Fragments of calcareous organisms. The composition changes through geologic time due to evolution.
Source of Ions (Weathering, Seawater)
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Biological Secretion (Shells, Reefs) + Chemical Precipitation (Ooids, Travertine)
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Primary Carbonate Sediment (Aragonite + Calcite Mud/Grains)
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Burial & Diagenesis
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Limestone Dolomite (via Mg-rich fluids)
Dolomite forms only by diagenetic replacement of limestone under high temperature (hydrothermal) or evaporitic conditions (the "Sabkha model"). origin of carbonate sedimentary rocks pdf new
This section is crucial in modern texts (PDFs). Diagenesis refers to the physical and chemical changes occurring after deposition. Fragments of calcareous organisms
No discussion of carbonate origin is complete without addressing dolomite [CaMg(CO₃)₂]. The classical problem: Dolomite is abundant in ancient rocks (Precambrian to Paleozoic) but extraordinarily rare in Holocene sediments. Dolomite forms only by diagenetic replacement of limestone
| Texture | Interpretation | |---------|----------------| | Clotted micrite with peloids | Thrombolite (microbial) | | Laminated micrite | Stromatolite | | Radial fibrous ooids | Abiotic, high-energy | | Concavo-convex micritized grains | Bioeroded, reworked | | Fenestral fabric (birdseyes) | Tidal flat, desiccation | | Saddle dolomite | High-temperature burial cement |