Lecture 11 - Sedimentary notes PDF

Preview

Summary

Lecture 11: Carbonates- Need a shallow marine seas to make carbonates- 20-25% of stratigraphic record- In Pre C and Palaeozoic dolomite isdominant over calcite- Host a lot of fossil records- hosts of petroleum and zinc/lead deposits- Remember this equation ——>i Calcite (CaCO3)- Rhombohedral/t...

Description

Lecture 11: Carbonates

- Need a shallow marine seas to make carbonates - 20-25% of stratigraphic record - In Pre C and Palaeozoic dolomite is dominant over calcite

- Host a lot of fossil records - hosts of petroleum and zinc/lead deposits - Remember this equation ——> i Calcite (CaCO3) - Rhombohedral/trigonal crystal structure - Layers are equidistant along the c axis - CO32- groups have same orientation within an individual layer but this reverses in alternating layers

ii Aragonite (CaCO3) Orthorhombic crystal structure Only significant in Canozoic and modern carbonates Micrite = lime mud Mostly surrounded by a micrite envelope

-

iii Dolomite (CaMg(CO3)2) - Rhombohedral/trigonal - Ca and Mg cation layers alternate with layers of carbonate anions - Simular to calcite - 1st described in 1791 in NE Italy by Count Déodat du Dolomieu - Looks very much like limestone but not effevesce with acid - If more 75% dolomite = dolostone - Never been synthesised in the lab but is very common in stratigraphic record - How calcite/ aragonite —-> dolomite actually occurs?? - Dolostone can retain fabric or completely destroy it - If it is retained = Mimetic dolomite (see nanocrystal of dolomite — haven't destroyed the fabric at all) - Can also have = Limpid dolomite (crystals overgrown on inclusion rich cores — destroy the fabric) - If have curved crystals = Saddle dolomite (mostly associated with sulphates/sulphides ores)

- Factors inhibiting dolomite: low temperature waters, very slow process so need a lot of time -

-

and stability, sea water has pretty low Mg:Ca ratio, relatively low carbonate ions, also sulphate present Factors that dolomite favours: where temperatures are high (sabbkas), have stable environment, bonds between Mg-OH break if its hot, carbonate is carbonate instead of bicarbonate, sulphate reduction, removed in precipitation Calcite and aragonite can be replaced by dolomite if conditions are good

- Can find small amounts of dolomite but never as large that is found in the stratigraphic record Ionic substitution in carbonates: - cationic substitution is common - can stain rocks to see what different types of carbonates are present - Can stain with syanide

- Carbonate classification are the same as last year Structures and textures of limestones: - Cross bedding can occur - Can get algal matting/ fabrics occurs - Get layers of algal holding sediment down...