Groundwater dolocretes in a lake-marginal environment: an alternative model for dolocrete formation in continental settings (Danian of the Provence Basin, France)



In the long continental history of the Provence Basin which extended from Santonian to Oligocene times, a major period of palaeosol development occurred in the Danian. Dolocretes developed within floodplain silts, and partly from palustrine limestones. Dolocretes are overlain by pedogenic facies: calcretes and palustrine limestones. Gradational lower and upper limits of profiles, succession of nodular, coalescent and massive horizons, the epigenesis of quartz by dolomite, the unimodal crystal size and the euhedral dolomite fabric, as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone.

The exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines, which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering. The term halo dolocrete is proposed to describe this type of dolocrete deposit.

Subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization. Calcitization was either fabric-destructive (type I) or fabric-preserving (type II) and took place during very early diagenesis, i.e. concomitant with calcrete formation or palustrine limestone deposition. Fabric-destructive calcitization is attributed to a drop in the lake level, when the upper part of the dolocrete was subjected to vadose zone processes, whereas the fabric-preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level.