Current research topics
mineralogical, and geochemical composition of sediments: A
quantitative tool for predicting sediment composition and
reconstructing sediment provenance
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
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,
Some first results:
global model of evolution of sediment lithic composition with grain
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
Exploring the geometry of
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.