THE CHEMISTRY AND BIOTA OF THE DEVONIAN SUBMARINE HYDROTHERMAL VENTS FROM THE EASTERN ANTI-ATLAS, MOROCCO

An unusual submarine hydrothermal system has been recognized in the Devonian of the eastern Anti-Atlas, Morocco. It developed on the passive continental margin of Gondwana, and hence is not a counterpart to hydrothermal vents widespread in the modern oceans. During the Early Devonian, a submarine volcanic eruption created an elevation on the sea floor (exhumed as a topographic ramp Hamar Laghdad in the present-day landscape), which subsequently became a site of extensive carbonate production. Lack of calcareous algae, microborings and micritic envelopes suggests deposition within an aphotic zone. Spectacular conical mud mounds were formed along synsedimentary faults that served as conduits for migration of hydrothermal fluids to the sea floor. Within the mounds fluids flowed through a system of chimneys and fissures. Geochemical data suggest that mud mound carbonates precipitated from brines comprising a mixture of hydrothermal fluids and seawater. Fluid inclusion measurements indicate low temperatures of hydrothermal fluids (below 150̊C) from which carbonates were predominantly precipitated. Metalliferous and sulphide phases are absent. Vents were episodically active during a time of approximately 18 Ma (from the late Emsian until the early Frasnian), but large mud mounds developed only during the Emsian. The fossil vent community includes thermophilic rugose corals, clusters of bizarre spiny tubes (pogonophorans?), we term Hamaria, small gastropods, small sponges, monoplacophorans, trilobites, and locally extremely frequent ostracods. Most of the fossil taxa are new to science. The peculiarity of the Hamar Laghdad vent system is that during the Eifelian and early Givetian hydrothermal phase hot fluids contained additionally thermogenic methane derived presumably from the underlying basaltic intrusives. Hydrocarbons contributed to a rapid in-situ cementation of the carbonate mud and led to development of a seep community dominated by bivalves.