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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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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)

Red and green chert in the Marin Headlands Terrane of the Franciscan Complex

Tropical Radiolaria

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Highly Contorted Bedded Chert

Marin Headlands, Franciscan

Red Bedded Chert

Mount Diablo

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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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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 25Ma)

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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Sabkha (Playa),Death Valley

Salt Pan

Peritidal carbonate environments

Peritidal marsh

Tidal flat

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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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Permian Basin Reef

geologic map of theGuadalupe Mounts

Guadalupe Mounts, Capitan, etc.

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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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