Hydrodynamic behaviour of organo‐phosphatic bioclastic sediments: An experimental study on linguliform brachiopod shell fragments

ABSTRACT Bioclastic phosphorites composed of linguliform brachiopod shell debris remain largely unexplored from a sedimentological, process‐oriented perspective. Understanding the hydrodynamic behaviour of these low‐density, platy sediments is essential for elucidating the concentration mechanisms driving their accumulation. The present experimental study delves into this issue through the determination of settling velocity and critical shear velocity of organo‐phosphatic bioclasts derived from extant Lingula anatina shells (Brachiopoda: Linguliformea). A total of 137 settling velocity determinations were obtained through stroboscopic photography of the particles' trajectories in a settling tube. In all, 30 critical shear velocity determinations of five sieve fractions were acquired from flow profiles measured in flume experiments. Additionally, L. anatina shell density is here reported for the first time. Organo‐phosphatic bioclasts present low particle densities (1040 to 1734 kg/m 3 ), which combined with their platy shape grant them low settling velocities and critical shear velocities comparable to siliciclastic sediments. They thus present a tendency to be easily transported once set in motion coupled with significant resistance to erosion, defining a dual hydrodynamic behaviour like the one reported in platy carbonate sediments. Their resistance to erosion, despite their relatively low submerged weight, is here hypothesized to be explained by shape effects such as elevated intergranular friction forces, large pivoting angles and a low degree of exposure to the flow. In terms of settling, gravel‐sized organo‐phosphatic bioclasts are equivalent to fine‐to‐coarse‐sized siliciclastic sand; this reflects the textural characteristics of many ancient phosphorite deposits, suggesting that settling equivalence played a role in their deposition and that the common conception of phosphatic particles as a heavy, relatively immobile component is not readily applicable to organo‐phosphatic bioclasts. These results shed light on the sedimentology of ancient bioclastic phosphorites and contribute to the still underexplored field of biogenic sediment transport.