Bacterially Induced Sulfur and Carbonate Formation Formed During Epigenesis in a Gypsum Caprock of a Zechstein Diapir, Northwest Hills, Germany

Henning Peters and Joern Peckmann
Bremen University, Research Center for Ocean Margins (RCOM), 28334 Bremen, Germany

Salt dome environments belong to the most productive settings concerning epigenetic mineral generation and biogeochemical cycling in the subsurface. A gypsum caprock in northwestern Germany contains native sulfur and authigenic carbonates. The onset of diapirism of Zechstein salt was induced by extensional tectonics in the Late Oxfordian. Later inversion tectonics related to Alpine convergence resulted in the uplift of the entire area. The mushroom-shaped salt dome consists of the following lithologies from base to top: (1) a succession of layered anhydrite with minor amounts of fine-grained gypsum and selenites (2) massive to nodular gypsum and (3) a carbonate cap overlain by Early Cretaceous blue-grey clays. The upper part of the massive gypsum unit is penetrated by veins of elemental sulfur and carbonates embedded in an organic-rich matrix. Porespace in the carbonate cap is entirely filled by pyrobitumen. A unique crystal mosaic, that records the epigenetic processes, consists of sulfur disseminated between spheroidal to spindle-shaped calcite crystals and minor amounts of pyrite. Sulfur and carbon isotope values of the elemental sulfur, sulfides, residual sulfates and carbonates indicate an origin from bacterial sulfate reduction. In contrast to earlier models for caprock formation assuming an aerobic mechanism, we propose a generation of elemental sulfur under anaerobic conditions via sulfate reducing bacteria favored by high C/H ratios in the biodegraded petroleum. A similar mechanism was yet only described for thermochemical sulfate reduction.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005