The middle-upper Eocene Loza-Portilla Formation (Western Pyrenees, North Spain): Palustrine-lacustrine carbonate-dominated sequences in a piggy-back basin under compressional tectonic stress

The Loza-Portilla Formation consists of 100–300 m of alluvial siliciclastics and palustrine-lacustrine carbonates and minor evaporites, infilling a tectonically active syncline basin (Miranda-Trebiño Basin, Basque-Cantabrian Pyrenees), which developed over the south Pyrenean Thrust Front during middle to late Eocene times. Detailed facies characterisation and correlation allowed the reconstruction of the stratigraphic architecture of the formation, which consists of three unconformity-bounded depositional sequences, representative of three successive phases of carbonate lake development. A model of low-energy shallow lake with ramp margins and wide palustrine fringes fits well with the architecture of the three stacked carbonate-dominated sequences, with stratal architectures characteristic of shallow balanced-fill lake basins. A range of limestones representative of different palustrine and lacustrine environments is distinguished, commonly arranged in metre-thick facies sequences and cycles reflecting water-level changes and marked shoreline shifts, likely associated with prevailing tectonic and climate conditions. Compressional tectonism and associated halokinetic movements exerted major control on depositional trends and distribution of the main depocentres. The Loza-Portilla Formation is of major palaeogeographical significance in the tecto-sedimentary evolution of the Pyrenean domain, labelling the first phase of uplift and emersion that affected large areas of the Basque-Cantabrian (western) Pyrenees, prior to end-Eocene generalised continentalisation.