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Geologisch-Paläontologisches Institut, Sigwartstrasse 10, D-72076 Tübingen, Germany
During sea-level highstand in late Mid-Devonian time, a carbonate ramp was established on the vast shelf of the Northwest African Craton, in the Ahnet and Mouydir Basins of southern Algeria. The ramp was at least 400 km wide and 700 km long and sloped, in front of a local shoal of rugose-tabulate-stromatoporoid dominated biostromes, towards the west into the Reggane Basin and towards the north into the Béchar Basin. Mid- and inner ramp zones were presumably situated in the area of the present Hoggar Massif and have been removed by erosion. Subsidence rates during Mid-Devonian time were considerably lower than during the preceding and subsequent periods of clastic sedimentation in Early and Late Devonian time. Spectacular mud buildups formed in an outer ramp setting and developed from small, lens-shaped, to steep, conical mounds, which eventually amalgamated into mud ridges and mud ‘atolls’. Orientation of mound rows and ridges matches older NW-SE and N-S tectonic lineaments, which were probably rejuvenated during Mid-Devonian time. Stable isotope values of early marine cements, composition of the buildup fauna, and the emplacement and orientation of neptunian dykes do not indicate, however, that buildup growth was triggered by hydrothermal activity or cold seeps.
