SPRING 2013GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityDiagenesis of LimestonesDiagenesis begins very early in limestones, right on the seafloorLimestone Diagenesis• Compaction and Cementation (mostly calcite) similar to that in sandstones• Pressure solution - dissolution caused by pressure of one grain on another• Replacement of Aragonite by Calcite• Local replacement of limestone by chert• Replacement of limestone by dolomite, called “dolomitization”Pressure Solution• Load pressure causes some calcite to dissolve• In some limestones, as much as 40% of original carbonate may have dissolved• Insoluble things (clay, organic matter) get concentrated or left behind and may form black marks called stylolitesStylolites• Irregular surface of interpenetrating “fingers” marked by concentrations of insoluble clay or organic matter• In cross section, they look like the writing of a stylus• See walls of bathroom stalls in White Hall (no more!!)StylolitesStylolitesBruce RailsbackU. of GeorgiaCalcite and Aragonite are polymorphs of CaCO3Calcite is more stable at Earth-surface conditions than Aragonite, but some organisms make their hard-parts out of aragonite anywayPressureTemperature (oC)AragoniteCalciteG=2.9G=2.720040005000 atm(depth)~15 kmAragonite changes to Calcite during diagenesis• Exposure to fresh water speeds up aragonite to calcite conversion• Paleozoic limestones don’t have any aragonite leftDolomitization• Dolomite is rare in modern carbonates• Makes up about 1/4 of Paleozoic limestones• Makes up about 3/4 of Precambrian limestones• Why?• When and where does dolomite form?Observations about Dolomite• Almost all dolomite forms by replacement of pre-existing calcite and aragonite• Dolomite rhombs cross-cut allochems• Dolomite obscures fine structures in limestones• Dolomite crosscuts bedding planes• Dolomite is commonly associated with evaporitesDolomite Rhombs in Thin SectionTo form dolomite by replacement of calcite or aragonite you need• Water of the right composition and• A mechanism to move that water through the limestone• There are two proposed mechanismsEvaporative Reflux• Requires periodic flooding of an exposed tidal flat or “sabkha” over a limestone• Evaporation that causes evaporites (especially gypsum, CaSO4·2H2O) to precipitate• Two effects:– increased density of brine, so it sinks through the limestone– increase in the Mg/Ca ratio of brineEvaporative Reflux:Ca-depleted (Mg-rich) brine moves through the limestone, and Calcite (CaCO3) is replaced by Dolomite (CaMg(CO3)2)Works for dolomites associated with evaporitesDolomites without Evaporites require a different model• Mixing of fresh water and seawater, called “Dorag” which means “mixed blood” in Persian• A mixture with ~5% to ~70% seawater is under-saturated with Calcite (wants to dissolve Cc), and super-saturated with Dolomite (wants to precipitate Do)Calcite and Dolomite Saturation% seawaterundersaturatedsaturatedoversaturated~5% ~70%5-70% seawater, Dorag works, Dolomite replaces CalciteDorag Dolomite• Landward of the shoreline, there is a zone of mixing of fresh groundwater and seawater• It should be a dolomitizing zone• Zone migrates landward as sea-level rises (during transgression)• Zone migrates seaward as sea-level falls (during regression)• This model is attractive for dolomites with no evidence of evaporitesDolomite formation on the north side of Jamaica - Dorag modelRainwaterGroundwater% seawaterundersaturatedsaturatedoversaturated0 50 100freshwater seawaterEvaporative Reflux explains Dolomites associated with EvaporitesDorag Model, north
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