Evaporites
Subtidal shelf carbonate facies model
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Very dynamic environment, considerable variation from the model is expected
Two possible models include:
• Thick homogenous skeletal/pelletal mudstone
• Shallowing upward sequence
Carbonates vs. Siliclastics
Influence of sea level change
on siliclastic and carbonate margins
What causes
cyclic
shallowingupward
sequences
Cyclic carbonate production models?
• Eustatic model - cycles arise from changes in sea level
• Tectonic Model cycles arise from tectonic activity
• Autocyclic model - production of carbonate in subtidal zone produces cycles
Difficulties w/ autocyclic model:
• Assume source of carbonate is the subtidal area
• Assume carbonate environments may not keep pace with sea level rise
Bedrock slope can influence reef progradation
style during marine
transgression
“chicken wire” Fabric
Found in evaporite minerals like Gypsum and Anhydrite that
grow from nucleation points, displacing carbonate or clay minerals
to form dark, thin stringers in the process.
Laminated Evaporites
• Thin lamina of gypsum or anhydrite interbedded with dolomite or organic matter.
• Can extend for >100 km!
• Formation processes?
– Shallow basins
– Deep water brines
–Merging of nodular evaporites
Enterolithic structure
• Observed in modern sabkha environment
• Growth of nodules of evaporative minerals results in increasing pressure
due to crowding
• Resulting layers become contorted
Modern Evaporitic Environs
• Continental Sabkha -playa
• Salt pan
• Coastal Salina
•Interdune
• Coastal Sabkha
• Tidal Delta
Salinas
• Form in shallow depressions
• Often associated with Sabkha
• Precipitation from a surface brine
• Gypsum dominant mineral
• Many include halite
The Sabkha Environment
• Evaporative mudflats
Fluvial-lacustrine
Marine
• Height above water level determines mineral
precipitation
Evaporitic tidal salt flats (sabkha)
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Example: Persian Gulf
Surf zone
– Lower foreshore
• Poorly sorted shell hash with micrite; longshore trough crossbeds
– Upper foreshore
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Subtidal lagoon sands & muds
Intertidal mud flats
Supratidal Sabkha - evaporites
Eolian carbonate dunes
• Well sorted lime sand & gravel; planar x-beds, 15° dip seaward; beachrock
Persian Gulf, 1
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2
Persian Gulf, 2
Area of 350,000 km
• Average water depth 20-80m
• Restricted environment with lower wave energy generates less
skeletal sand than Bahamas
• Associated with evaporite minerals
Intertidal mudflats
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Rippled intertidal carbonate sands with stromatolites
Hamelin Pool,Western Australia
Closeup of calcified algal mats (stromatolites)
Same location
Bird’s eye Limestone
• Calcified algal mats (bindstone or biolithite)
• Internal structure of stromatolites from tidal flats
• Fenestral porosity results from burrows and gas bubbles
Intertidal to supratidal mudcracks
• Gypsum, other evaporite minerals form in cracks
• Cracks can fill with sand
• Mudcracks can flake off as mudchips
Mudchip pebbles
• Mudchips flake off mudcracks
• Transported by tides, currents, waves
• Cement to form intramicrite or intrasparite limestone
Evaporite Models
Four principle models:
• Shelf
• Shallow basin, shallow water
• Deep basin, shallow water
• Deep basin, deep water
• Based on interpretation of the sedimentary record
• Some lack modern analogs