Hydrothermal origin of Devonian conical mounds (kess-kess) of Hamar Lakhdad Ridge, Anti-Atlas, Morocco

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 st ructures, The mounds are cone shaped, steep sided, circular to subelliptica l 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 ma ssif, Stable isotope analyses of first-stage isopachous nonluminescent ceme nt have yielded marine values, In contrast, the finely crystalline carbonat e that makes up the bulk of the mound and the internally sedimented mud bet ween crusts of early marine cements have significantly lower delta(18)O val ues, whereas values of delta(13)C are similar to those of the early marine cement. Strontium isotope ratios also clearly distinguish the nonluminescen t early marine cements from the finely crystalline material. The Sr-87/Sr-8 6 values for the former (0.707934-0.709392) are in or near the Devonian mar ine range, whereas the ratios for the latter (0.708515-0.709656) indicate a more radiogenic Sr source. On the basis of their lithology, architecture, isotope geochemistry; and clustered occurrence on top of a volcanic pile, w e propose that the finely crystalline material that forms the mounds and th e intermound beds was precipitated from hydrothermal fluids, and that accre tion of the material was driven by hydrothermal venting.