Etude geochimique d'une sequence des 'terres noires'; application au probleme de l'ecologie de spongiaires constructeurs

The 'black earths' of Diois (French Prealps) and the Laragne region are a monotonous succession of argillaceous and marly beds of late Bajocian to early Oxfordian age. They contain platy limestones and associated lentils of abundant sponge spicules, apparently bioherms, especially near the middle of the sequence. Variable but high sulfur content, constant high boron content, and constant contents of Cr, V, Ba, Sr, and Ga in these rocks indicate euxinic conditions of deposition. The platy limestones represent influxes of colder, more oxygenated water, brought into the deep part of the basin by intermittent currents, making possible the development of the sponge bioherms under semi-euxinic conditions.