20_sedrocks_mud pores and cement-h
TRANSCRIPT
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Components of a Carbonate rock
Matrix< 0.03 mm
Non-skeletal grains
Skeletal grains
Cement
Pores
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Matrix = Lime Mud
• What is it?– silt & clay-sized carbonate sediment (<62 μm)
• Why is it important geologically?– constitutes 50% of carbonate strata by volume
• How is it Formed?– precipitation from seawater– post-mortem decay of calcareous algae– Biomechanical breakdown
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Modern Shallow Water Depositional Environment
• High porosity (50-80%)• Aragonite & HMC• 62-5 μm size, broken shells• 5-10 μm size, Aragonite needles
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Origin of Lime Mud: Decay of calcareous Algae
• Modern aragonite muds (< 10 μm) produced by the post-mortem disintegration of calcareous green algae
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Origin of Lime Mud: Skeletal Breakdown
• Browsers/scrapers eat substrate, make pellets that decay• Results of boring endolithic organisms
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Porosity in Carbonate Rocks
• Primary– voids left during original deposition between or within
grains
• Secondary– voids formed by dissolution after original deposition
• Recognizing the difference can inform us about the history of the rock/sediments
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Porosity in Carbonate Rocks
Intraparticle Interparticle
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Porosity in Carbonate Rocks
Moldic – voids left by dissolved grains
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Porosity in Carbonate Rocks
Vugs – voids larger than dominant grain size
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Porosity in Carbonate Rocks
Fractures can create large interconnected voids
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Porosity in Carbonate Rocks: Lucia• Porosity Classifications:
Interpretive/Descriptive (Lucia, AAPG 1995)– 2 Types of Porosity– Interparticle
• Intergranular• Intercrystalline
– Vuggy• Separate vugs
– interconnected only through the interparticle porosity.
• Touching Vugs– form a interconnected pore system of
significant extent
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Carbonate Cements• Crystalline carbonate that fills voids and/or replaces grains in
(carbonate) rocks• Cements can form early or form late• Cements can form in 5 different environments
– Marine Vadose (marine waters - possibly hypersaline)– Marine Phreatic (~salty groundwater - below seafloor)– Meteoric Vadose (freshwater - above water table)– Meteoric Phreatic (freshwater - below water table)– Deep Basin (basinal brine)
• The form, mineralogy, spatial arrangement can help distinguish environments of origin
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Carbonate Classification
• Classification Schemes– Folk (descriptive)– Dunham (descriptive/interpretive)
• Components– Allochems– Lime mud– Cement– Porosity
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Folk’s Classification Scheme
• (+) prefix:Fossils = bio- biomicrite or biospariteOoids = oo- oomicrite or oosparitePeliods = pel- pelmicrite or pelspariteIntraclasts = intra- intramicrite or intrasparite 15
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Dunham’s Classification Scheme• Dunham devised an alternative carbonate classification
scheme in 1962, which has become the most widely used system.
• Divided Carbonates rocks into 4 components:– particles
• (≥ sand size)– Fine matrix
• (lime mud)– Pore space
• (open or filled)– Organic Framework
• Key Concept– Mud vs. grain support
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