Quaternary crusts on slopes of the Mediterranean Sea: A tentative explanation for their genesis

Quaternary deep-sea crusts, i.e. lithified omission surfaces, are widespread on slopes and subsea relief features in the Western and, especially, the Eastern Mediterranean Sea. The lithification is induced by the early precipitation of a magnesian calcite cement in the uppermost few centimetres of a calcareous ooze. Based on their thickness and degree of hardening, the collected samples have been divided into three main types, each of them corresponding to a different degree of lithification. Except in rare, probably old crusts collected at water depths below 2000–2500 m, and in infills of some sponge skeletons or in the case of inferred internal tension, recrystallization and (generally) mineral alterations are very limited. The precipitation mechanisms appear to be essentially physicochemically controlled, and ions involved in cementation come from the seawater reservoir by means of an inward diffusion of solutions. Diffusion is enhanced by bottom currents, these currents maintaining a low or zero rate of sedimentation. Moreover, lithification also depends on the chemical characteristics of the seawater; i.e. supersaturation with respect to low-magnesian calcite, and low contents of phosphates and dissolved organic matter which would inhibit crystal growth. Local factors and primary composition of the original sediment also appear to influence the speed of early lithification. When conditions of lithification are favourable, a few hundreds of years to a few thousand years at the most seem sufficient for the first two stages of lithification to be reached. The submarine lithification was active during the last glacial episode and, as exhibited by numerous samples, especially towards the end of this period. The previous cold stages were probably also episodes of efficient lithification. This correlation with Quaternary climatic cycles certainly reflects an intensification of thermohaline and bottom-current circulations during periods of severe climatic conditions. There is also some evidence of Holocene and even present-day crusts.