KESS KESS carbonate mounds, Hamar Laghdad, Anti-Atlas, SE Morocco - A FIELD GUIDE

Content Provider	Semantic Scholar
Author	Cavalazzi, Barbara
Copyright Year	2006
Abstract	Spectacular conical Early Devonian carbonate buildups up to 55 m high that crop out in the eastern Anti-Atlas of southern Morocco are microbially mediated carbonate mud mounds that were surfaced by soft- bottom communities dominated by small tabulate corals. They developed on the Hamar Laghdad elevation, which was created by a submarine volcanic activity, and were associated with a network of synsedimentary radial and tangential faults that originated by uplift of the intrusive laccolithic body underlying the Kess-Kess Formation. These faults served as conduits for the migration of hydrothermal fluids to the sea floor. Most mounds developed over cross-points of radial and tangential faults. Vents were episodically active until the Famennian, however extensive vent carbonate production occurred only during the Emsian. Preliminary geochemical results document that mud-mound carbonates and calcite cements in neptunian dikes precipitated from brines comprising a mixture of hydrothermal fluids and seawater. In addition, carbon isotope compositions (delta 13 C as low as -18 per mil PDB) suggest a contribution from thermogenic methane presumably derived from underlying basaltic intrusives. Aerobic bacterial oxidation of methane is likely the main process driving carbonate precipitation and rapid lithification of the mounds. Journal of Sedimentary Research 68 (1998), 368-377 Hydrothermal origin of Devonian conical mounds (kess-kess) of Hamar Lakhdad Ridge, Anti-Atlas, Morocco D. Mounji, P.A. Bourque, M.M. Savard ABSTRACT Various interpretations have been proposed for the origin of these cone-shaped, finely crystalline, Devonian carbonate mounds, ranging from shallow-water reefs to deep-water mud mounds, formed by biogenic and/or hydrodynamic processes. This study is the first integrated sedimentological and geochemical analysis of these structures. The mounds are cone shaped, steep sided, circular to subelliptical in plan view, and exhibit internal crude bedding parallel to the mound's outer surface. They occur in a cluster of 48 mounds on top of a volcanic massif. Stable isotope analyses of first-stage isopachous nonluminescent cement have yielded marine values. In contrast, the finely crystalline carbonate that makes up the bulk of the mound and the internally sedimented mud between crusts of early marine cements have significantly lower delta 18 O values, whereas values of delta C are similar to those of the early marine cement. Strontium isotope ratios also clearly distinguish the nonluminescent early marine cements from the finely crystalline material. The Sr/ Sr values for the former (0.707 934-0.709 392) are in or near the Devonian marine range, whereas the ratios for the latter (0.708 515-0.709 656) indicate a more radiogenic Sr source. On the basis of their lithology, architecture, isotope geochemistry, and clustered occurrence on top of a volcanic pile, we propose that the finely crystalline material that forms the mounds and the intermound beds was precipitated from hydrothermal fluids, and that accretion of the material was driven by hydrothermal venting. Various interpretations have been proposed for the origin of these cone-shaped, finely crystalline, Devonian carbonate mounds, ranging from shallow-water reefs to deep-water mud mounds, formed by biogenic and/or hydrodynamic processes. This study is the first integrated sedimentological and geochemical analysis of these structures. The mounds are cone shaped, steep sided, circular to subelliptical in plan view, and exhibit internal crude bedding parallel to the mound's outer surface. They occur in a cluster of 48 mounds on top of a volcanic massif. Stable isotope analyses of first-stage isopachous nonluminescent cement have yielded marine values. In contrast, the finely crystalline carbonate that makes up the bulk of the mound and the internally sedimented mud between crusts of early marine cements have significantly lower delta 18 O values, whereas values of delta C are similar to those of the early marine cement. Strontium isotope ratios also clearly distinguish the nonluminescent early marine cements from the finely crystalline material. The Sr/ Sr values for the former (0.707 934-0.709 392) are in or near the Devonian marine range, whereas the ratios for the latter (0.708 515-0.709 656) indicate a more radiogenic Sr source. On the basis of their lithology, architecture, isotope geochemistry, and clustered occurrence on top of a volcanic pile, we propose that the finely crystalline material that forms the mounds and the intermound beds was precipitated from hydrothermal fluids, and that accretion of the material was driven by hydrothermal venting. Geology 26 (1998), 1123-1126 KESS KESS carbonate mounds, Hamar Laghdad, Tafilalt, Anti-Atlas, SE Morocco – A FIELD GUIDE MOROCCO, 01-05 December 2006 15 4. PLATE 7 – Outcrops and stratigraphic log of kess kess mounds Outcrop image of the Middle Devonian Hollard Mound, eastern part of Hamar Laghdad. It is interpreted as hydrocarbon- seep mound with a recognizable core facies. From Cavalazzi (in press). Outcrop images of Early Devonian mounds, Hamar Laghdad Ridge. Photos show a field of exhumed Early Devonian mounds (small cones in the central part of the image) exposed in the central area of Hamar Laghdad Ridge. These mounds, that in foreground are about 30 m high, show crude bedding and steep flanks. Facies and interfacies mounds consist of fossiliferous limestones. From Cavalazzi et al. (submitted). Semplified stratigraphic log of the Hamar Laghdad. Modified after Belka (1998). Kess-Kess Formation (Brachert et al., 1992) KESS KESS carbonate mounds, Hamar Laghdad, Tafilalt, Anti-Atlas, SE Morocco – A FIELD GUIDE MOROCCO, 01-05 December 2006 16 4. PLATE 8 – Macrofaunas, Devonian mounds, Hamar Laghdad Ridge Dense accumulation of trilobite’s pygidia, Scutellum sp. Scabriscutellum sp., closed to veins cutting side of Early Devonian mounds. Dense accumulation of brachiopods and corals, locally associated with veins cutting the Early Devonian mounds. Dense accumulation of articulated bivalves and trilobites, Hollard Mound hydrocarbon seep, Middle Devonian. KESS KESS carbonate mounds, Hamar Laghdad, Tafilalt, Anti-Atlas, SE Morocco – A FIELD GUIDE MOROCCO, 01-05 December 2006 17 5. LIST OF PARTICIPANTS Participant countries Belgium Italy Morocco Netherlands Portugal Russia Switzerland
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