Latest Oligocene–Late Miocene lacustrine systems of the north-central part of the Ebro Basin (Spain): sedimentary facies model and palaeogeographic synthesis

In the central part of the Ebro Basin (Spain), north of the Ebro River, three allostratigraphic or tectosedimentary units differentiated within the latest Oligocene–Late Miocene (upper Agenian–Vallesian) interval are sedimentologically characterized. These deposits represent carbonate and sulphate lacustrine environments fed by Pyrenean alluvial systems (Luna system and Huesca system). In each unit, several facies associations or lithofacies were mapped. These interfinger through time: gypsum lithofacies occupy the southernmost part of the study area and are surrounded by carbonate and/or detrital lithofacies to the north. Each lithofacies consists of different carbonate, sulphate and/or detrital facies, arranged in simple sequences, decimetres to metres thick. The four types of sequences characterized reflect a complex shallowing evolution involving carbonate or sulphate contexts. Thus, each lithofacies represents the sedimentation in a particular lacustrine or alluvial subenvironment. A single lacustrine system is proposed for the central part of the basin. Its evolution followed cycles of water level variations, which caused kilometre-scale migrations of the lake shoreline. In this model, two distinct situations or lakes alternated through time. (a) One represents high levels associated with a single body of dilute carbonate water, where massive carbonate facies formed influenced by organism proliferation. These lakes had wide palustrine vegetated margins, in which bioturbated facies developed. (b) The other represents low levels that correspond to a playa-lake model. Laminated sulphate deposits formed in the lake waters, while nodular gypsum originated in the surrounding saline mud flats and within the exposed, previously deposited carbonate sediments. Transitions from one situation to the other, including minor water level fluctuations within carbonate sedimentation conditions, were marked by the development of laminated and stromatolitic facies. Lake level variations were caused by climatic cycles and gave rise to the simple sequences and to the lithofacies lateral relationships recognized in this study. Correlation among tectosedimentary units in the studied Pyrenean domain and in the Iberian domain of the Ebro Basin shows that for units N1 and N2 the facies that resulted from dilute carbonate waters are rare in the lacustrine areas of the Iberian margin. This indicates that a strong hydrological contrast existed between both regions. On this basis, a theoretical asymmetrical facies model is proposed for the single lacustrine basin that extended over these regions.