Mud buildups on a Middle Devonian carbonate ramp (Algerian Sahara)

Abstract

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 B6char 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.

Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)

“…Considering the climatic optimum during this interval (Copper, 2002), such a position is feasible. In fact, coral-stromatoporoid buildups of this age are known from the southern border of the Tindouf Basin (Gevin, 1960), from the former Spanish Sahara (Dumestre & Illing, 1967), southeastern Morocco (Fröhlich, 2003) and the Gourara of central Algeria (Wendt & Kaufmann, 1998). A more northerly position of about 20 • S was obtained from Givetian limestones in the southern Ahnet Basin by Smith, Moussine-Pouchkine & Ait Kaci Ahmed (1994).…”

“…Stromatoporoids, tabulate and colonial rugose corals are extremely rare and have been found only in a few places (e.g. Gautier & Chudeau (1907) were the first to mention the spectacular mud buildups, which were constructed during the early Givetian on the central and marginal ridge (Figs 13, 16;Moussine-Pouchkine, 1971;Wendt, Belka & Moussine-Pouchkine, 1993;Belka, 1994;Wendt et al 1997;Wendt & Kaufmann, 1998;Kaufmann & Wendt, 2000). The entire fauna indicates a shallow, open marine environment which passes laterally, with an insignificant relief, into basinal areas with prevailing shale sedimentation.…”

Benthic carbonate factories of the Phanerozoic

“…Devonian, Azzel Matti, Algeria. Construction probably by microbially mediated organomineralization and biomineralization, influence of methane seeps is possible (see Wendt and Kaufmann 1998). Photo courtesy of Bernd Kaufmann related empty bucket may be a diagnostic criterion for the tropical factory.…”

“…A likely example of this transition is the interfingering of Devonian and Carboniferous mounds of the Sahara with skeletal sands of the intermound areas (Kaufmann 1998;Wendt and Kaufmann 1998;Wendt et al 2001). The absence of euphotic biota and the position on a ramp strongly suggest that these sands represent an autochthonous cool-water assemblage that surrounded the mounds.…”

“…However, the evidence is circumstantial, and the interpretation remains speculative. It has been challenged in favor of hydrocarbon seeps (Peckmann et al 1999) or normal marine, microbial origin (Wendt and Kaufmann 1998). At present, there is little evidence that mudmounds dominated by hydrothermally precipitated carbonate are a volumetrically important phenomenon in the rock record.…”