Erosional and Depositional Patterns in the Valencia Mouth: An Example of a Modern Channel-Lobe Transition Zone Channel A. PALANQUES 1, N.H. KENYON 2, B. ALONSO l, and A. LIMONOV 3 l Institut de Ciencies del Mar ( C.S.I.C.), Passeig Joan de Borb6 s/n, Barcelona 08039, Spain. 2 Institute of Oceanographic Sciences, Deacon Laboratory, Brook Road, Wormley, GodaIming Surrey, GU8 5 UB, UK. 3UNESCO Center for Marine Geology and Geophysics, Faculty of Geology, Moscow State University, Lenin Hills, Moscow 119899, The Russian Federation (Received 26 July 1995; accepted 26 July 1995) Key words: Bedforms, channel-lobe transition zone, Valencia Fan Abstract. Recent deep-towed,high resolution sidescan sonar records and seismicprofiles have been collected on the lower Valencia Fan (Northwestern Mediterranean). Three morphological zones, chan- nelled, transition and unchannelled, have been recognized in the Valencia Channel mouth. Sonographs from the transition zone show a progresivetransversal gradation from depositional to ero- sional bedforms. This asymmetry may be due to the lateral inputs of sediment flows from the Rhone deep-sea fan and to the effect of the Coriolis force,which could have diverted the flowsto the south- west. Bedforms recorded in the study area include trains of starved ripples and dunes, sand ribbons, and fields of elongated scours. Most morphological features, bedforms and seismic characteristics of the ValenciaChannel mouth are typical of channeMobetransition zones. 1. Introduction Deep-sea fans have a variety of morphological and sedimentological features that define each of their mor- phological divisions (upper, middle and lower). In the upper and middle fan, large-scale relief features such as channels are readily seen on seismic profiles and have been widely studied. In contrast, the lower fan areas are fairly flat (Nelson and Nilsen, 1974; Nelson et al., 1978) and hence poorly defined morphologically from low and intermediate resolution seismic records. The higher resolution of the modern exploration methods permits the observation of more complex morphological and sedimentological features on the relatively flat areas of the lower fan. The areas beyond channels are sometimes called lobes (Muttti and Nor- mark, 1991), and are considered to be one of the main depocentres of sandsized material in turbidite systems. The transition zone between channels and lobes is believed to have erosional features and sandy depo- sitional bedforms as a consequence of the increased influence of turbidity currents undergoing a hydraulic jump (Komar, 1973; Mutti and Normark, 1991). The Valencia Fan that has developed in the north- western Mediterranean is fed by the complex system of canyons incised into the Iberian margin that merge with the Valencia .Channel (Palanques and Maldo- nado, 1985; Maldonado et al., 1985a; Alonso et al., 1994). The Valencia Fan has been active not only dur- ing lowering and low stand sea-level periods but also during sea-level rise, and probably to some extent dur- ing high stand periods (Palanques et al., 1994). The upper fan has a deeply incised fan channel whose course is partly controlled by the presence of volcanoes. The middle fan shows prominent levees and overbank deposits. In the northern side of the distal middle fan some sinuous distributary channels have been recog- nized (Maldonado et al., 1985b). The lower fan, how- ever, has not been studied in detail up to now. This paper focusses on the lower fan from the area of the mouth of the Valencia Channel to the unchannelled zone (Figure 1) in order to study the transition between channelled and unchannelled deposits. 2. Procedures This study has been carried out by combining analysis of three different types of acoustic data: (1) Sonographs (sidescan sonar images), (2) 5 kHz sub-bottom profiles and (3) Sparker records. Sonographs of a wide strip of the sea bottom up to 20 000 m wide were obtained by a long range OKEAN system. Sonographs and 5 kHz sub-bottom profiles were obtained by the deep towed MAK I system. The MAK I sidescan sonar was used at 104