Sedimentology and Paleontology Lecture 19: Chemical Sediments Flashcards
Define Precipitation
The act of a solid coming out of solution, typically resulting from a drop in temperature or a decrease of the dissolving material.
Define SIliceous organisms
Organisms that use silica to form skeletons through a process known as biomineralization. Diatoms and radiolarians have evolved to uptake silica in the form of silicic acid, Si(OH)4.
For example
Radiolaria - Marine zooplankton, Cambrian to Recent
Diatoms - Marine and nonmarine phytoplankton, Cretaceous to Recent. Some lake deposits are entirely made up of diatoms (called Diatomites)
Sponge spicules - (these spicules make up interal skeleton of sponge) Marine, Cambrian to Recent
List biological silica minerals
Chalcedony
Microquartz
megaquartz
opal
biogenic opal
amorphous opaline silica
Define Chert
a hard, dark, opaque rock composed of silica (chalcedony) with an amorphous or microscopically fine-grained texture. It occurs as nodules (flint) or, less often, in massive beds.
Chert very hard and dense, conchoidal fractures
Composed of microcrystalline or cryptocrystalline quartz,[1] the mineral form of silicon dioxide (SiO2).
Chert/Microquartz/Opal CT
Describe the CCD and what happens above and below it
The Carbonate compensation depth is depth where carbonates begin to fully dissolve./ Rate of carbonate dissolution is equal to the supply
DIssolution increases with depth and lessening temperature
Different minerals have different compensation depths, for example Aragonite = 1-2km and Calcite = 3-4km
Describe how Chert bedding occurs
Under CCD, Calcareous nanoplankton/ or anything containing CaCO3
Thus the beds are made up of only Chert which does not dissolve
Above CCD calcite can form so there are only of nodules of chert (flint) in the beds.
Describe Non biogenic cherts (hydrothermal)
Cherts can be found in Alkaline lakes in Volcanic regions (e.g. East African rift valley lakes)
Sio2 dissolved from volcanic bed rock, concentrations 1000x what they would be in seawater
Chert is precipitated from evaporation
Define an Evaporite mineral
An evaporite is a water-soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution.
Precipitated in limited environments, little life present
Evaporites = crystalline chemical sediments (salts)
precipitated from brines
Brines = Waters that have been concentrated by evaporation, Water either fresh or marine
List some relevant/important evaporite minerals
Gypsum
Anhydrite
Halite
Potash salts
Describe Gypsum
(CaSO4!2H2O)
Can form as Selenite (has swallow tail twins)
Gypsum in XPL
Low relief and low order birefringence in thin section. Thin tabular shape, they all form next to one another.
Very soft, can scratch with fingernail
Describe Anhydrite
(CaSO4)
Chicken-wire anhydrite Buckled anhydrite
Anhydrite in XPL
Prismatic with moderate relief and
high birefringence
Describe Halite
NaCl
Carnalite
Halite
List some subaqueous fabrics
Water column deposits = laminated and fine-grained
Gypsum selenite supercones
Halite chevrons
Describe some Reworked subaqueous evaporites
Irregular laminated gypsum
Ripple cross-laminated gypsum
Structures in reworked gypsum
Where do evaporites form
Evaporites can form on land/continental environments as well as marine.
How do evaporites form in continental environments
Playa lakes = Arid zone lakes that episodically dry out. Are reflooded after heavy water and by groundwater
Playa lakes generate Halite surface crusts, Gypsum precipitate out of from sediment, Although deposits are thin due to infrequent replenishment
Salt pans = Partially dried out lakes where salt deposits from large arid flats. Occur in mountains and rift valleys. Mostly fed by groundwater
How do evaporites form in coastal environments
Coastal Salinas = hypersaline lagoons. Either natural or man made for the production of salt.
Sabkhas = Supratidal (above mean high tide). Marginal Lacustrine flats that are episodically flooded and then dry out to leave precipitated salts.
Give some examples of modern continental salt deposition
Brine pools in the Atacama desert, Chile
Lake Magadi = soda lake
Lake Natron = soda lake
Describe how zonation work in the precipitation in an ephemeral Salina
Ephemeral salinas have a distinguished cutoff from the sea.
Least soluble salts will precipitate out of the brine first, while more soluble salts will precipitate later on and the lake dries out.
Basically Less soluble salt deposits will be situated on the shallower banks of the lake while the more soluble salt deposits will be situated on the deeper banks of the lake
From birds eye view looks like bullseye, with most soluble salts in middle/bullseye.
Describe how zonation of evaporite minerals works in a Perennial embayment
A perennial embayment has the similar structure of a lake nearby the coast however the sea level is too high so the water of the lake and the sea are connected.
In this the most solube and saline salts are deposited the furthest away from the sea in the lake basin, whilst the least soluble is positioned the closest to the sea water.
From birds eye view looks like a tear drop or a funnel with the narrow point on the sea water side.
Describe the two types of ancient saline giants
Desiccated basins
* Bottom growth of evaporateswith lateral zonation
* Shallow subaqueous and subaerial fabrics
* Desiccation features
Perennially flooded basins
* Deep water fabrics
* Thick gypsum-anhydrite sequences with some halite and few K-salts
Define Salt Diapirs
Salt diapirs = ancient salt deposits flow up by buoyancy and breach the surface
