SEDS 3-4

• The carbonate factory is born not made. The majority of carbonate sediments are precipitates from organisms that produce skeletons and shells
• The carbonate factory optimal conditions are shallow, warm, nutrient rich, sunlit water
• Pelagic factory is surface ocean with microscopic carbonate producers that fall to seafloor when they die
• The benthic carbonate factory is on the seafloor
• Carbonate grains are silt to gravel sized and are called allochems
• Allochems have skeletal particles ( benthic or planktic), coated grains: ooids pisoids oncolites, peloids and intraclasts
• Peloids are grains composed of microcrystalline clacite or aragonite. Smaller than ooids and have no internal structure. Usually fecal pellets
• Ooids are coated carbonate grains with a nucleus
• Pisoids are similar to ooids but are larger than 2mm and irregular. Can have algal origin
• Intraclasts are fragments of lithified or partly lithified sediment that was transported only a short distance
• Extraclast - fragments consisting of lithology not represented in the immediate depositional environment
• CARBONATE MUDCan form by direct precipitation or by breakdown of skeletal components (e.g. some green algae)
• STROMATOLITESFormed of microbial mats (e.g. cyanobacteria) Sediment gets trapped in fine filaments in mats, microbes grow up and around sediment to formnew mat Algal laminations are crinkly Responsible for oxygenating ouratmosphere!
• ALLOCHEMS: ONCOIDSMicrobial origin Coated irregular layers bound by CyanobacteriaDevelops in a more energetic environment than stromatolites Energetic subtidal environments
• Matrix vs cement: mud looks brownish between grains and cement looks more clear and crystalline
• IF LESS THAN 10% OF ALLOCHEMS ARE > 2 MM (SAND-SIZED)If it contains no mud: GrainstoneHas mud but is grain supported: PackstoneIs mud supported and more than 10% of rock is composed of grains: WackestoneContains less than 10% grains: Mudstone
• POROSITY IN CARBONATE ROCKSLimestones are especially vulnerable to diagenetic change (chemical processes that occur post deposition)Skeletal fragments are in equilibrium with seawater, but not with freshwater or groundwaterAfter burial, start dissolving allochems and precipitating cement
• Supratidal: more equivalent to backshore. Flooded during highest tides only
• Intertidal: exposed and flooded during tidal cycles
• Subtidal : below low tide
• PERITIDAL ENVIRONMENTSSupratidal environments classified based on climate. Intertidal environments can be high or low energy. Broad range of facies possible in subtidal settings
• Sabkha: an arid supratidal environment
• Semi - arid intertidal settings will have stressed stromatolites and oolitic sands with waver ripples
• EVAPORITESIf we evaporate half of a volume of seawater, we can precipitate CaCO3If we evaporate more we start to precipitate gypsumNeed to evaporate 90% to get halite
• ARID PERITIDAL SYSTEMS: Northern margin of United Arab Emirates is the best modern example of an arid peritidal system where evaporites are forming
• BOUNDARY BETWEEN SUPRATIDAL AND INTERTIDAL ENVIRONMENTS: View looking from supratidal environment towards open ocean. Supratidal only flooded during extremely high tides, but evaporation is so intense it pumps ground water through to continuously precipitate halite
• ARID INTERTIDAL: Similar tidal channels, but no mangroves. Microbial mats along margins of channels. Gypsum and anhydrite
• PERITIDAL CYCLES:Peritidal carbonates tend to form peritidal cycles: shallowing upward cycles with stacked subtidal, intertidal, and supratidal deposits. Source of cycles could be autogenic (e.g. aggradation or shifting around of environments) or allogenic (driven by sea-level cycles)
• Reef Attributes: extensive in-place biological fixation of carbonate growth cavities with internal sediment synsedimentary cementation
• WHERE AND WHY ARE CARBONATE SEDIMENTS FOUND IN THE WORLD’S OCEANS?Consequence of solubility differences between different carbonate minerals. Ocean circulation. Changes in seawater chemistry with depth
• DISTRIBUTION IN GLOBAL OCEAN:Sediments from pelagic factory end up in deep ocean basins. Why does the Pacific (N.Pacific especially) have less calcareous deepwater sediment than the Atlantic?
• Dissolution in deep ocean increases exponentially at the lysocline
• lysocline: when we first start to see the corrosion/dissolution in carbonate particles
• Carbonate Compensation Depth (CCD):the depth at which carbonate sediment no longer accumulates. Rate of accumulation = rate of dissolution
• THERMOHALINE CIRCULATION:The seawater ages with time. Water at seafloor in N. Pacific is the oldest water in circulation system. Older water hashigher CO 2 content
• CARBONATE SLOPE ENVIRONMENT:Two sources of deepwater carbonate sediment:Long periods of pelagic rain out Interrupted by gravity-driven transport (re sedimented)
• Pelagic Factory:Suspension rain of calcareous phytoplankton and zooplankton
• Periplatform:Sediment derived from the shallow-water carbonate factory (platform)and redeposited in deeper waterThis material is called a re-sedimented deposit
• WHEN DID THE PELAGIC FACTORY EVOLVE?

  Didn’t get going in a considerable wayuntil the Jurassic. When you see carbonate mud in rocks older than Jurassic in deepwater - it formed on the platform (peri-platform ooze) and has been re-sedimented
• YPES OF CARBONATE SLOPE CONGLOMERATES:ResedimentedCoarser deposits found higher on slopeSame transport processes assiliciclastic gravity flows