Evaporite Diagenesis
Gypsum vs Anhydrite
Gypsum CaSO4.H20
Hydrated anhydrite essentially
Anhydrite
CaSO4
No water of crystallisation
Anhydrous
Important Q:
Is this a...
Secondary gypsum possible origins:
Gypsum
dehydrated with burial
rehydrated on uplift
Anhydrite
rehydrated with uplift
forms gypsum
Primary fabric - initial precipitate ?
Secondary fabric -
result of diagenesis / recrystallisation ?
Gypsum
Cycle
If Gypsum = original mineral
- Begin with gypsum at surface
lagoon/shelf - twinned selenitic
basin = laminated
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
back-sabkha - gypsum from anhydrite hydration
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)
Primary Anhydrite Textire
Nodular deposits
Fine, equant mosaics
Lath arrangements felted/parralel/subparallel
Recrystallisation Texture Anhydrite
Due to burial (of equant/lath anhydrite)
Coarse granular mosaics
Large fibrous crystals
Fibro-radiating aggregate
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'
Landward Sabkha
Enterolithic texture
Thin layers / coalesced nodules of anhydrite
= Contorted / buckled
Anhydrite rehydration
possible in most landward (back-) sabkha
forms gypsum
Hydrous - water in crystal structure