Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Formation and Composition of Siliceous Sedimentary Rocks, Phosphorites, and Evaporites, Lecture notes of Geochemistry

An in-depth exploration of siliceous sedimentary rocks, including chert and silica geochemistry, as well as phosphorites and evaporites. Topics cover the origin and transformation of these rock types, their geochemical properties, and the conditions necessary for their formation. The document also discusses the economic importance of phosphorites and the role of upwelling zones in the formation of these sedimentary rocks.

Typology: Lecture notes

2021/2022

Uploaded on 09/12/2022

mjforever
mjforever 🇺🇸

4.8

(25)

258 documents

1 / 24

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
1
Lecture 11: Non-Carbonate Biogenic
and Chemical Sedimentary Rocks
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 (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
pf3
pf4
pf5
pf8
pf9
pfa
pfd
pfe
pff
pf12
pf13
pf14
pf15
pf16
pf17
pf18

Partial preview of the text

Download Formation and Composition of Siliceous Sedimentary Rocks, Phosphorites, and Evaporites and more Lecture notes Geochemistry in PDF only on Docsity!

Lecture 11: Non-Carbonate Biogenic

and Chemical Sedimentary Rocks

• Siliceous Sediments & Chert

• Phosphorites

• Evaporites

• Banded Iron

Siliceous Sedimentary Rocks

Fine-grained, dense , hard rocks composed predominantly of SiO 2 minerals quartz, chalcedony , and opal + 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- μm)
  • Biogenic Silica - amorphous Silica/Opal A (SiO 2 *H 2 O) - Readily transforms to chert

Silica Geochemistry

Amorphous SiO 2 - highly soluble

  • Groundwater
    • 100-200 ppm
    • Source: feldspar to clay 2KAlSi 3 O 8 + 2H+^ + 9H 20 ⇒ H 4 Al 2 Si 2 O 9 + 4H 4 SiO 4 + 2K+
    • Solubility increases in Alkaline (hi pH)water

Silica Geochemistry

Amorphous SiO 2 - highly soluble

  • Seawater (H 4 SiO 4 )
    • <1 to 11 ppm
    • Highly undersaturated!
    • Organic coatings preserve shell opal
    • Accumulation occurs only where fluxes are high
    • Diatom/radiolarian oozes

Leg 199 Sites: Si & Ca Wt% & mass accumulation rates (MARs)

Si mass accumulation rates (MARs) in the mid- Cenozoic

Meridional Pattern

Biogenic Opal to Chert Transformation

  • 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

Cretaceous Cherts

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

Cretaceous Hawasina Group, Oman

Chert (Radiolarites)

A: radiolarite. B.spiculite, C. lutecite, D. chalcedony (fiberous microquartz replacement)

Phosphorites

  • Rocks that are significantly enriched in phosphorus
    • 15% P 2 O 5 , 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

Phosphorites: Composition

  • Ca phosphate minerals ( apatite )
    • Fluorapatite - Ca 5 (PO 4 ) 3 F 5
    • Chlorapatite - Ca 5 (PO 4 ) 3 Cl
    • Hydroxyapatite - Ca 5 (PO 4 ) 3 OH
    • Carbonate hydroxyl fluorapatites (10% PO 4 is replaced by CO 3 )
    • 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

Phosphorite Origin/Deposition

  • 100-1000 m water depth (i.e., shelf, slope)
  1. Upwelling of nutrient rich waters
  2. Hi organic carbon flux, burial
  3. Slow decay releases PO 4 , consumes O 2
  4. Pore waters - saturated
  5. Phosphorite precipitates on grains

Peru Margin, ODP Leg 201

Deep Sea Core - Pore water chemistry (interstitial water)

Miocene Phosphorites (Southeastern US)

  • Early Miocene (18 to 25

Ma)

Paleocene (55-60 Ma)

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

Peritidal carbonate environments

Stromatoliths in

peritidal zone

(Hamling Pool,

Western Australia)

Sabkha environment (Persian Gulf)

Evaporites: Composition

  • Marine Evaporites:
    • Halite (NaCl)
    • Anhydrite (CaSO 4 )
    • Gypsum (CaSO 4 • H 2 O)
    • Calcite
  • Non-Marine Evaporites:
    • May include the above, but tend to have less Cl, more HCO 3 and Mg

Gypsum and Anhydrites

  • Deposited mainly as Gypsum
    • Rapid dehydration or during burial ( compaction) - Anhydrite
  • Anhydrites - CaSO 4
    • 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!?