Geochemical variability induced by entrainment sorting: quantified signals for provenance analysis

A substantial part of the mineralogical and chemical heterogeneity of clay-poor sands, at the bed scale, results from tractive redistribution of the grains in the bedload. Since coarse and light particles are more easily entrained than denser and smaller heavy minerals, the latter commonly concentrate in grain scale lags, that alternate with sand depleted in these minerals. When sampled and analysed at the centimetre scale, this primary heterogeneity translates into nice geochemical covariations between the minor/trace elements that concentrate in some heavy minerals (Zr, Ti, P, Cr, V, Y, Ce among others). Regressing these variations allow us to derive two chemical fingerprints of the sand: one records the proportions between some heavy minerals, the other the average composition of framework grains, i.e., a sand composition corrected from the contribution of heavy minerals. Both relate to provenance and allow different sand beds to be compared in appropriate spider diagrams. The construction of these diagrams, their potential use and their limitations are discussed using two contrasting examples. Continental channel sandstones from the Ravenscar Group (middle Jurassic, Yorkshire, UK) illustrate how these geochemical fingerprints vary from the local to the Group scales and how robust they are against diagenetic overprint. At Ravenscar, the Zr vs. Ti, V, Cr covariations record the presence of Ti oxides and magnetite together with zircon and of volcanic lithoclasts at the deposition stage, despite a high degree of diagenetic alteration. Overbank sandstones (lateral to channels) from the deep marine Grès d'Annot Formation (Eocene–Oligocene, Alps) illustrates how apatite and monazite delivered by a dominantly granitic source generate covariations between Zr and P, Y, Ce and Th contents with characteristically steep slopes.