The Lower Aptian D.S. is made up of materials which settled in shallow carbonate ramp enviroments. This sequence shows a retrogradational evolution, that is, from internal to mid ramp enviroments. The vertical evolution of this sequence and its sedimentological features allowed us to differenciate three system tracts (L.S.T, T.S.T and H.S.T). The Low System Tract appears only in the western area of the basin and it consits of facies of typical shallow marine enviroments. Transgressive System Tract and Highstand System Tract are identified in the central and eastern areas of the basin and characteristic facies associations stand for opener sea enviroments. Finally, the paleogeography of the basin in the Lower Aptian period shows that Las Parras subbasin was opened up eastwards. This fact is due to the thickness that raise south and eastwards and to the opener sea enviroments that are spreaded out in the easternmost area of the basin
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