Dewońskie budowle węglanowe (Carbonate buildups) Sahary Środkowej i ich związek z podmorskimi źródłami termalnymi

Zdzisław Bełka




Perfectly exposed, carbonate mud buildups of Givetian age occur in the southern Ahnet Basin of the central Sahara (Fig. 1). They are different in respect of geometry, size, and biota from those generally described from the Devonian. Their geometries range from isolated mud mounds to mud ridges up to 8 km long (Figs 6 - 7). Both the mounds and the ridges yield the same internal structure. They are built of a massive core surrounded by mostly coarse-grained capping beds. Apparently, these cores gained a considerable relief (up to 60 m) during their growth, but they did not produce any coarse-grained debris material deposited as talus on the flanks. The rocks are auloporoid-dominated bioclastic wackestones to bafflestones. Typical is the total absence of stromatoporoids. There are at least three cement generations associated with pyrite crystals and Fe-impregnations in the spar-filled voids. The capping beds are distinct from the core both deposition ally and diagenetically, and the boundary between them is sharp. These wellbedded, poorly cemented crinoid packstones overlie the core with a discordant contact and can additionally be found in neptunian dykes cutting the cores. The mud mounds and ridges are perfectly aligned along three directions (Figs 4,7-8) that correspond exactly with the lineament and fault system known from the Pan-African (Precambrian) basement around the Ahnet Basin. A bathymetric differentiation of the seafloor due to a system of active faults, however, is questionable. It seems more possible, however, that the faults served as a conduit for fluid migration and that the mud buildups have thus formed at sites of concentrated hydrothermal venting. This hypothesis is based on several coinciding geological observations including the presence of the hydrothermal mineralization (pyrite, barite, apatite, and dolomite) within the buildups and the isotopic signature of carbonates. Moreover, the occurrence area of the Givetian mud mounds and ridges corresponds well with the position of the Pan-African formations in the basement, which display characteristics typical for of island arcs and active continental margins. The evolution of mud mounds into mud ridges cannot only by explained by near-by placement of centres of venting along tectonic lines. It is suggested that the formation of mud ridges was favoured in those places where predominant currents were directed parallel to the orientation of venting fissures (Fig. 5 and 7).