Catchment evolution of the continental strike-slip as Pontes basin (Tertiary), NW Spain): constraints from the heavy mineral analysis

The Oligocene-Early Miocene As Pontes basin is a non-marine strike-slip basin, filled with small-sized alluvial fan deposits and related coal-bearing lacustr'ine-palustrine successions. The heavy mineral associations were determined in samples collected from two coal exploration wells, which cut the whole basin infill succession. The resulting data enabled us to reconstruct the evolution of the relief in the source areas that supplied sediment to the basin. The source areas were small and located in Precambrian to Palaeozoic (Cambrian-Silurian) rocks belonging to a basement structured during the Variscan orogeny. The main factor controlling the final heavy mineral assemblage in the Tertiary sequences was the initial source area lithology. Climatic variations and transport do not seem to have played a role in modifying the resulting heavy mineral composition. The environmental conditions in the basin led to a pervasive, early diagenetic sulfide neoformation in the terrigenous facies, whereas other diagenetic processes were not significant. The catchment areas which fed the As Pontes basin were mainly controlled by the major Pedroso-As Pontes-Moiñonovo fault system. They reached their maximum extent eastward, whereas their areal extent was more restricted northward and westward. Source area uplift and relief rejuvenation are recorded in the sedimentary infill by an increase in contribution of detrital grains ofultrastable heavy mineral (mainly zircon and tourmaline) fed from the metamorphic Precambrian 0110 de Sapo domain. Moreover, the progressive incision and down cutting ofthe drainage networks are recorded by progressive variation in the relative metamorphic heavy mineral percentages (biotite, gamet, andalusite, staurolite, sillimanite and kyanite