Bulk geochemical analysis of different lithological types (arenites, wackes and shales) demonstrates a strong relationship between texture and chemical composition of the rocks, even taking into account the immobile elements. The largest differences are between shales and arenites, while the wackes have intermediate compositions. The CIA (chemical index of alteration) indicates partial remobilisation of oxides from source rocks and enrichment of aluminium and potassium in the shales, which reflects their potassic clay mineral composition. Overall, siliciclastics of the Cerro Agua Negra Formation have a similar composition to the upper continental crust (with slight net enrichment of SiO2 in the arenites and of Al2O3 in the shales). Rare earth element profiles reflect terrigenous contributions, since they are enriched in light-REE, have a pronounced negative Eu anomaly and a relative depletion of the heavy-REE. The results show that discrimination plots commonly employed to infer provenance and tectonic setting from siliclastics are compromised where bulk chemical composition is strongly dependent on grain size and sorting.