Provenance of Oligocene–Miocene alluvial and fluvial fans of the northern Ebro Basin (NE Spain): an XRD, petrographic and SEM study

Abstract

This paper combines stratigraphic and sedimentological data with XRD, petrographic and SEM techniques in order to investigate the fluvial and alluvial fans that were developed during the Oligocene and the Lower Miocene in the central area of the northern Ebro Basin (Spain), and their respective source areas. The proximal sectors of the coeval depositional systems can be separated on the basis of lithofacies criteria. Conglomerate features reveal that the source areas for the alluvial fans were limited to the South Pyrenean Sierras, whereas those of the fluvial fans included more northern Pyrenean areas, such as the Axial Zone or the clastic Tremp-Graus Basin. Far from the basin margin, the distal alluvial facies interdigitate with the middle-distal fluvial deposits. In these areas, only detailed mineralogical and petrographic analysis of the rocks can determine their origin. Thus, the methods used have allowed us to differentiate between authigenic and detrital phases. On the basis of these observations, three different types of coarse sandstone petrofacies have been distinguished: (1) carbonate-rich sandstone, (2) high-quartz sandstone and (3) high lithic-fragment sandstone (rich in shale and feldspars grains). The first corresponds to the alluvial samples and corroborates the South Pyrenean Sierras as alluvial fans source area. The high-quartz and the high lithic-fragment sandstone correspond to the fluvial samples and evidence that the drainage basin of the fluvial fans included the clastic Tremp-Graus Basin and the Axial Zone. The high lithic-fragment petrofacies is representative of the most ancient fluvial rocks whereas the high-quartz sandstone corresponds to the recent and nonconforming fluvial rocks. This change in the fluvial petrofacies allows us to recognise the changing importance of each region as a source area of the fluvial fans over time, which can be correlated with Pyrenean tectonic activity during the Oligocene.