Geol 285 - Dr. Helen Lang, West Virginia University, Spring 2009 Diagenesis of Limestones Diagenesis begins very early in limestones, right on the seafloor Limestone Diagenesis • Compaction and Cementation 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 stylolites Stylolites • 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!!Stylolites Calcite and Aragonite are polymorphs of CaCO3 Calcite is more stable at surface conditions – see handout Aragonite changes to Calcite during diagenesis • Exposure to fresh water speeds up aragonite to calcite conversion • Paleozoic limestones don’t have any aragonite left Dolomitization • 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 carbonates • Dolomite rhombs cross-cut allochems • Dolomite obscures fine structures in limestones • Dolomite crosscuts bedding planes • Dolomite is commonly associated with evaporitesDolomite Rhombs in Thin Section To 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 mechanisms: Evaporative Reflux Model • 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 brine Evaporative Reflux: as the Ca-depleted brine moves through the limestone Cc (CaCO3) is replaced by Dolomite (CaMg(CO3)2) Evaporative Reflux explains Dolomites associated with Evaporites Dolomites without Evaporites require a different model • Mixing of fresh water and seawater, called "Dorag" for mixed blood in Persian• A mixture with ~5% to ~70% seawater is under-saturated with Calcite (wants to dissolve it), and super-saturated with Dolomite (wants to precipitate it) Calcite and Dolomite Saturation (see handout) Dorag Dolomite • Landward of the shoreline, there is a zone of mixing of fresh groundwater and seawater • Should be a dolomitizing zone • Zone migrates landward as sealevel rises (during transgression) • Zone migrates seaward as sealevel falls (during regression) • This model is attractive for dolomites with no evidence of evaporites Dolomite formation on the north side of Jamaica - Dorag model – see
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