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Ironstone formation (Iron Oxide -Red Beds (Red beds (result of diagenesis…
Ironstone formation
Iron Oxide
-Red Beds
= accumulations of Fe-rich material
Close to source (generally)
Bauxite
result of weathering
often red sandstones & bauxites with oxide deposits
Red beds
result of diagenesis
sands altered & turned red
Iron oxides Where?
2. Deltaic
envmts
ecomomically important deposits
oolitic ironstones common in Jurassic from deltas
cross beds & channelised
1. Lake envmts
rarely economically important
3. Beach envmts
current swept
physical processes can cause concentration of resistant Fe minerals
placer deposits
concentrates of enriched minerals
e.g. magnetite
4. Continental shelf
5. Deep ocean
manganese crusts
hydrothermal deposits associated with ridge volcanism
Jurassic Ironstones
Yorkshire
Cyclical sealevel ∆
important effect on salinity
3 main Fe-rich horizons (pyritic & sideritic)
Rosedale
Cleveland ironstones
Jet rock
Interbedded with shales
Depositional Model - Hallam
Deltaic environment
- adjacent to tropical rainforest
Ironstones form on highs/ swells
in off margin setting
Ironstone deposits
only few metres thick
found on highs
Basin acts as sediment trap
collects coarser material
Fe-rich clay sized sediment accumulates on highs
Condensed (reduced) sedimentation results
Trapping - only if sealevel right level
Too high
barrier doesn't work anymore (submerged)
Too low
island forms on high
Just right
Clay-sized material winnowed out
shallow water thus reduced sedimentation
condensed sedimentation
Flaw in
Hallam Model
Extensional faulting forms highs/swells
during basin opening
∴ sealevel + structural control (tectonic)
Hallam only considered sealevel
Appalachian Basin Silurian Ores
Appalachian Mtns Formation
during Silurian
shed sediment Westward into basin
Interior = carbonate environment
Appalachian
Basin envmts
Basin Interior
= carbonate platform
River input
from mtns
= sandstone river deposits
Where delta meets carbonate platform
i.e. Interface btwn terrigenous & carbonate envmts
Iron ore deposits form thin layer
Rich in
chamosite - most valuable
haematite - nr hills
glauconite - nr sea
Weathering
can cause appreciable change in rock appearance
i.e. reddening of sstones
Ancient Ironstone Formation
Iron Oxide Formation