Astronomically-tuned cyclicity in the alluvial sediments of the Guadix Basin (Betic Cordillera, S Spain)

In the central sector of the Guadix Basin (southern Spain) a cyclic pattern can be inferred from the
interbedding of different continental sediments. Those sediments, Late Pliocene to Quaternary in age, consist of
alluvial fan and fluvial-lacustrine deposits. During this time span, the basin was endoreic and characterized by a
marginal border of alluvial fans connecting transversally with an axial valley.
The marginal alluvial fan system (so-called Internal Transverse System, ITS) giving the cyclic interbedding
sourced from the Sierra Nevada area (the main relief of the Betic Cordillera) and connected transversally with
the fluvial-lacustrine facies of the longitudinal system (the Axial System, AS).
The ITS was made up of a series of large alluvial fans deposited by a network of low sinuosity channels,
coalescing between them to form a bajada system. The alluvial fans had a large radius (10-11 km), over half the
width of the basin (which was locally 15 km wide), since they lay against the tectonically active margin of the
basin (meaning a high sediment supply/subsidence ratio).
At least sixteen progradation phases of the ITS fans into the AS facies have been identified in the central
sector of the Guadix Basin. Although the facies of both systems were connected laterally, the soft slope of the
progradations (almost subhorizontal) resulted in a vertical alternation of the reddish sediments of the transverse
alluvial fans and the greyish to white fluvio-lacustrine sediments of the axial drainage system.
The estimated age for this alternation is based on the previous data provided by litho-, bio- and
magnetostratigraphic studies carried out in the area. This estimation gave an average value of 100 ka per
progradation phase. When correlating the alluvial fan progradation phases with the Laskar04 insolation curve,
the estimated ages coincided in most of the cases with the eccentricity maxima peaks. This fact lead us to
establish a cyclicity of ca. 100 ka, falling into the band of Milankovitch high-frequency eccentricity. These phases
are interpreted to be the result of eccentricity maxima, during which winter periods would be longer and cooler
and precipitation values would be higher. The increase in precipitation would enhance the erosion in the source
area of the alluvial fans, allowing them to prograde cyclically, invading the axial valley each 100 ka.