Abstract

Various interpretations have been proposed for the origin of peculiar cone-shaped, finely crystalline, Devonian carbonate mounds of the Hamar Lakhdad Ridge, Anti-Atlas, Morocco, 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 δ18O values, whereas values of δ13C 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 87Sr/86Sr 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.

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