Current research topics

Modeling granulometric, mineralogical, and geochemical composition of sediments: A quantitative tool for predicting sediment composition and reconstructing sediment provenance

Numerical modeling has become a powerful tool in several fields of geosciences, but a comprehensive model to describe the composition of clastic sediments is still lacking. This project aims at developing a numerical-statistical model that allows for describing quantitatively the petrographic, chemical, and granulometric changes related to the overall transfer of material from the original source rock to the sediment at the final site of deposition. These relative changes of sediment composition (as compared to the initial source) are largely controlled by tectonics, physiography, climate, and transport energy. The model is intended for both inductive and deductive approaches. Potential applications of the model might be, for example, the prediction of sediment composition under well-known geologic conditions and the reconstruction of sediment provenance based on final composition. To achieve this general goal, we are working in two subgoals: (1) the broadening of our theoretical understanding of so-called multi-way compositions (e.g., data sets simultaneously reporting grain size, chemistry, and mineralogy of sediment samples) and (2) modelling of multi-way compositional change in several case studies.

Keywords: compositional data analysis, diagenesis, log-linear models, quantitative provenance analysis, weathering.

• Basics about compositional data analysis: Compositional data in a nutshell

Some first results:

• A global model of evolution of sediment lithic composition with grain size variations. Blatt, Middleton and Murray (1972) published a plot where they conveyed "the probable relationship between grain size and detrital fragment composition, based on the limited data currently available". The plot gives, for the range of grain sizes occurring in nature, the composition on five parts (rock fragments, poly-crystalline quartz, mono-crystalline quartz, feldspar and mica). Our goal here is to fit an Aitchison (1986) trend to this data, by means of regression of a composition on grain size. The theory underlying the solution to this problem is available here, and a nice presentation here. A paper (under revision) has been accepted in Mathematical Geosciences.
• Exploring the geometry of granulometric curves. Granulometric curves (equivalent to probability densities) can be treated within the framework of the Hilbert space structure proposed and developed by Egozcue and Diaz-Barrero (CoDaWork'03), and by van den Boogaart (CoDaWork'05). One takes logs of the measured density curve, and regresses the result against a (low) number of orhonormal polynomials. The chosen polynomials should be orthogonal with respect to a given reference density (exponential if the dominant process is comminution, normal if the dominant process is sorting vs. mixture, uniform for unknown dominant process). A preliminary talk is available here, and a more evolved material will be presented at CoDaWork'08.
• A local model of dependence of geochemistry on grain size. That the geochemistry of a sediment depends on its grain size is a well-known fact. The factors underlying that control are also quite known, but nevertheless still poorly quantified. Of goal here is to derive a predictive model, able to tell us what should we expect to find when measuring the geochemistry of a sediment given its grain size fraction (and explain why). The data set used was obtained from moraines and streams of a glacial area in the Alps (in a granitic/granodioritic massif): samples were sieved in 11 one-unit-phi intervals, and each fraction was analysed for major oxide and trace element concentrations. Results suggest that the strongest control in this case is the inherited characteristic crystal size of each mineral in the parental rock. No chemical weathering is observed, as the resulting geochemical evolution can be perfectly explained by invoking three main characteristic crystal sizes (from -1 to 4, from 4 to 7 and finer than 7 phi) and a slight difference on the mechanical properties of phyllosilicates against other components. This material will be presented at CoDaWork'08. The necessary theory regarding regression of compositional responses has been submitted to Computers & Geosciences.