THE ROLE OF EROSION AND DEPOSITION IN THE CONSTRUCTION OF ALLUVIAL-FAN SEQUENCES IN THE GUADIX FORMATION (SE SPAIN)

A study is made of a Pliocene alluvial fan in the Guadix Basin (Betic Cordillera, Spain). On the basis of the characteristics of the sequenc es and the position of the Arroyo de Gor section within the fan, a mod el is proposed to explain the sequential trends, taking into account t he importance of the erosional processes associated with lateral displ acement according to a pendular pattern of a fluvial system made up of a trunk channel and lateral, secondary channels. Lateral migration of channels in a constant direction is a consequence of the preferential attachment of bars to one bank, as shown by recent systems. Change in the direction of migration is caused when channels reach the basin ma rgin or the opposing slope of an adjoining fan. This can also occur if tilting towards the centre of the basin takes place. According to thi s model, a marginal position in a fan would be recognised by a series of fining and thinning upward (FU + ThU) sequences delimited by clear erosion surfaces. The backsets of the bars accumulated in the channels invariably dip towards the centre of the fan. A central position is c haracterised by a higher number of FU + ThU sequences not reaching com pletion at thp ton (and, therefore, With a lower proportion of overban k facies). The backsets dip alternately in opposite directions. This m odel may also be used for the evolution of the sediment supply/subside nce ratio in a particular zone of a basin.