrich 1992; Koss et al. 1994). areas under the influence of base level. Until recently, controls Upland portions of river systems are generally unaffected by on fluvial incision and rates of incision in mountain belts have the localized perturbations that accompany base level change. been neglected. Burbank et al. (1996) showed that the Indus The section of the Rio Cinca, which is the focus of this study, River of the northwestern Himalayas incises through bedrock is some 150–200 km away from the Mediterranean and its at some of the highest rates of fluvial incision in the world "1 associated sea level fluctuations. It is therefore, highly unlikely (2–12 mm a ). They identify that in rapidly deforming that perturbations in the terrace and river longitudinal profiles regions, an equilibrium is maintained between bedrock uplift were caused by knickpoint recession over this distance. It is and river incision, with landsliding allowing hillslopes to adjust necessary to turn to other regional changes to explain the to rapid river down cutting. Such a conclusion has often been observed patterns of incision, particularly localized thrust inferred by geomorphologists and shows the important role movement, differential rates of tectonic uplift, fluctuating that tectonics plays in understanding fluvial incision in sediment supply and rates of denudation. mountain belts. As the upper Rio Cinca is within the tectonically active mountain belt of the Pyrenees, tectonics must be exerting a strong influence on patterns of fluvial incision in the Plio-Pleistocene. Incision along the Rio Cinca Paired terraces, such as those observed in the Rio Cinca, Tectonics and incision. The effects of tectonic uplift on the record the progressive abandonment of earlier river beds as the longitudinal profile of the upper most portion of the Rio Cinca system incises (Figs 3 & 4). If time since abandonment can be can be clearly seen in the area north of the town of Ainsa inferred and the height of the terrace above the valley bottom (Figs 4 & 5). The profile gradient gradually increases to the are known, a minimum mean rate of river incision can be north of Ainsa, but with several perturbations, each corre- calculated. A minimum estimate of the rate of incision for the sponding to an individual thrust along the southern margin of proto-Rio Cinca since the latest Pliocene has been calculated the Axial Zone. For example at Bielsa an abrupt change in as between 2 and 30 cm ka"1. This estimate is based on the river gradient corresponds with a reverse thrust by which proto-Rio Cinca downcutting 350 m from the latest Pliocene Devonian metasediments are displaced against Mesozoic lime- to recent. These results are comparable with those for several stones (Fig. 6). However, further downstream within the other rivers in tectonically active areas (e.g. Colorado, USA; Abizanda to Barbastro area, the effects of tectonic uplift and Nahal Zin and Nahal Paran, Israel; Atenguillo, Mexico). general thrust tectonic activity can still be readily identified.
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Fig. 5. Simplified structural map (a) and longitudinal profile (b) along the middle to upper reaches of the Rio Cinca. Many of the thrusts can be correlated to breaks in slope/gradient along the river course. Understanding the bedrock geology and structure along the Fig. 4. (a) North–south river profile along the northern margin of profile of a river is an important consideration as it governs the rate the Ebro foreland basin to the source of the Rio Cinca in the Axial and behaviour of fluvial incision. Zone of the Pyrenees (location of profile (b) is shown in boxed area). (b) Detailed profile between the town of Barbastro and north of Abizanda showing the relatively smooth profile of the Rio Cinca, the four terrace levels and the location of the cross-sections across the Rio Cinca valley. (c) Three cross-sections along the Rio Cinca valley from north of Abizanda to Barbastro in the south. The location of each of the cross-sections is shown in (b), and illustrates the incised form of the Rio Cinca valley with many paired gravel terraces.
All the terrace levels have been gently tilted by 2–5) (Fig. 2) and perturbations along the longitudinal profile correspond to blind thrusts and changes in bedrock lithology (Fig. 5). Experiments which simulated the effects of uplift and sub- sidence on river character have shown that changes in slope imposed on the longitudinal profile of a river are compensated initially by changes in sinuosity with the along-channel slope remaining constant (Ouchi 1985; Burnett & Schumm 1983). However, the extent to which rivers can adapt in this way is limited and prolonged uplift results in changes in gradient and incision. An important observation for understanding tectonic controls on fluvial systems is that in upland areas, rivers becomes fixed in position in relation to the geoid as they incise Fig. 6. The upper reaches of the Rio Cinca at Bielsa (see Fig. 1 for into bedrock. Additionally, tectonic uplift in mountain belts location). Sketch geological map of the Bielsa and the Valle de leads to increased sediment supply whose rate of mass transfer Pineta area. to the fluvial system must relate directly to rates of incision and sediment transport. Uplift also influences climate which number of stimuli, and it is often difficult to determine whether feeds back into incision rates through weathering and river a given change in character reflects tectonic or climatic influ- hydraulics. ences, or both (Frostick & Reid 1989). Attempts to unravel causes are often thwarted by feedback among climate change, The role of climate change. It is well known that river form and tectonism and sea level fluctuations (Shanley & McCabe 1994). processes vary with climate (see Schumm 1977; Bull 1991; Despite these difficulties, river character can be a valuable, and Summerfield 1991 for reviews). However, rivers react to a sometimes unique, guide to climate-induced environmental
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Fig. 7. Graphs illustrating the effects of climate upon sediment supply through denudation rates. (a) Various estimates of the relationship between (mechanical) denudation rate and mean annual precipitation. (b) Generalized relationship between mean annual precipitation and biomass based on various studies. Both graphs adapted from Ohmori (1983).
change. Along the southern flanks of the Pyrenees there was a processes, which are most effective but less frequent on the dramatic change in the character of fluvial systems at the end bare-ground of arid zones and the soil protection afforded by of the Miocene and more subtle changes in fluvial character vegetation growth as rainfall increases. Climate change in the have also occurred during the last 6 Ma. These changes are Pyrenees is likely to have altered sediment supply to the rivers. accompanied by known shifts in climate (Calvo et al. 1993; High rates of sediment supply during semi-arid Oligo-Miocene Krijgsman et al. 1994). times contrasts with the slower erosion rates during the Throughout the Oligo-Miocene in the south central temperate-humid Plio-Pleistocene. Pyrenees the climate was persistently arid to semi-arid. Evi- The role of climatic fluctuations controlling terrace for- dence for such a climate comes directly from the sediments and mation and fluvial aggradation and incision was first discussed fauna preserved within the Ebro Basin and along its northern by Huntington (1907), who recognized that valley erosion margin (Salvany et al. 1994). Several attempts have been made takes place under wet conditions and aggradation takes place to relate denudation rate and mean annual precipitation under more arid conditions. More recently there has been (Fig. 7). Although there are differences in the detail of the considerable debate about the possible correlation of terrace relationships, all suggest an initial peak of erosion in semi-arid formation with climatic fluctuations during the Pleistocene environments and a progressive increase in denudation above a (Green & McGregor 1987; Bridgland 1994; Blum et al. 1994). mean annual precipitation of around 1000 mm. The curve of It is now generally accepted that terraces develop in response Ohmori (1983) is based on the largest data set and suggests a to climatic deterioration. sediment removal maximum at rainfalls of between 300 and When long profiles of terraces of the River Thames and 400 mm a"1, a minimum at 600 mm a"1 and then a progres- other major British rivers are compared with those from the sive increase to a maximum at around 1000 mm a"1 (Fig. 7). Pyrenees there is a striking similarity. Although there is an This pattern reflects a balance between rainfall-runoff obvious difference in tectonic setting between many British
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Fig. 8. Schematic diagram of the effects of climatic fluctuations upon river behaviour; i.e. whether a river shall incise or aggrade. During glacial periods when more sediment was available, due to the lack of vegetation to trap sediment and stabilise erosion, rivers aggrade with accompanying terrace formation. However, during interglacial periods vegetation is able to re-establish itself and restrict sediment supply to rivers, thus promoting incision and vertical downcutting. Time scale adapted from De Jong (1988). Fig. 9. Simplified graphs of palaeohydraulic data comparing the Oligo-Miocene () and Plio-Pleistocene () gravel-bed river rivers and those of the Pyrenees, Bridgland (1994) suggested systems (Jones 1997). (a) Relative bedload transport rates that many of the terraces in the lower reaches of the River (kg a"1 m"1) as a function of stream power (W m"2). (b) Relative Thames are primarily base level controlled. This contrasts to efficiency (%) in transporting bedload sediment as a function of "2 the terraces in the middle to upper reaches which directly stream power (W m ). correspond to glacial periods when aggradation was dominant. The coincidence of river response in the two areas suggests a and thereby determining the volume of sediment in an individ- common, climatic cause. During glacial periods the drainage ual accretionary unit of a gravel bar per flood event. As with basin was less well vegetated favouring catchment erosion, an all palaeohydraulic reconstructions inaccuracies exist such increase in sediment supply to the river and within-channel as the removal of some sediment by erosion, variability in aggradation. By contrast interglacials were warmer and more porosity of the gravel and the inability to quantify accurately humid leading to more vegetation, a reduction in sediment the amount of suspended sediment per flood event. However, supply and vertical incision. The four terrace levels along the the palaeohydraulic results still provide a valuable data set Rio Cinca correspond very well with the main glacial and allowing a better understanding of fluvial behaviour. interglacial palaeoclimatic fluctuations (Fig. 8; De Jong 1988). Differences between the Oligo-Miocene and Plio-Pleistocene river deposits are highlighted in bivariate plots of relative Palaeohydraulics and sediment flux. If climate is exerting a bedload transport rates against stream power (Fig. 9). The controlling influence on incision and aggradation through its distinction between the two data sets is remarkable. At low control on sediment supply it should be possible to detect this values of stream power, relative bedload transport rates for the from palaeohydraulic and palaeosediment flux reconstruc- Plio-Pleistocene Rio Cinca are as much as 400 times greater tions. The methods used are discussed elsewhere (Ryder & than those for the Oligo-Miocene system. Church 1986; Laronne & Reid 1993; Jones 1997) but are based Calculations of the efficiency of the two systems show the on measurements of maximum clast size to infer flow velocity, proto-Rio Cinca to be the more efficient (mean efficiency is measuring width:depth ratios of channels and thicknesses of 1.63% against a mean of 0.07% for the Oligo-Miocene systems individual beds, estimating the gradient of the proto-Rio Cinca over approximately the same range of hydraulic con- following the methodology of Paola & Mohrig (1996) and ditions). This suggests that differences in sediment supply is an estimating a minimum bedload transport rate (Jones 1997). A important factor. bedload transport rate was determined by measuring the It has been suggested that differences in the transport cross-sectional area and lateral extent of an accretionary unit efficiency of gravel-bed rivers are a function of bed structure.
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the Valencia (Bartrina et al. 1992) and the late Miocene Messinian desiccation crisis. Both would have lowered regional base levels and may have initiated the headward erosion of the proto-Ebro River. However, this study has shown that climate and tectonics are the primary controls on fluvial incision or aggradation in the middle to upper reaches of rivers whereas base level changes are more dominant in the lower reaches of fluvial systems. The observed behaviour of the Rio Cinca during the Plio- Pleistocene was controlled by extrinsic factors that determined the rate of incision and intervening terrace aggradation episodes, as follows: (1) In the middle to upper reaches of the Rio Cinca thrust tectonics controlling bedrock geology exerted an important local control on fluvial incision. Uplift increased local gradients steepening segments of the stream and causing downcutting. (2) The transverse rivers along the southern flank of the Pyrenees responded to documented changes in climate from Fig. 10. Summary schematic diagram of the main controls on fluvial cold semi-arid to temperate-humid. This led to a rapid incision along the entire course of a hypothetical river system. It decrease in sediment supply and an accompanied increase in highlights the controls in the middle to upper reaches of the Rio vegetation causing the Plio-Pleistocene rivers to incise. It is Cinca, but it equally applies to river systems in other mountain belts suggested that at a finer resolution the fluctuations in climate (adapted and redrawn from Shanley & McCabe 1994). from glacial to interglacial controlled the four phases of terrace formation. ffi Laronne & Reid (1993) show that the e ciency of ephemeral (3) Palaeohydraulic and palaeoefficiency calculations are gravel-bed rivers is an order of magnitude higher than that of used to differentiate between deposits of aggradational and perennial counterparts as a function of the absence of an predominantly degradational river systems. The bedload armour layer. At first appearance, the Rio Cinca data might be transport rate calculated for the proto-Rio Cinca show that explained by an ephemeral to seasonal river during the Plio- differences can be attributed to climatic change. Pleistocene with a more perennial Oligo-Miocene system. Despite acknowledged difficulties in understanding controls However, field examinations reveal armouring within both on the behaviour of river systems in their middle to upper ffi sequences, suggesting perennial rivers. As e ciency is funda- reaches, the character of river deposits preserved in terraces mentally linked to sediment supply, the aggradational phases can be a valuable, and sometimes unique, guide to the controls of the Plio-Pleistocene sequence must be linked to a flush of exerted by tectonic uplift and climate change on river detritus through the system. Periodic increases in sediment flux aggradation and incision. from the Himalayas have been linked to phases of thrust movement (Burbank 1992; Burbank et al. 1996). Tectonic We thank D. Lawrence for early discussions of the palaeohydraulic activity had been waning since mid-Miocene times and for the data, S. Vincent and J. Howell for constructive reviews and K. Plio-Pleistocene deposits of the Pyrenees climate is the most Davis for assistance with drafting. D. Pirrie is thanked for his probable cause of changes in sediment supply. Incision occurs careful editing. S.J.J. was supported by a NERC research once the river becomes over-powered with respect to a reduced studentship GT4/94/242/G. This paper is University of Reading sediment supply in response to climate change. PRIS Contribution No. 689.
Conclusions References
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Received 30 January 1998; revised typescript accepted 1 December 1998. Scientific editing by Martyn Pedley and Duncan Pirrie.
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