Lecture 12 - Sedimentary notes PDF

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Lecture 12: Carbonate diagenesis(15 MINS LATE)Stable Isotopes:- using oxygen and carbon isotopes- Oxygen has 3 stable isotopes: 16,17,18 (O17 has a verylow concentration)- Measure O18:O16 ratio, analysed against a standard- Calculate the change in O18 value- Most come through water- Low magnesium ca...

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Lecture 12: Carbonate diagenesis (15 MINS LATE) Stable Isotopes: - using oxygen and carbon isotopes - Oxygen has 3 stable isotopes: 16,17,18 (O17 has a very low concentration) - Measure O18:O16 ratio, analysed against a standard - Calculate the change in O18 value - Most come through water - Low magnesium calcite is the most stable carbonate - Target planktic forams (Mesozoic and younger) or articulate brachiopods as these secrete Low Magnesium Calcite - High amount of evaporation get a high value of O18 - Also Carbon isotopes are measured: 12, 13 and radioactive 14 - Measure C13:C12 ratio - δ13C is –ve in cements is precipitated in soil — vegetation results in 12C rich soils and 12C is incorporated into the calcite precipitate - δ13C value low where higher plants are involved - Lighter isotope biomineralised or taken up preferentially - Can record the history of carbonates Ionic content of seawater: - Seawater is 300x more saline than river water = 30% - Can get up to 45% in arid conditions - In order of decreasing abundance in seawater: Na+, Mg 2+, Ca2+, K+, Cl -, SO42- and HCO3 -

Stability of carbonates in marine waters - Temperature and solubility of carbonates vary - Aragonite precipitates in the warmest waters — but dissolves away only in zone 1 - If you add in latitude - only see zone 1 in tropical areas (where aragonite precipitates out) - Zone 1 = precipitation of aragonite - Zone 2 = dissolution and predication of calcite - Zone 3 - zone of active dissolution of calcite - Zone 4 zone of no carbonate - Take depth and latitude into account Marine diageneis - eogenesis: - Cements form in gaps and slopes Meteroic and early burial diagenesis: - Where got a mixing zone of seawater and fresh water will get a mixture - Different opportunities for different cements to form

Deep burial diagenesis: Mesogenetic environment If see compaction of stylites occurs in mesogenetic environment Chemical compaction occurs by pressure solution Begins few 100s of m below ground. Interstitial fluids not rapidly or readily exchanged. Precipitation is slower and grains larger. Voids then fill; early rims of bladed calcite then coarser mosaic infills in remaining spaces. Can sometimes date cement from the compaction Lots of micrite is deposited in carbonate environments — will filter down to the seafloor Micrite neomorphose to spar

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Telogenesis: - Taking it up to the surface — see lithified limestones...