Settling velocity of Lingula anatina shells: an experimental approach to bioclastic granular phosphorites

DUPERRON, MARÍA; MOUAZÉ, DOMINIQUE; TESSIER, BERNADETTE; SCASSO, ROBERTO ADRIÁN; TAKEUCHI, TAKESHI

Praga

Congreso; 35º IAS Meeting of Sedimentology; 2021

International Association of Sedimentologists

Granular phosphorites composed by linguliform brachiopod shells andsiliciclastic sediments represent a globally distributed phosphorus resource. Thebrachiopod bioclasts are characterized by an organo-phosphatic composition andlaminar shape which defines a particular hydrodynamic behaviour that has notbeen investigated yet. Their study is crucial in understanding theconcentration mechanisms acting in the formation of such phosphorites, as wellas for enrichening the knowledge of mixed-sediments and sediment dynamics ingeneral. We present the results of settling velocity measurements made on wholeand fragmented shells of Lingula anatina, a modern linguliformbrachiopod species. The results will be applied to a case study of ancient bioclasticphosphorites (Ordovician of northwestern Argentina). The shells are characterized by a complex shape and variable densityacross them. The study and classification of their fragments includedtaphonomic observations on their breakage patterns, bulk density andmorphometric measurements, in order to interpret their settling behaviour. PVCsheets were also used as possible analogs to Lingula shells.Density measurements yielded very low density values (range: 1013 –1330 kg/m3), with the thickest, more mineralized shells and shellfragments showing higher values. 3D nominal diameter values ranged between 1 –9 mm, at the coarse sand to gravel size. Settling velocities ranged between0.02 – 0.08 m/s. Shape factor ranged between 0.01 – 0.05. The whole set of settlingvelocity results show a wide dispersion when plotted as a function of particlediameter. When the particles are classified according to their characteristics,clear trends in settling velocities are revealed. Particles with higher densityand shape factor, show higher settling velocities (0.05 – 0.08 m/s), whereas thosewith lower density and shape factor, show lower settling velocities (0.02 –0.05 m/s). The behaviour of the platy PVC sheets is noteworthy: they show lowsettling velocities (≈ 0.04 m/s) in spiteof higher density (1324 kg/m3); this means that, for the set ofparticles under study, variations in shape might weigh more heavily on settlingvelocity than variations in density. With respect to their settlingequivalence, the sets of lower- and higher- shape factor particles have falldiameters corresponding respectively to fine-to-medium and to medium-to-coarse quartzsand. The Ordovician phosphorites are composed of linguliform brachiopodfragments, comparable in size with the experimental clasts, and siliciclastic coarsesilts to fine sands. These sediment fractions appear either well-mixed orinterlaminated. Siliciclastics are finer than the bioclast fall diameter: themixed sediments were thus not equivalent in terms of settling, the phosphaticbioclasts being the faster-falling. In the interlaminated deposits suspensionsettling was probably the main depositional mechanism, producingmillimetric-scale compositional segregation in absence of coarser-grainedsiliciclastics. In the mixed deposits, suspension settling was probably not themain depositional mechanism. Differences in settling behaviour between bothsediment fractions probably contributed to their segregation during transport. Althoughsedimentary processes other than settling probably acted in the formation ofphosphorites, contrasting settling behaviour between the phosphatic bioclastsand the siliciclastics may have been important in controlling the depositsformed.