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Controls Silica Diagenesis (Chert on Geological Tscale (No Paleozoic opal…
Controls Silica Diagenesis
Chert Formation
Takes a very long time (>20Myr)
Chert not present in surface sediment
Need sediment atleast 20 Myr before chert develops
Chert on Geological Tscale
Opal-CT in Eocene age layer (40-50 Myr) in atlantic
Most opal-CT in Tertiary
(not long enough to form chert yet)
Little during Mesozoic
Next to none from Paleozoic (transformed to chert)
No Paleozoic opal-CT
Palaeozoic = such long time ago
All opal-CT has converted to chert
Paleozoic chert = quartzitic chert
Replicating silica diagenesis in lab
Difficult given long timescales for chert formation
Diagenetic processes take many Myr
Controls Diagenetic
Δ Silica
burial depth
pH
temp
conc dissolved SiO2
host sediment
nature of sediment influences silica diagenesis
Host sed dependence
Carbonate sediments
i.e. oozes
Opal-CT formation occurs at
relatively shallow depths
Muddy
Sediments
Opal-CT / quartz formation =
strongly delayed
Inhibition by magnesium ions in clays
Delays silica diagenesis advancing
Surface productivity
Most important source silica in cherts =
oceanic
especially upwelling areas
Diatoms = important silica secretors
Siliceous sediment requires
both
silica productivity
silica accumulation
Mismatch exists btwn areas silica productivity & accumulation i.e. Antarctic high production & precipitation despite high dissolution rate
Upwelling & Silica
Upwelling =
rise of nutrient rich waters to surface
High productivity correlates with upwelling areas
High productivity areas include:
Antarctic / Arctic
Central Pacific (just N of equator)
West Continental margins
Silica Accumulation
Circumantarctic - huge accumulation
North Central Pacific - quite a lot
(Coastal areas - not so much)
Less accumulation
at coastal regions
Dilution
Silica often diluted by terrigenous material
Lots of silt/sand washes off continent forming mudstone using lotsa silica
Dissolution
of silica
Only 1-10% silica reaches seafloor
Lots of silica dissolves in surface waters