Integrated stratigraphy of the upper Paleocene-lower Eocene successions of the Tremp and Ager basins (south-central Pyrenees, Spain)

This work investigates the problems in the correlation between two piggy-back basins located in the South-Pyrenean foreland (Tremp and Ager basins, pr. Lleida, Spain). These basins formed sedimentary troughs, elongate in a NW-SE direction, and constricted between sub-parallel structural synsedimentary highs. These formed because of compressional stresses associated with the advancement of thrust faults. The studied interval (Paleocene/Lower Eocene) includes the period during which the two basins initially got separated; concurrent long-term sea-level rise and tectonic subsidence of the axial sectors allowed the deposition ad preservation of thick continental and marine successions. These record, through angular unconformities and growth geometries the combined effects of tectonics and eustatism on the development of sedimentary successions. The main problems in the Tremp-Ager correlation are related to poor chronostratigraphic coverage, prolonged hiatuses and/or condensed successions and to the strong N-S deformation of the Ager basin. Conversely, in the Tremp basin outcrop orientation and a minor degree of deformation complicate the reconnaissance of the different syntectonic stratigraphic units. These problems were faced, from one side, by reviewing the available chronostratigraphic informations, on the basis of updated age models, and by integrating them through geochemical and, marginally, magnetostratigraphic analyses; these were aimed to the recognition of specific undocumented chronostratigraphic horizons (Paleocene-Eocene Thermal Maximum in the Ager basin and in the southern sector of the Tremp basin; base of chron C24n in the Ager basin), and to obtain local correlations. The latter mainly concerned the continental deposits of the Paleocene of the Ager basin, where a characterization of the isotopic composition (carbonate C and O stable isotopes; organic matter C stable isotopes) was performed on different lithostratigraphic units. An intense field activity was also conducted, that concerned the facies analysis and measurement of many stratigraphic logs, for a total thickness of some 4000m, with the aim of building a stratigraphic correlation panel upon which chronostratigraphic data could be integrated in a physical-stratigraphic framework; contextually to this activity, a portion of the southern sector of the Tremp basin was studied in detail, in order to investigate the effects of synsedimentary growth of the Montsec high (the structure that separates the two basins) on the carbonate ramp deposits that formed during the lower Eocene, within a transect perpendicular to the original depositional profile. The results of these activities allowed the construction of an evolutionary model, which describes the separation of the two basins and the first stages of development of the Montsec high. Following an initial compressional stage, possibly concluded in the Lower Paleocene (Middle-Paleocene Unconformity), that brought to the initial development of a morphological threshold between the two basins (†œPaleo-Montsec†�), the main deformational stages took place in the Thanetian (Upper-Paleocene Unconformity) and in the Lower Ypresian; the latter is recorded by growth geometries, better than sharp angular unconformities, in the southern Tremp basin; two hypotheses are proposed concerning possible surfaces or equivalent units in the southern sector of the Ager basin.