The main goal of this paper is the study of the geometry and distribution of the units that made up the infilling of the southwestern basin margin. The drilling analysis and geological cross section define an eastern-western orientation of the axis basin, and deepening basement to the east and a some structural blocks with different deepening and trend, these latter are: the Corrales and Topas blocks are the shallowest, the Cuarena block is intermediate and Alaejos block is the deepest. They are limited by NNE-SSW, ENE-WSW, WNW-ESE fault systems. Five lithostratigraphyc units, these demostrates that fluvial-lacustrine sedimentation is controlled by the basin dinamics and the prograding alluvial fans that came from the basin margins: the Zamora Sandstones Formation, the Lutites Ehtrala Formation, the Corrales Sandstones Formation, the Aldearrubia Sandstones Formation and Molino del Pico Formation, can be distinguished