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Evaporite Diagenesis (Sabkha Displacive Anhydrite Precip (Gypsum…
Evaporite Diagenesis
Sabkha Displacive Anhydrite Precip
Gypsum precipitated in shelf/ lagoon/intertidal
rosette selenite / twinned crystals
Sediment pore waters
derived from
surface flooding
with seawater largely
Intense evaporation
causes pore fluid concentration in sabkhas
Gypsum replaced by
fine mush of equant anhydrite laths
Continued displacive precipitation
Closely packed anhydrite nodules
with only thin strings of host sediment
Nodule texture = 'Chicken-wire anhydrite'
Recrystallisation Texture Anhydrite
Due to burial (of equant/lath anhydrite)
Coarse granular mosaics
Large fibrous crystals
Fibro-radiating aggregate
Gypsum vs Anhydrite
Gypsum
CaSO4.H20
Hydrated anhydrite essentially
Hydrous - water in crystal structure
Anhydrite
CaSO4
No water of crystallisation
Anhydrous
Important Q:
Is this a...
Primary fabric - initial precipitate ?
Secondary fabric -
result of diagenesis / recrystallisation ?
Secondary gypsum possible origins:
Gypsum
dehydrated with burial
rehydrated on uplift
Anhydrite
rehydrated with uplift
forms gypsum
Gypsum
Cycle
If Gypsum = original mineral
Begin with gypsum
at surface
lagoon/shelf
- twinned selenitic
basin
= laminated
back-sabkha -
gypsum from anhydrite hydration
2. Burial diagenesis -
remains as gypsum for some depth
Even if recrystallises
3. Gypsum dehydration during deep burial
4. Recrystallises as anhydrite at depth
If Anhydrite = original mineral
1. Begin with anhydrite
at surface
sabkha
- nodular / chickenwire
2. Burial diagenesis
-
Anhydrite recrystallised
to form new fabric
Uplift of Anhydrite
(both)
Anhydrite at depth formed by
gypsum dehydration
anhydrite recrystallisation
During uplift
Groundwater hydrates anhydrite
forming secondary gypsum
as veins/fine-grained gypsum (alabastine)
Landward Sabkha
Enterolithic texture
Thin layers / coalesced nodules of anhydrite
= Contorted / buckled
Anhydrite rehydration
possible in most landward (back-) sabkha
forms gypsum
Primary Anhydrite Textire
Nodular deposits
Fine, equant mosaics
Lath arrangements
felted/parralel/subparallel