The role of erosion and deposition in the construction of alluvial fan sequences in the Guadix Formation (SE Spain)

Abstract

A study is made of a Pliocene alluvial fan in the Guadix Basin (Betic Cordillera, Spain). On the basis of the characteristics of the sequences and the position of the Arroyo de Gor section within the fan, a model is proposed to explain the sequential trends, taking into account the importance of the erosional processes associated with lateral displacement 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 margin or the opposing slope of an adjoining fan. This can also occur if tilting towards the centre of the basin takes place. According to this 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 characterised by a higher number of FU + ThU sequences not reaching completion at the top (and, therefore, with a lower proportion of overbank facies). The backsets dip alternately in opposite directions. This model may also be used for the evolution of the sediment supply/subsidence ratio in a particular zone of a basin.