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 tectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature of the source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) can be established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the Iberian Range; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin. Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm 67F 4Lt 29 and Rs 79Rg 3Rm 18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range. Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm 57F 34Lt 9) with diverse rock fragments (Rs 34Rg 23Rm 43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates of the Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclastic sand modes (Qm 60F 33Lt 7; Rs 4Rg 21Rm 75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif. Province D is quartzofeldspathic (Qm 55F 39Lt 6) with a dominance of phaneritic rock fragments (Rs 5Rg 33Rm 62) 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. The abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of the Tajo River drainage basin faithfully 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 trunk 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 underrepresented or not represented by sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation 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 confirms the rapid loss of these grains during transport because of their labile nature.