Download Lecture 11: Non-Carbonate Biogenic and Chemical Sedimentary

Lecture 11: Non-Carbonate Biogenic
and Chemical Sedimentary Rocks
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Siliceous Sediments & Chert
Phosphorites
Evaporites
Banded Iron
Siliceous Sedimentary Rocks
Fine-grained, dense, hard rocks
composed predominantly of SiO2
minerals quartz, chalcedony, and
opal + minor impurities
• Occur throughout the rock record
– Most common in Jurassic,
Cretaceous, Paleogene rocks (18040 Ma)
– Bedded
– Nodular
• Chert - microcrystalline quartz,
w/minor calcedony/opal
– Grain sizes/shapes variable (1-50
µm)
• Biogenic Silica - amorphous
Silica/Opal A (SiO2*H2O)
– Readily transforms to chert
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Silica Geochemistry
Amorphous SiO2 - highly soluble
• Groundwater
– 100-200 ppm
– Source: feldspar to clay
2KAlSi3O8 + 2H+ + 9H20 ⇒ H4Al2Si2O 9 + 4H4SiO4 + 2K+
– Solubility increases in Alkaline (hi pH)water
Silica Geochemistry
Amorphous SiO2 - highly soluble
• Seawater (H4SiO4)
– <1 to 11 ppm
– Highly undersaturated!
– Organic coatings preserve shell
opal
– Accumulation occurs only where
fluxes are high
– Diatom/radiolarian oozes
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Origin of Chert
2 Types:
1. Biogenic Chert
2. Nonfossiliferous Chert
• Requirements:
Chlorophyll contents in the Pacific
1. Silica Source
2. Precipitation mechanism
• Supersaturation
Origin of Biogenic Chert
1. Silica Source:
Upwelling zones-high
productivity (diatoms)
Chlorophyll contents in
the Pacific
ODP Leg 199
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Leg 199 Sites: Si & Ca Wt% & mass accumulation rates (MARs)
Si mass accumulation rates (MARs) in the mid- Cenozoic
Meridional Pattern
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Biogenic Opal to Chert
Transformation
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Rapid accumulation of diatom/rad ooze
Compaction
dissolution of opal frustules (unprotected)
Rate of dissolution >> rate of diffusion
Pore waters - Si saturation ~1000 µM
Pore Water Chemsitry from Site 1218.
Biogenic Opal to Chert Transformation
Solution-Reprecipitation Process
• Opal A - amorphous
• Opal Ct - cristobalite (metastable phase)
• Chert (microcrystalline)
Transformation from A to Ct can occur at low temperatures
<45°C, and burial depths (~50 m)
– Absence of detrital impurities speeds up the process
Bedded
Chert
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Biogenic Opal to Chert
Transformation
• Chert replaces limestone
chert
limestone
oolitic limestone has been completely replaced
by quartz
Partial Silicification of calcite with the
development of radially fibrous or
botryoidal quartz (e.g., chalcedonyfibrous)
Silicification of calcite with
complete replacement of the
limestone fabric with quartz
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Cretaceous Cherts
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Nodular Chert
• Typical of shallow water environments
– Continental shelves
– Especially in carbonates (replacement)
Bedded Chert
• Typical of clastic starved basins
– Pelagic setting (deep sea)
– Shelf edge (upwelling)
Tropical Radiolaria
Red and green chert in the Marin Headlands Terrane of the Franciscan Complex
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Highly Contorted Bedded Chert
Marin Headlands, Franciscan
Red Bedded Chert
Mount Diablo
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Chert (Radiolarites)
Cretaceous Hawasina Group, Oman
A: radiolarite. B.spiculite, C. lutecite, D. chalcedony (fiberous microquartz
replacement)
Phosphorites
• Rocks that are significantly enriched in phosphorus
– >15% P2O5, or 6.5%P
– Average sediments <0.5%
• If <15%, ~ “phosphatic”
• Small fraction of the sedimentary rocks
• Economically important
– 80% of the worlds phosphate
• Occur in rocks of all ages
– Concentrated in certain regions (ie., central, SE Asia; eastern
Europe, N Africa, SE US (florida)
• Modern:
– Coastal Peru, Chile, Baja, SW Africa
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Phosphorites: Composition
• Ca phosphate minerals (apatite)
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Fluorapatite - Ca5(PO4)3F5
Chlorapatite - Ca5(PO4)3Cl
Hydroxyapatite - Ca5(PO4)3OH
Carbonate hydroxyl fluorapatites (10% PO4 is replaced by CO3)
Accessory components - Detrital qtz, authigenic chert, opal-ct,
dolomite, glauconite, zeolites
Phosphorite Deposits
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mm scale laminae to meter scale beds
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Phosphoria Formation, ID & WY - several hundred meters thick
Interbedded with shales, cherts, limestones, dolomites
Textures:
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ooids, peloids, fossils (bioclasts), clasts or nodules
sand size most common
4 types of deposits:
1. Bedded Phosphorus
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Nodular Phosphorites
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Brownish to black, diameter (cm-m), layered (concentrically banded)
Modern upwelling zones
Pebble-bed phosphorites
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Varying thickness, interbedded, fish debris
Phosphoria (Permian), Australia, N. Africa
Phosphatized fragments, fossils, nodules
Florida (Miocene)
Guano deposits
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Bird and bat excrement - leached to form insoluble Ca phosphate
Eastern Pacific
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Permian Phosphoria Formation
• Bedded Phosphorites (420 m thick)
Phosphorite Origin/Deposition
Pacific Ocean (150°W) : Dissolved PO4 (µmol/kg)
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Phosphorite Origin/Deposition
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100-1000 m water depth (i.e., shelf, slope)
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Upwelling of nutrient rich waters
Hi organic carbon flux, burial
Slow decay releases PO4, consumes O2
Pore waters - saturated
Phosphorite precipitates on grains
Peru Margin, ODP Leg 201
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Deep Sea Core - Pore water chemistry (interstitial water)
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Lake Neosho Shale Member, St. Louis Missouri, Middle Pennsylvanian
Limestone lens with phosphate nodules (from bioclastic shale bed)
Lake Neosho Shale Member, St. Louis Missouri, Middle Pennsylvanian
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Miocene Phosphorites (Southeastern US)
• Early Miocene (18 to 25
Ma)
Paleocene (55-60 Ma)
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Evaporites
• Sediments (rocks) composed of minerals (salts) precipitated
from saline solutions concentrated by evaporation
• All ages
– Common in Cambrian, Permian, Jurassic, and Miocene
• Marine and non-Marine
– Marine - thicker and more extensive
• Semi-enclosed Basins & Arid climate
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Salt Pan
Sabkha (Playa),
Death Valley
Peritidal carbonate environments
Peritidal marsh
Tidal flat
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Peritidal carbonate environments
Stromatoliths in
peritidal zone
(Hamling Pool,
Western Australia)
Sabkha environment (Persian Gulf)
Evaporites: Composition
• Marine Evaporites:
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Halite (NaCl)
Anhydrite (CaSO4)
Gypsum (CaSO4•H2O)
Calcite
• Non-Marine Evaporites:
– May include the above, but tend to have less Cl, more HCO3 and
Mg
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Gypsum and Anhydrites
• Deposited mainly as Gypsum
– Rapid dehydration or during burial ( compaction) Anhydrite
• Anhydrites - CaSO4
– Nodular Anhydrites
• Lumps in halite, clay, or carbonate matrix
• Carbonate or clayey sediments - growth of
gypsum
• Sabkha environment
– Laminated Anhydrites
• Thin layers - alternate w/dark laminae of
dolomite/organic matter (seasonal varves,
Permian Formation)
– Massive Anhydrites
• Semi-enclosed Marine Basin (Mediterranean)
Evaporite origin and deposition
Evaporation Sequence
• 50% remaining
– Carbonate
• 20%
– Gypsum
• 10%
– Halite
– Dolomite
• <5%
– MgCl, KCl
Evaporation of 1000m SW
will produce 15 m salt
– Some evaporite deposits
>2 km thick!?
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Permian Basin Reef
Guadalupe Mounts, Capitan, etc.
geologic map of the
Guadalupe Mounts
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Castille Formation
(Ochoan)
Laminated and nodular (secondary)
evaporites
Laminated basin evaporites
(annual)
nodular anhydrite (dark gray) in GrayburgSan Andres dolomite
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Messinian Crisis (late Miocene)
between 5.96 and 5.33 m.y.
laminated gypsum with soft-sediment deformation, Villadoro, Corvillo Basin
Messinian
Proposed Mechanisms for
Isolation
1) a 60 m global drop in
sea level due to
glaciation,
2) horizontal squeezing,
and
3) tectonic uplift
4) ????
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