Geochemical variability induced by entrainment sorting: quantified signals for provenance
analysis
Abstract
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 Gre`s 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.