15. Limestone/Dolomite
Orthochemical Sediment (Matrix)
- Most produced by microscopic hard parts from aragonite needles of green algae --> crystalline to micrite calcite - Same produced by precipitation from supersaturated solution or organisms - Mostly microcrystalline calcite (1-5 µm) = micrite - 5-15 µm = microspar (recrystallization & size increase) - Secondary calcite (>20 µm): fill spaces btw. grains
Coated grain - rhodoliths
- formed when calcifying red algae coat the grain surface - precipitation a skeletal structure
Coated grain - compound grains
- grapestone: aggregates of fecal pellets (peloids) that are stuck together - intraclasts: fragment of older material incorporated into limestone from same basin (rip-up clasts) - lithoclasts: "extraclast" from outside depositional basin & associated with detrital quartz, feldspar, lithic fragments; found in tectonical-active areas
Coated grain - ooids
- have central nucleus surrounded by multiple layers of mud, crystals, skeleton material - most common: quartz or calcite nucleus, almost spherical, tidal bars, sand shores, beaches' environment - > 2mm diameter = pisoliths
Limestone/Dolomite General Infos
- make up by CaCO3/CaMg(CO3)2 - 10-15% vol. of sedimentary rock - limestone react with cold HCl (faster with cold) - dolomites tend to weather dull brownish-yellow apperance
Types of grains
- presence/absence of carbonate mud - presence/absence of coarse calcite cement - In carbonates ("Allochem"): fossils, coated grains, peloids, compond grains
Fossils
- range from peleypods, brachiopod, ostracods, crinoids, bryozoans, corals, oysters... - Calcite: bryozoans, oysters, brachiopod - Aragonite: green algea, gestropods, scleractinian corals (mold later can be fill by other cement)
Carbonate Factory
- shallow: with in photic zone - clear: no terrigenous sediment - warm: tropical/sub-tropical - normal salinity: 4‰
Coated grain - peloids
- silt- to sand- size of microcrystalline CaCO3 that lack internal structure - ellipsoidal to roughly spherical shape - fecal pellets: contain organic matter and assorted detritus normally ingested by organisms during feeding
Dunham (1962) method
Hand samples; Emphasized the presence/absence of grain support and mud matrix (more commonly used today) - Increasing energy of deposition --> lack of mud matrix between: mudstone (<10% grains)> wackestone (>10% grains) > packstone (content mud, grain-supported) > grainstone (no mud, grain-supported)
Folk (1959) method
Thin section petrography; emphasized nature of allochems (used in academica) - fossil - "bio-" - peloid - "pel-" - ooids - "oo-" - intraclast - "intra-" - microcrystalline calcite - "mic-" - saprry - "spar-" - gravel size allochem - "-rudite"
Coated grain - oncoliths
coated with non-calcifying microbes that trap sand grains in successive layers
