The significance of hiatus beds in shallow-water mudstones: An example from the middle Jurassic of Switzerland

Limestone beds formed in nearly carbonate-free shallow-water mudstones by d iscontinuous sedimentation and erosion are called hiatus beds. Anaerobic ox idation of organic matter bg microbes provided excess alkalinity, inducing carbonate precipitation. A multiphase history of such beds is documented fr om the Swiss Jurassic by several cementation and dissolution phases. Four c ement types occur: micrite as earliest cement (delta(13)C -10 to -20 parts per thousand), stellate calcite between micrite-cemented parts (delta(13)C -5 to -10 parts per thousand), fibrous calcite cement in dissolution caviti es (delta(13)C < -30 parts per thousand), and blocky calcite in remaining p ores (delta(13)C -5 parts per thousand). Except for the late blocky cement, all cements contain pyrite indicating carbonate precipitation within the s ulfate reduction zone. After early cementation by micrite, the beds moved r elatively upwards into a shallower geochemical zone and some even to the se afloor because of erosion. Cavities formed during reburial by dissolution i n the upper part of the sulfate reduction zone and in the upper part of the methanogenic zone. Strongly reduced sedimentation rates provided the requi site stable geochemical conditions for at least several thousands of years, which permitted precipitation and dissolution of carbonate by biochemical processes and diffusion. This happened on short-lived sn ells caused by dif ferential subsidence and rotation along listric faults when basement struct ures became reactivated during the extensional stress regime from opening o f the Tethys, During the Jurassic and Cretaceous breakup of Pangea the shel f area increased, and differential subsidence on these newly formed shelves was the main factor responsible for the observed post-Paleozoic maximum in hiatus beds and hiatus concretions.