HYDRODYNAMIC BEHAVIOR AND SORTING MECHANISMS OF ORGANO- PHOSPHATIC BRACHIOPOD SHELLS: PRELIMINARY RESULTS OF EXPERIMENTAL STUDIES

DUPERRON, MARÍA; MOUAZÉ, DOMINIQUE; TESSIER, BERNADETTE; WEILL, PIERRE; SCASSO, ROBERTO ADRIÁN; LAGNIEL, ÉMILIE; TAKEUCHI, TAKESHI

Brest

Congreso; 18ème Congrès Français de Sédimentologie; 2022

Association des Sédimentologistes Français

Phosphorites composed by linguliform brachiopod shells represent a poorly known phosphorus source. As mineral precursorsfor P-fertilizers, they are an increasingly important resource for agricultural activities. Linguliform brachiopod shells have astratiform organo-phosphatic microstructure, which produces bioclasts characterized by their low density and laminar shape; as ofyet, no other studies on their hydrodynamic behaviour have been carried out. The objective of this work is to shed some light on theconcentration mechanisms acting on organo-phosphatic particles and finally leading to phosphorite formation; and also, to betterunderstand general processes of segregation in mixed bioclastic-siliciclastic sediments. In this work we present preliminary resultsof experiments made on fragments of Lingula anatina shells, a modern linguliform brachiopod species. We performed settlingvelocity and entrainment velocity measurements, as well as current-flow experiments on a mixed-sediment bed to investigateparticle segregation.Lingula anatina shells show a variable degree of mineralization, and thus density, throughout the shell: measured densities rangedfrom 1010 – 1330 kg/m3 in marginal areas to 1370 – 1730 kg/m3 in the central part of the shell. Their fragments have platy shapes,with shape factors between 0.02 – 0.39, a range similar to the one reported for platy carbonate bioclasts (Li et al., 2020). Settlingvelocity values were about 50% of similarly shaped and sized carbonate bioclasts, and 20% of similarly sized quartz sand. Settlingvelocities are controlled jointly by particle diameter, shape and density, which are strongly variable in Lingula anatina bioclasts;when plotted against each other, these parameters show high dispersion and complex interrelationships. By contrast, the ratiobetween the particle mass and its maximum projected area (M:A) shows a strong correlation with settling velocity. Drag coefficientswere calculated by nonlinear regression. The obtained values are comparable with those calculated from the data of Li et al. (2020),suggesting that the lower settling velocities in organo-phosphatic bioclasts are explained mostly by their lower density and not bydifferences in their falling behaviour. Threshold near-bed velocities (u*c) ranged between 1.0 – 4.3 cm/s for organo-phosphaticbioclasts ranging between 0.7 – 5 mm (sieve-diameter). The obtained values are comparable with those of carbonate bioclasts andquartz-sand with similar diameters (Weill et al. 2013, Rieux et al. 2018). Dual-behaviour has been noted in carbonate bioclasts, wherelow settling velocities and significant resistance to erosion contributes to their segregation from siliciclastic sediments (Weill et al.2013, Li et al. 2020); this type of behaviour should be more pronounced in organo-phosphatic bioclasts given their smaller settlingvelocities.Entrainment and segregation experiments were conducted on a mixed-sediment bed composed of fine siliciclastic sand andorgano-phosphatic particles (3 mm-sieve diameter). For the bed set-up, a concentrated slurry of siliciclastic and organo-phosphaticparticles was set on the empty flume, which was later filled with water. At sub-threshold velocities, little sand transport took placewith no bioclast motion; this was followed by incipient migration of small sandy ripples, occasional bioclast motion and imbrication.At this stage, armoured bioclast-rich patches developed through sand winnowing. Above threshold velocities the sandy ripples grewtaller; bioclasts were not easily eroded, excepted in high-shear and turbulence areas such as stoss faces and downstream of lee facesof ripples. Once incorporated into the flow they were rapidly transported downstream and redeposited in bioclast-rich areas (withhigher bed rugosity) or out of the test section. By contrast, sand seemed to be transported with a lower velocity corresponding tothat of the migrating ripples. The final rippled bed was noticeably sandier than the original mixed-sediment massif; by contrast, abioclast-rich deposit was formed downstream of the test section. Contrasting hydrodynamic behaviour in terms of settling andentrainment velocities favoured the segregation between both sediment fractions; however, sorting mechanisms are also stronglydependent on less predictable sedimentary processes such as bed armouring, and increased form-related shear stress and turbulencedue to ripple migration. Extensive experimental studies should be carried out to further investigate segregation in mixed bioclastic-siliciclastic sediments