Sand composition in an Iberian passive-margin fluvial course: the Tajo River

Abstract

The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multiple Province D is quartzofeldspathic (Qm55F39Lt6) with a dominance of phaneritic rock fragments (RssRg33Rm62) and corresponds to the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem-Lisboa basin are the main sources.Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continental block fields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminate the defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy.Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in the source areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies is the main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the main bedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sand and the low significance of inherited sandy load from previous provinces. * Corresponding author. Tel.: +39 984 835528; fax: +39 984 835319. E-mail addresses:emilia.lepera@irpi.cnr.it(E.LePera).anibas@geo.ucm.es (l Arribas).The abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of the Tajo River drainage basin faithfu11y represents the relative abundance of granodiorite+monzogranite bedrock exposure in each subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rock fragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main tnmk river. This is manifested by differences of 18 23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand from Provinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is lUlderrepresented or not represented by sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This lUlderrepresentation causes a difference between means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands from Province A, B and C+D, respectively. This fact confmns the rapid loss of these grains during transport because of their labile nature.

Composition of clastic sediments in the Somosaguas area (middle Miocene, Madrid Basin): insights into provenance and palaeoclimate

“…Comparing these data with the average values obtained by Palomares and Arribas (1993) for different mixed source ar- rocks (Table 2) indicates a mixed composition source area (Mack, 1981). However, as several authors have suggested (Palomares and Arribas, 1993;Le Pera and Arribas, 2004), the abundance of these grains in the sediments is not directly related to their abundance in the source area. Even when each lithology have a different Sand Generation Index (Palomares and Arribas, 1993;Arribas et al, 2000;, we can define the extent of intervention of each lithology in the final mixture based on other features.…”

“…Several studies have addressed the relationship between climate and sandy deposit composition in present-day sediments Basu, 1976;Suttner et al, 1981;Franzinelli and Potter, 1983) which serves as a base for palaeoclimatic interpretations of older sediments. To better understanding the roles of these factors in determining sand composition, actualistic research has addressed fluvial to transitional and marine deposits worldwide (Ingersoll and Suczek, 1979;Le Pera and Critelli, 1997;Le Pera et al, 2001) including river sediments of the Madrid Basin (Arribas et al, 2000;Le Pera and Arribas, 2004). This paper addresses sand petrofacies formed during the Middle Aragonian (14.2Ma) at the Somosaguas palaeontological site (Madrid Basin), the evolution of their source area (Spanish Central System) and the climate that prevailed over the middle Miocene in this area.…”

Petrography of Middle Jurassic to Early Cretaceous sandstones in the Kutch Basin, western India: Implications on provenance and basin evolution

“…Petrographic investigation of sandstone is widely used to infer tectonic setting, source rock composition, transportation and environment of deposition (Dickinson 1970(Dickinson , 1985Dickinson and Suczek 1979;Dickinson et al 1983;Ingersoll and Suczek 1979;Le Pera and Arribas 2004;Le Pera et al 2001;Zuffa 1985Zuffa , 1987. Sediments derived from cratonic sources (low-lying, granitic or gneissic with recycled platform sediments) are generally quartzose sands with high monocrystalline quartz (Qm)/polycrystalline quartz (Qp) and K-feldspar (K)/plagioclase feldspar (P) ratios, and deposited in continental interiors or passive continental margins (Dickinson 1985).…”

Provenance of fluvial sandstones at the start of late Jurassic–Early Cretaceous rifting in the Cameros Basin (N. Spain)

“…Despite the low percentage of slate-schist fragments (b2%), the presence of these rock fragments is highly significant. Several studies on modern sediments (Palomares and Arribas, 1993;Le Pera and Arribas, 2004) have demonstrated that sandstone composition does not allow direct quantitative estimations of the lithologies at the source, owing to the different capacities of rocks to generate sands (Palomares and Arribas, 1993). Indeed, slates and schists show a very low sand-generating capacity (Sand Generation Index, Palomares and Arribas, 1993) and are often underrepresented, because of their low rock-fragment content.…”