Flow paths of hydrothermal vent fluids in the Devonian Kess-Kess mounds, Anti-Atlas, Morocco

Spectacular Early Devonian carbonate buildups exposed in the eastern Anti-Atlas of southern Morocco and widely known as the Kess-Kess mounds constitute a classical example of deep-water mud mounds interpreted to be related to hydrothermal venting. They developed on the Hamar Laghdad elevation, located near Erfoud, created by a submarine volcanic eruption and composed of peperites and tuffs displaying geochemical characteristics of typical intra-plate basalts. During late Emsian time, reactivation of magmatic processes caused doming of the volcanic complex and the overlying sedimentary strata, and in consequence, a network of radial and tangential faults originated. Most of the mounds developed over the cross-points of these faults. One of the characteristic features of the Kess-Kess mounds is the presence of small caves and rock shelters. They represent fragments of larger fissures, originally open and connected to the seafloor through vent outlets. This system has formed simultaneously with the formation of mounds or immediately after. The irregular and/or pipe-shaped geometry of fissures points to chemical corrosion processes rather than to a tectonic mechanism for their origin. Dissolution of carbonates appears to be in contradiction with enhanced carbonate production, characteristic for the rapid mound growth, but it can easily be explained by mixing of two different carbonate solutions, hydrothermal fluids and seawater. It is suggested that during the formation of the Kess-Kess mounds dissolution took place in zones dominated by hydrothermal fluid-rich mixtures, and precipitation progressed with dilution of the vent fluids by seawater.