Carbonate reaction and condition for carbonate precipitation or dissolutionoThe solubility of CO2 in water is greatest at low temperatures and high pressuresSurface water is less acid than deeper water, because the deeper the water, the lower T and higher PWhat is dolomitization and conditions that favor itoThe process by which limestone is altered into dolomiteThis process creates secondary porosityoConditions:High Mg2+/Ca2+ ratio in fluidHigh temperatureLack of sulfate (SO42-)Three forms of carbonateoCarbonate grainsSilt to sand-sized grains, but larger particles of fossil shellsEither skeleton or non-skeleton grains5 types:Skeletal particlesoSkeletal grainsSkeletons differ widely in size and shapeSkeletal materials are subject to mechanical abrasion & destruction by waves and currents; and bioerosion by boring organisms such as sponges, bivavles, algaeMost common kind of grain in carbonate rocksCarbonate clasts oNon-skeleton grainsoCarbonate rock fragmentsDerived by erosion of ancient limestone exposed on land(‘extraclasts’) or derived by erosion of carbonate
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sediments within the same depositional basin (‘intraclasts’)Range in size from very fine sand to gravel, show some degree of rounding, indicative of transportOoidsoNon-skeleton grainsoRound grains, have a concentric internal structure: layers of carbonate around a nucleusoFormed through chemical or biological precipitation by the rolling action of waves on beachesoRequire agitated or constantly moving wateroSediment consisting of ooids are called oolites—excellent reservoirsPeloids oNon-skeleton grainsoSmall (<2mm) spheroidal or oval particles of fine grained carbonate with nointernal structureVarious originsMost originate: fecal pellets or microbial breakdown of other particlesoFecal pellets- organic excrements generally produced by mud-eatinggrazing animals (crustaceans, worms, mollusks)Most 0.1-0.4 mm in diameterRound to oval shapeDark brown color AggregatesoMicrocrystalline calcite Fine grains (<0.03mm)Carbonate mud (or lime mud) may contain small amounts of fine-grained detrital minerals such as clay minerals, quartz, feldspar, and organic mattersMicrite as matrix among carbonate grains or form most of limestoneBreakdown of calcareous algaeChemically precipitatedModern Ca-mud origin organic processes, mainly aragoniteDeposition under quiet-water conditionsoSparry calcite Crystals in pore-spacesLarge crystals (0.02~0.1mm) formed during diagenesisDistinguished from micrite by their larger size and clarity and from carbonate grains by their crystal shapes and lack of internal textureIndicates that grain framework voids were emptyDeposition under high-energy flow conditionCan also form by recrystallization of primary depositional grains and micrite during diagenesisDepositional environments: Dr. Wilson’s lecture
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