Chemical Sedimentary Rocks
Evaporites - formed from evaporation of seawater in restricted basins or from evaporation of water from desert lakes
Chemical Sedimentary Rocks

• restricted basin could be large - Persian Gulf, Gulf of Mexico (Louann salt - Jurassic), Mediterranean Sea
• common in newly opening ocean (GOM)
• precipitation sequence from seawater: 1) calcite, 2) gypsum, 3) halite

• Limestone - mostly calcite; often composed of biogenic grains which may recrystallize
• typically deposited where there is little or no clastic sediment
• can vary from free of fossils to fossiliferous

• Coquina - poorly cemented shells and shell fragments

• Chalk - soft, friable, porous; micro-fossils
•  coccoliths, 1-celled plants
• foraminifera, 1-celled animals

• Oolite - composed of sand-sized spheres - ooids (like limestone “snowballs”) (book calls chemical sediment)

• Stromatolite
• formed by trapping of sediment by algae
• some of the earliest fossils
• Reefs - biogenic buildup; carbonate secreting organisms; marine
• atoll - circular chain of coral islands

• Dolomite/dolostone - CaMg(CO3)2,
•  “Dolomite problem”: dominant in ancient rocks; progressively less common in younger rocks
• Forms most often by chemical alteration (diagenesis) of limestone after deposition
• diagenesis - physical and chemical changes in a sediment after deposition at lowT (< 200°C)

• chert - SiO2, micro- or cryptocrystalline
• remains of silica-secreting microorganisms living in ocean (diatoms, radiolaria)
• Also forms from silica precipitation in pores after burial
• coal - plant remains; swamp
• peat -> lignite -> bituminous -> anthracite

• bedding, strata - bedding planes form due to change in conditions, sediment type
• cross-bedding - need sandy sediment
• Water current
• Wind - Eolian - larger (see Fig. 7-18)

• ripples - asymmetrical
• Water moves in one direction

• ripples - oscillatory - form from oscillating waves (water moves back and forth)

• graded bedding - grain size decreases upward
• caused by slowing of current

• mud cracks - desiccation; clay-rich sediment (Fig. 7-22)