Variabilidad en la composición y procedencia de las areniscas turbidíticas del Grupo Hecho (Pirineo Sur-Central)

Resumen

The Eocene turbidite systems of the Hecho Group (tectosedimentary units, TSU-2, 3, 4 and 5) of the Ainsa-Jaca foreland basin (South-Central Pyrenees) are characterized by spectacular outcrops and good preservation of the original field relationships between the fluvio-deltaic, shell deposits and turbidites. The aim of this work is the petrographic characterisation of turbiditic carbonate arenites in order to unravel provenance signatures and their evolution. Ternary compositional diagrams based on carbonate grains types, allow us the recognition of a maturity trend in the Hecho Group turbidites. The lowermost turbidite system, TSU-2, present the highest abundance in quartz grains and the lowest quantity in carbonate grains, both extra-and intra-basinal. Toward the top, in TSU-5, quartz abundance decresase and conversely carbonate grains increase. Non-carbonate rock fragments indicate the dominance of low-and medium-rank metamorphic source rocks toward the uppermost TSU-5. In addition , back scattering lectron imaging on feldspar grains, confirm the presence of granites, gneisses and metamorphic rock in the source area. Thus, the obtained results reflects the exposition and erosion of the Pyrenean crystalline basement (granites, gneisses and metamorphic rocks) and its progressive replacement as sourece rock by carbonate rocks from the thrust-folds belt, due to tectonism evolution and sea-level variations.