lecture 11: non-carbonate biogenic and chemical sedimentary rocks
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Lecture 11: Non-Carbonate Biogenicand Chemical Sedimentary Rocks
• Siliceous Sediments & Chert• Phosphorites• Evaporites• Banded Iron
Siliceous Sedimentary RocksFine-grained, dense, hard rocks
composed predominantly of SiO2minerals quartz, chalcedony, andopal + minor impurities
• Occur throughout the rock record– Most common in Jurassic,
Cretaceous, Paleogene rocks (180-40 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 GeochemistryAmorphous SiO2 - highly soluble• Groundwater
– 100-200 ppm– Source: feldspar to clay2KAlSi3O8 + 2H+ + 9H20 ⇒ H4Al2Si2O9 + 4H4SiO4 + 2K+
– Solubility increases in Alkaline (hi pH)water
Silica GeochemistryAmorphous 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 Chert2 Types:1. Biogenic Chert2. Nonfossiliferous Chert• Requirements:
1. Silica Source2. Precipitation mechanism
• Supersaturation
Chlorophyll contents in the Pacific
Origin of Biogenic Chert1. Silica Source:
Upwelling zones-highproductivity (diatoms)
Chlorophyll contents inthe 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- CenozoicMeridional Pattern
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Biogenic Opal to ChertTransformation
• 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 TransformationSolution-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 ChertTransformation
• Chert replaces limestone
chert
limestone
Partial Silicification of calcite with thedevelopment of radially fibrous orbotryoidal quartz (e.g., chalcedony-fibrous)
Silicification of calcite withcomplete replacement of thelimestone fabric with quartz
oolitic limestone has been completely replacedby quartz
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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)
Red and green chert in the Marin Headlands Terrane of the Franciscan Complex
Tropical Radiolaria
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Cretaceous Hawasina Group, Oman
Chert (Radiolarites)
A: radiolarite. B.spiculite, C. lutecite, D. chalcedony (fiberous microquartzreplacement)
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; easternEurope, N Africa, SE US (florida)
• Modern:– Coastal Peru, Chile, Baja, SW Africa
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Phosphorites: Composition
• Ca phosphate minerals (apatite)– 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• mm scale laminae to meter scale beds
– Phosphoria Formation, ID & WY - several hundred meters thick• Interbedded with shales, cherts, limestones, dolomites• Textures:
– ooids, peloids, fossils (bioclasts), clasts or nodules– sand size most common
4 types of deposits:1. Bedded Phosphorus
– Varying thickness, interbedded, fish debris– Phosphoria (Permian), Australia, N. Africa
2. Nodular Phosphorites– Brownish to black, diameter (cm-m), layered (concentrically banded)– Modern upwelling zones
3. Pebble-bed phosphorites– Phosphatized fragments, fossils, nodules– Florida (Miocene)
4. Guano deposits– 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/DepositionPacific Ocean (150°W) : Dissolved PO4 (µmol/kg)
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Phosphorite Origin/Deposition• 100-1000 m water depth (i.e., shelf, slope)1. Upwelling of nutrient rich waters2. Hi organic carbon flux, burial3. Slow decay releases PO4, consumes O2
4. Pore waters - saturated5. Phosphorite precipitates on grains
Peru Margin, ODP Leg 201
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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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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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Peritidal carbonate environmentsStromatoliths inperitidal zone(Hamling Pool,Western Australia)
Sabkha environment (Persian Gulf)
Evaporites: Composition
• Marine Evaporites:– 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 depositionEvaporation Sequence• 50% remaining
– Carbonate• 20%
– Gypsum• 10%
– Halite– Dolomite
• <5%– MgCl, KCl
Evaporation of 1000m SWwill produce 15 m salt– Some evaporite deposits
>2 km thick!?
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Castille Formation(Ochoan)
Laminated basin evaporites(annual)
Laminated and nodular (secondary)evaporites
nodular anhydrite (dark gray) in Grayburg-San Andres dolomite
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between 5.96 and 5.33 m.y.
Messinian Crisis (late Miocene)
laminated gypsum with soft-sediment deformation, Villadoro, Corvillo Basin
Messinian
Proposed Mechanisms forIsolation
1) a 60 m global drop insea level due toglaciation,
2) horizontal squeezing,and
3) tectonic uplift4) ????
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