Subtidal Shelf Carbonate Facies Model

Evaporites

Subtidal shelf carbonate facies model

•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 changeon siliclastic and carbonate margins

What causescyclic shallowing-upward 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 marinetransgression

“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)

•Example: Persian Gulf

•Surf zone

–Lower foreshore

•Poorly sorted shell hash with micrite; longshore trough crossbeds

–Upper foreshore

•Well sorted lime sand & gravel; planar x-beds, 15° dip seaward; beachrock

•Subtidal lagoon sands & muds

•Intertidal mud flats

•Supratidal Sabkha - evaporites

•Eolian carbonate dunes

Persian Gulf, 1

Persian Gulf, 2

•Area of 350,000 km2

•Average water depth 20-80m

•Restricted environment with lower wave energy generates less skeletal sand than Bahamas

•Associated with evaporite minerals

Intertidal mudflats

•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