Sedimentology and Paleontology Lecture 21: Carbonate processes and environments and Diagenesis Flashcards
Describe how the carbonate reaction is an equilibrium reaction
The carbonate reaction is an equilibrium reaction
- If CO2 is removed (e.g. by photosynthesis or degassing) – the reaction shifts to the left and drives CaCO3 precipitation
- If CO2 is added (e.g. by bacterial decomposition of organic matter) – the reaction shifts to the right and drives CaCO3 dissolution
Define Hypercalcifiers
Hypercalcifiers are organisms that can make calcium very efficiently.
- Scleractinian/hermatypic corals have a symbiotic relationship with photosynthetic brown algae
- Bleaching caused when the algae die off or leave the coral
Describe the Zones of optimal carbonate production
Zones of optimal carbonate production
* Shallow tropical
* Pelagic
* Temperate
Describe Stromatolites
Stromatolites consist of alternating layers of biofilms and sediment
Abundant across shallow marine niches in Precambrian, as there was no natural predators like gastropods to eat them
Mostly restricted environments today, they thrive in very saline environments where predators like gastropods can not survive, e.g. like Lagoons.
Define Whitlings
Whitings = micrite in the water column
Give some Post depositional environments
Bioturbation - the physical movement of soil by fauna or plant roots.
Hardgrounds - surfaces of syn sedimentarily (deformation soon after depostion) cemented carbonate layers that have been exposed on the seafloor.
Mangroves
Stromatolites
Desiccation
Evaporites
Subaerial exposure on tidal flats can result in fenestral fabrics
Describe Skeletal reefs
Shallow water
Frame building organisms (Coral)
E.g. Coral, Stromatopoid, Red algae, Stromatolite
E.g. Great Barrier reef
Describe reef mounds
Reef mounds = mud-rich build-up of delicate skeletal +/or abundant microbialite and marine cement
Deep water
Upright/vertical organisms
e.g. Crinoids, Sea grasses, Codiacean green algae
Describe Mud mounds
Mud mounds = very few skeletal grains but abundant bacterially produced sediment
Variable water depth
Microbial mats
Describe Modern reefs
Most in shallow tropical zones
* Fringing reefs and atolls
* Patch reefs
* Barrier reefs
Deep cool-water patch reefs
Deep water reef mounds
Describe the construction and destruction of reefs
Construction by skeletal growth as well as sediment baffling and binding
Destruction by Rasping by fish and boring by sponges can damage and weaken the reef framework.
* Leads to the breakdown of the reef and generation of
sediment
Define rudist bivalves
Weird uncoiled bivalves (Late Jurassic to Cretaceous) that displaced corals into deeper water
Describe reef rock terminology
Dunham: boundstone
Embry and Klovan for reef rocks
Bioherm = in situ build-up with topographic relief
Biostrome = bedded mass of reefal debris
Describe the relationship between increasing wave energy and decreasing sedimentation rate in barrier reef frame builders
As wave energy increases the sedimentation rate decreases.
Give the two types of mechanical compaction
Sloughing of ooid cortices
Nested fabric
Give the types of Chemical compaction
Interpenetrated and sutured grain contacts – fitted fabric
Deformed twins in calcite spar
Define chemical compaction
Dissolution with increasing stress and burial
Define Styolites
Dissolution concentrated on laterally extensive and irregular planes in pure limestones
serrated surfaces within a rock mass
Define Anastomosing seams
Describe Carbonate cements in relation to water type
Marine Aragonite and Fibrous, bladed,
Mg-calcite and prismatic
Meteoric Calcite Equant and syntaxial
Burial Calcite and Equant and poikilotopic
Fe-calcite
Describe the dissolution and replacement of Carbonates
Calcite is stable
* Retains original skeletal structure
Aragonite is metastable
* Replaced by calcite or Fe-rich calcite with loss of
texture
Mg-calcite is metastable
* Replaced by calcite or Fe-rich calcite with no loss of texture
Define Dolomitisation
Dolomitisation is a low temperature diagenetic replacement reaction
- Fluids import Mg2+ and export Ca2+
- Substitution by Fe2+ gives a brown weathering colour
