Channel migration patterns and related sequences in some alluvial fan systems

Abstract

The pendular displacement of a channel system, consisting of an area of higher channel density (trunk channel zone) on both sides of which channel density progressively decreases (secondary channel zone), can, under favourable subsidence conditions, lead to the development of alluvial fans. Characteristic sequences are found in these fans, depending on their position in the sedimentary body. A marginal position in the fan is thus recognized by the superposition of fining and thinning upwards (FTU) cycles, the upper part of which is made up of important thicknesses of overbank fines, all the backsets of bars dipping in the same direction (towards the centre of the cone). On the other hand, a central position is characterized by a higher number of FTU cycles, which are incomplete due to erosion of the upper parts (corresponding to the higher concentration of overbank fines), and the backsets in each cycle dip alternately in opposite directions. The displacement of the channel system in a constant direction may be caused by the preferential accumulation of bars on one of the banks of the channel. The change in migratory direction giving rise to the pendular movement is caused by the trunk channel reaching the basin margin or the sedimentation area of an adjoining fan. Palaeogeographic reconstructions of fans using this technique contribute to the analysis of ancient basins: small-radius fans with a high sweep angle (A S) are characteristic of basin margins subjected to a low sediment supply/subsidence ratio (S S /S B), whereas large-radius fans with a low A S characterize periods with a high S S /S B ratio on the basin margin. This model can be applied in economic geology studies, as the location of an ancient cone permits delimitation of the axial strip (with a higher proportion of coarse, highly porous, channelled facies) and the marginal sectors (where thick layers of less porous overbank fines are intercalated). We here present the example of the alluvial fans in the Guadix Fm. (Betic Cordillera, Spain), which lie on a basin margin (ESE) where the S S /S B ratio increased progressively throughout the Lower Pliocene. The detailed analysis of palaeocurrents on the basis of different criteria throughout the Arroyo de Gor section helps to locate more precisely and confirm the palaeogeographical distribution of the Pliocene cone, as deduced from sequential analysis.