CAJUN Postscript Prologue for Automating Pdfmarks (PPT

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; Munõz 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 761 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/761/4887077/gsjgs.156.4.0761.pdf by guest on 30 September 2021 762 S. J. JONES ET AL. 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 Boltanã oblique ramp anticlines 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 Boltanã 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- Penã 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 Ribagorçana 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/761/4887077/gsjgs.156.4.0761.pdf by guest on 30 September 2021 FLUVIAL INCISION AND AGGRADATION 763 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 palaeohydraulic 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.

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