Transition from alluvial to fluvial systems in the Guadalentín Depression (SE Spain) during the Holocene: Lorca Fan versus Guadalentín River

Abstract The Lorca alluvial fan (SE Spain) constitutes an excellent example of transition from alluvial to fluvial sedimentary environments in a semiarid region. The fan body represents a large gently-sloping alluvial fan of mainly sandy–silty composition. It is presently dissected throughout its length by the Guadalentin River, which worked as the main fan channel and still operates as a torrential rambla-type channel responsible for historic large flood events. Three different outcrops are analysed at fan-apex, mid-fan and distal-fan locations, taking also into account the different inset terrace levels developed within the main fan trench at + 8–9 m, + 5–6 m, + 3–4 m and + 1–2 m above the present thalweg. Available radiocarbon data and archeological remains collected in fan sediments and inset terraces enable the construction of a preliminary hypothesis about the progressive incision of the fan body from pre-historical to historical times. Proximal fan aggradation took place during the Early Holocene before ~ 7900–7700 cal. yr. BP and continued until the Late Neolithic to Calcolithic Period (~ 5500–4200 cal. yr. BP) when the fan surface stabilised, and channel incision generated a first fanhead trench. Subsequent infilling of the fan trench led to the development of the upper terrace level (+ 8–9 m) which merged with the palustrine environments 23 km downstream of Lorca, feeding distal fan lobes during Late Bronze Age and Post-Roman times. The second terrace level (+ 5–6 m) is a cut & fill episode of about 5 m thickness developed from Roman times, and dated to ~ 1700 to 950 cal. yr. BP. The geometry and altimetry of the upper terrace level, together with the occurrence of different steps in the longitudinal profile of the river at mid to distal fan locations, strongly suggest that the incision of the ancient fan channel was assisted by intersection point headcut development (sensu [Harvey, A.M., 1978. Dissected alluvial fans in southeast Spain. Catena 5, 177–211.]) at proximal-mid fan locations, but by headward erosion exerted by neighbouring drainage basins at distal palustrine environments. Therefore, the Lorca fan exemplifies the behaviour of large-sized fans in its transition from pure alluvial environments to fluvial ones, and its eventual incipient integration into a regional drainage network subject to a dynamic base-level during the historic period.

[1]  A. Harvey.  THE ROLE OF ALLUVIAL FANS IN THE MOUNTAIN FLUVIAL SYSTEMS OF SOUTHEAST SPAIN: IMPLICATIONS OF CLIMATIC CHANGE , 1996 .

[2]  S. Black,et al.  Interpreting the response of a dryland river system to Late Quaternary climate change , 2004 .

[3]  A. Rachocki,et al.  Alluvial fans : a field approach , 1990 .

[4]  A. Mather.  Impact of headwater river capture on alluvial system development: an example from the Plio-Pleistocene of the Sorbas Basin, SE Spain , 2000, Journal of the Geological Society.

[5]  Roberto Tomás,et al.  Engineering-geological model of the Segura River flood plain (SE Spain): a case study for engineering planning , 2003 .

[6]  J. García,et al.  Evolución del poblamiento desde época ibérica hasta los inicios de la romanización en el casco urbano de Lorca (Murcia) , 1999 .

[7]  W. Bull,et al.  The alluvial-fan environment , 1977 .

[8]  M. Walker,et al.  The palaeoecoloical potential of pollen records in caves: the case of Mediterranean Spain , 1999 .

[9]  J. J. Martínez-Díaz,et al.  The Alhama de Murcia fault (SE Spain), a seismogenic fault in a diffuse plate boundary: Seismotectonic implications for the Ibero‐Magrebian region , 2004 .

[10]  M. Kirkby,et al.  Dryland Rivers: Hydrology and Geomorphology of Semi-arid Channels , 2002 .

[11]  A. Harvey,et al.  Response of Quaternary fluvial systems to differential epeirogenic uplift: Aguas and Feos river systems, southeast Spain , 1987 .

[12]  I. Reid,et al.  Desert Sediments: Ancient and Modern , 1988 .

[13]  D. Thomas,et al.  Arid zone geomorphology. , 1989 .

[14]  J. Carrión,et al.  A Palaeoenvironmental Study in Semi-arid Southeastern Spain: the Palynological and Sedimentological Sequence at Perneras Cave (Lorca, Murcia) , 1995 .

[15]  R. Weijermars.  The Palomares brittle—ductile Shear Zone of southern Spain , 1987 .

[16]  Oliver A. Chadwick,et al.  Tectonically driven landscape development within the eastern Alpujarran Corridor, Betic Cordillera, SE Spain (Almeria) , 2003 .

[17]  Francisca Navarro Hervás.  El sistema hidrográfico del Guadalentín , 1991 .

[18]  I. Fuller,et al.  Correlation of fluvial sequences in the Mediterranean basin over the last 200 ka and their relationship to climate change , 2002 .

[19]  Robert Pocklington.  Estudios toponímicos en torno a los orígenes de Murcia , 1990 .

[20]  T. Bardají,et al.  Paleoseismic indications along ‘aseismic’ fault segments in the guadalentín depression (SE Spain) , 1997 .

[21]  L. Schulte.  Climatic and human influence on river systems and glacier fluctuations in southeast Spain since the Last Glacial Maximum , 2002 .

[22]  J. Bousquet.  Quaternary Strike-Slip Faults in Southeastern Spain , 1979 .

[23]  T. Bardají,et al.  Fault-generated mountain fronts in southeast Spain: geomorphologic assessment of tectonic and seismic activity , 2003 .

[24]  Caitlin E. Buck,et al.  Intcal04 Terrestrial Radiocarbon Age Calibration, 0–26 Cal Kyr BP , 2004, Radiocarbon.

[25]  C. Zazo,et al.  Fan-surface dynamics and biogenic calcrete development : Interactions during ultimate phases of fan evolution in the semiarid SE Spain (Murcia) , 1998 .

[26]  L. Somoza,et al.  Landscape response to strike-slip faulting linked to collisional settings: Quaternary tectonics and basin formation in the Eastern Betics, southeastern Spain , 1993 .

[27]  C. Denny.  Fans and pediments , 1967 .

[28]  Pierre Bordet,et al.  The Betic segment of the lithospheric Trans-Alboran shear zone during the Late Miocene , 1988 .

[29]  A. Mather,et al.  Tectonic origin and evolution of a transverse drainage: the Rı́o Almanzora, Betic Cordillera, Southeast Spain , 2003 .