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

Abstract

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 11) 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.

Distribution and origin of protodolomite from the late Miocene‐Pliocene Red Clay Formation, Chinese Loess Plateau

“…First, the groundwater replacement would produce carbonate that has meso-crystals or sparry crystals, whereas pedogenesis produces micrites (<5 mm in diameter). Second, horizontal elongate carbonate bodies are produced by deep groundwater or non-pedogenic calcretes, but primary pedogeneic carbonates produce pendent, laminar and pisolitic fabrics for carbonate calcretes or dolocretes [Colson and Cojan, 1996]. Third, the horizontal groundwater carbonates are larger and thicker, up to 10 m or more in thickness; primary pedogenic calcretes are typically 0.5-2 m in thickness [Wright and Tucker, 1991].…”

The origin and significance of pedogenic dolomite from the Upper Permian of the South Urals of Russia

“…4, Type C palaeosols, Table 1). These nodular dolocretes precipitated in the vadose zone as the lakes dried out (Colson & Cojan, 1996;Alonso-Zarza, 2003;Quast, Hoefs & Paul, 2006). Previous studies have inferred the presence of ephemeral lakes and ponds during Late Permian time in the study area from sedimentological evidence (Newell, Tverdokhlebov & Benton, 1999).…”

“…Primary dolomite, formed through surface and groundwater processes rather than diagenetic or metamorphic routes, is usually associated with lacustrine conditions (Watts, 1980;Colson & Cojan, 1996;Bustillo & Alonso-Zarza, 2007). The Type C palaeosols which contain dolomite are certainly associated with evidence of lacustrine conditions, such as those occurring with thin, ostracod-bearing limestones (Fig.…”

Palaeoenvironmental significance of palustrine carbonates and calcretes in the geological record

“…Groundwater doloc retes show a wide range of crystal size and include spheroidal (Spotl and Wright, 1992) and zoned dolo mite crystals, as well as dolomite with cloudy nuclei (pimentel et aI., 1996). Groundwater dolocrete for mation may also be favoured by the mixing of groundwaters and lake brines (Colson and Cojan, 1996) or with sea water (Williams and Krause, 1998).…”