International Geologiical Congress - Oslo 2008


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SES-05 Dynamics of complex intracontinental basins


The interrelations of tectonism and sedimentation in the Miocene deposits of Sepulveda-Ayllón sub-basin


Javier Luengo , Instituto Geológico y Minero de España (Spain)
Francisco Nozal, Instituto Geológico y Minero de España (Spain)
Manuel Montes, Instituto Geológico y Minero de España (Spain)
Ildefonso Armenteros, University of Salamanca, Spain (Spain)
Fabian Lopez-Olmedo, Instituto Geológico y Minero de España (Spain)


Compressional stresses due to the convergence of African and European plates deformed the Iberian Plate lithosphere, giving rise to intraplate Spanish Central System mountain range (SCS). This is a linear range trending ENE-WSW, in which the Variscan basement has been uplifted during Alpine compression. The SCS is a large pop-up structure bordered by a low angle thrust to the north-west (NBF, Central System northern border fault) and a steep reverse fault to the south-east. On both sides of the SCS there are two large sedimentary basins filled with continental Tertiary materials derived from the erosion of the uplifted belt, namely Duero and Tajo basins.
Within the SCS, the Sepulveda-Ayllón sub-basin (SAB) was an episodically closed basin connected to the Tertiary Duero Basin. The SAB lies in the south-eastern border of the Duero Basin, with a surface extent of 770 km2. It is elongated in a SW-NE direction, and fitted between the Palaeozoic-Mesozoic massif of Honrubia-Pradales to the northwest and Somosierra-Ayllón Range to the southeast, both of them parts of the SCS.
Neogene deposits of the SAB comprise different clastic systems that were fed up by uplifting structures related to successive movements of the SCS and to climatic conditions. During the compressive episodes, the Neogene record of the SAB was developed in the depression generated between the two main overthrusts; the Somosierra north border fault, that border on the south margin of the SAB and the Central System north border fault, which gave rise at the rear the Honrubia-Pradales amticlinorium.
The sedimentary response to the tectonism is keeped in the architecture of the SAB fill and in the distribution of the sedimentary facies. Lithostratigraphic research and facies mapping reveal three allostratigraphic units for the Miocene deposits. These units are composed of alluvial fan facies around the margin and mud flat facies at the centre. These units are separated by unconformities and/or sedimentary breaks, normaly marked over the top of each unit by a calcitic crust (Lacustrine/palustrine facies). Evidences of tectonism is held within the geometry of the fill, based on the recognition and interpretation of progradation/retrogradation cycles. Magnetostratigraphic analysis, together with a well-documented mammal fossil record, provide a rather precised chronology for the Miocene basin infill, spanning beyond chron C5AB to chron C4Ar.
This study examined the interrelationships between the timing rates, the depositional style and the deformational processes. It is as well possible to point out the movement cessation age of the main structures controlling this area of the Spanish Central System, according to the chronostratigraphic insight of the Miocene deposits, discriminating between deposits generated by regional tectonic processes or generated by other allocyclic process.


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