Reading Question
1. How are the chemical/biochemical sedimentary rocks distinguished from siliciclastics?
Carbonates
• Chemical / Biochemical Sedimentary Rocks
• Precipitation of minerals from water
– Chemical or Biochemical
• Limestones and Dolostones
• Agriculture, industry, building stone
• petroleum
Carbonates versus Siliciclastics
• Both siliciclastics and most limestones have clastic textures
• Limestone texture -
– Grain size, grain type, framework versus matrix, sorting
• Extra- versus Intrabasinal
• Carbonates grow
Carbonate Mineralogy
• Calcite (CaCO3)
– Low-magnesian calcite - very stable
– High-magnesian calcite - less stable
• Aragonite (CaCO3) - different crystal structure
– Metastable
– Ocean chemistry - Changes through time (Mg)
Carbonate Mineralogy
• Dolomite (CaMg(CO3) 2)
• Primary
– Arid - sahbka
• Replacement
– Problem = where does all the Mg come from?
• Most old carbonates = dolostones
Question
• What determines where carbonate sediments are deposited? Why?
• Chemical and biological factors
• Primary Controls =
– Tectonic setting, climate, sea level and rates of S/L change
Carbonate Textures
• Grains (framework) - skeletal and non-skeletal
• Mud/micrite (matrix) - mircrocrystalline calcite
• Spar (cement) - crystalline calcite
• Significance framework/matrix - environmental & diagenetic
Carbonate Grains
• Non-skeletal Grains (allochems)
• Ooids - carbonate sand
– Radial
– Concentric
• Peloids - fecal? - grapestone
• Intraclasts
– Rip-up clasts
– erosional
Carbonate Grains
• Skeletal Grains (allochems)
• Marine invertebrates
• Mineralogy and microstructure
– Horowitz and Potter
Micrite
• Microcrystalline calcite
• Modern = tiny aragonite needles
• Matrix or most of rock
• Sources
– Bioerosion, dissagregation of algae, mechanical dissagreagation, chemical
precipitation
Spar
• Crystalline calcite cement
• Forms during diagenesis
• Chemistry and type indicate diagenetic stage
• Early - rim cements, well washed, beachrock
• Late (burial) - course, ferroan
Carbonate Deposition
• Carbonate sedimentation - shallow water
• Exception = calcareous oozes
• Limits = CCD - concentrations of CO2
– Snow line
Diagenesis
• Dolomitization - the problem
• Dolomite (CaMg(CO3) 2)
• Penecontemporaneous - models
• Hypersaline - Persian Gulf - sabkhas
• Mixing zone models - Dorag
• Seawater model
• Microbial?
Diagenesis
• Dolomitization - burial models - still a problem
Diagenesis
• Changes in porosity, mineralogy, and chemistry
• Realms
• Marine - phreatic; vadose
• Meteoric - phreatic; vadose
• Subsurface burial
Diagenesis
• Processes
• Bioturbation
– Burrows, boring, micritization
• Cementation - early
– Beachrock, hardgrounds
Diagenesis
• Cementation - late burial
– Unstable mineralogy; drusy
• Dissolution
– Unstable mineralogy
– Water table
• Neomorphism - recrystallization/pseudospar
• Compaction - physical & chemical