Near-normal seawater dolomitization of Upper Jurassic carbonates in the Franconian Alb, southern Germany

Dolomite is widespread, but its formation remains debated, including the origin of the Upper Jurassic dolostones in the Franconian Alb, southern Germany. This study investigates a Kimmeridgian dolostone-bearing succession using field observations, microscopy, X-ray diffraction, stable carbon and oxygen isotopes, and trace element data. Dolomite abundance generally increases upward, from partially dolomitized basinal carbonates and marls to pervasively dolomitized reefal, massive facies. In the partially dolomitized interval, dolomite occurs preferentially in marls, matrix, and open or sediment-filled voids. Near and within the lower part of the massive facies, clay-rich dolostone type 1 (DS1) contains finely crystalline dolomite (FD) and medium crystalline dolomite type 1 (MD1), both characterized by lower stoichiometry and crystallinity, whereas clay-poor dolostone type 2 (DS2) in the upper part contains medium crystalline dolomite type 2 (MD2), characterized by higher stoichiometry and crystallinity. Petrographic relationships indicate that dolomitization began during very shallow burial, before marls and, locally, limestones were lithified. REE-Y patterns, together with Na and Sr concentrations, indicate that the dolomitizing fluids were near-normal seawater, consistent with published fluid inclusion salinities from adjacent areas. The fluids are interpreted to have migrated downward and basinward from shallower-water areas of the carbonate system. This hypothesis accounts for the observed facies-dolomitization arrangement in a depositional framework characterized by a general shallowing-upward trend and massive facies expansion. These dolostones are therefore comparable to other Upper Jurassic dolostones commonly associated with evaporation.