Multiple episodes of dolomitization and dolomite recrystallization during shallow burial in Upper Jurassic shelf carbonates: eastern Swabian Alb, southern Germany

The Upper Jurassic of the eastern Swabian Alb is composed of oolitic platfo rm sands with associated microbe-siliceous sponge mounds at the platform ma rgins. They are surrounded by argillaceous or calcareous mudstones and marl -limestone alternations, deposited in adjacent marl basins. Partial to comp lete dolomitization is predominantly confined to the mound facies. Six type s of dolomite, as well as one type of ankerite, document a complex diagenet ic history during shallow burial with multiple episodes of dolomite formati on and recrystallization. The earliest massive matrix dolomitization is Ca- rich, has slightly depleted oxygen isotope values relative to Late Jurassic seawater, and carbon isotopic values in equilibrium with Late Jurassic sea water. This initial massive matrix dolomitization occurred during latest Ju rassic to earliest Cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 degrees C. Hydrologic c onditions and mass-balance calculations indicate that burial compaction pro vided sufficient fluids for dolomitization. Mg is derived from negligibly m odified seawater, that was expelled from the adjacent off-reef strata into the mound facies. Position of the mounds along the platform margins control led the distribution of the shallow-burial dolomite. Covariant trends betwe en textural modification, increasing stoichiometry, partial changes in trac e element content (Mn, Fe, Sr) and depletion in stable isotopes as well as distinctive CL pattern illustrate two recrystallization phases of the precu rsor matrix dolomite during further burial at elevated temperatures. Strong Sr enrichment of the second phase of recrystallized dolomite is ascribed t o Sr-rich meteoric waters descending from overlying aragonite-bearing reef limestones or evaporite-bearing peritidal carbonates. Late-stage coarsely c rystalline dolomite cements occur as vug and fracture fillings and formed d uring burial. Ankerite, associated with sulphide and sulphate minerals, and saddle dolomite are assumed to have formed from hydrothermal waters that m oved to higher stratigraphic levels along fracture conduit systems that dev eloped during Late Cretaceous to Tertiary Alpine orogenesis. (C) 1998 Elsev ier Science B.V. All rights reserved.