[en] The sediment infilling of the Cameros Ilasin during the Latest Jurassic and Early Cretaceous can be separated into four depositional sequences, using unconformities as the key. Each sequence starts with rocks of fluviatile facies and ends with lacustrine carbonates. The Leza Fomiation belongs to the Depositional sequence 111 (Barremian - Aptian) and corresponds to the lacustrine facies that developed in the northeastern margin of the basin in the upper part of the sequence. It is a particularly suitable unit for studying the main factors that controlled sedimentation (tectonic and eustasy) because it shows marine influence at several levels (the age of which has been determined), and the synsedimentary tectonic (growth faults and slumps) were very active in that margin, due to the existence of a low angle main listric fault which limited the basin in the NE. The sedimentological analysis of facies and palaeogeographical maps show that the Leza Formation is divided into six bodies or lithosomes, corresponding to six paleolakes, limited by synsedimcntary faults. These were finally buried and preserved by the overlying formation. However, Ihe lakes were not isolated as the marine transgression, coming from the southeast (Tetbys), afected aH of them. This transgresion occurred during the Lower Aptian, the moment when the sea level was highest in the Iberian Trough during the Early Cretaceous, so that. the drainage from the continental aeras was insuficient (great amounts of water were retained in the basin and, therefore, led to the development of important acumulations of rocks of lacustrine facies). The major tectonic control was due to the movement of the low angle listrie fault (NW - SE) that dips southwestwards, that actuated as a detachment during the distensive stage (basin formation). That event was acompanied by the development of a systcm of parallel and orthogonal faults that produced the small basins where sedimentation of the rocks of the Leza Formation took place. During the inversion stage, the fault acted as a thrust and it produced a 30 km displacement to the north.

[es] La sedimentación durante el Jurásico terminal y el Cretácico inferior en la Cuenca de Los Cameros se desarrolla en cuatro Secuencias deposicionales de forma tal que todas comienzan con facies siliciclásticas fluviales y terminan (excepto la IV) con facies calcáreas de origen lacustre. El control tectónico durante la sedimentación es muy claro y se manifiesta de diversas maneras y escalas, pero el control eustático también parece jugar un papel importante. Para estudiar estos factores se ha analizado con detalle una de las etapas lacustres en la cuenca, concretamente la que termina la Secuencia deposicional III (Barremiense - Aptiense), y se ha elegido la franja mas nororiental de la misma. En esta unidad (la Formación Calizas de Leza), y en ese sector, se ha realizado un análisis sedimentológico detallado identificando cuatro grupos de facies que corresponden a la sedimentación en lagos carbonáticos costeros, con etapas de influencia marina neta. También se ha reconstruido la cuenca para la etapa Barremiense - Aptiense, que formaba una gran rampa basculada hacia el NE, en cuyo borde nororiental se desarrollaba la sedimentación lacustre- costera que nos ocupa. La influencia marina proviene del SE, del Tetis, ya que durante esa etapa en el resto de la cuenca Ibérica se produjo la mas importante transgresión del Cretácico inferior en la zona. Por otro lado, la Formación Leza está formada por seis litosomas carbonáticos claramente diferenciados, limitados por paleofracturas que funcionaron durante la sedimentación de la unidad y que fueron fosilizadas posteriormente. El control tectónico está ejercido por una fractura de bajo ángulo en el margen noreste, buzando hacia el sur, que durante la etapa distensiva, de formación de la cuenca, actuó como "detachment" y que durante la etapa de inversión alpina actuó de nuevo originando un cabalgamiento de cerca de 30 km claramente apreciable en los perfiles sísmicos disponibles.