An early carboniferous seep community and hydrocarbon-derived carbonates from the Harz Mountains, Germany

Early Carboniferous (latest Visean) seep deposits occur on top of the drown ed Middle Devonian-Late Devonian Iberg atoll reef, Hart Mountains, Germany. These deposits include limestone with a low-diversity but high-abundance f auna of rhynchonellid brachiopods and rare solemyid bivalves, as well as mi crobial limestone. Rhynchonellids form dense, autochthonous shell accumulat ions and are generally articulated. They are closely associated with hydroc arbon-derived carbonates. The carbonates exhibit delta C-13 values as low a s -32 parts per thousand, relative to the Peedee belemnite standard, reveal ing that they are predominantly hydrocarbon derived. The fauna, carbonate f abrics, and isotope signatures provide unequivocal evidence for a seep orig in of the Visean deposit. The occurrence of solemyid bivalves supports this interpretation as members of this family (1) are well known for their rela tionship with chemoautotrophic bacteria and (2) have been reported from anc ient and modern seeps. Possible hydrocarbon sources are thermogenic methane derived from the volcanic base of the Iberg reef or methane from a petrole um reservoir.