Week 9 - Carbonate factory & carbonate platforms

Subtidal carbonate factory

Driven by precipitated carbonate sediment in the shallow water due to high light levels and nutrients

Forms a shelf

Activity decreases with depth

What controls carbonate deposition in supersaturation (highest)

Abiotic control → high temperature, salinity and ocean restriction

Produces → Non skeletal grains, mud, microbes

What controls carbonate deposition in medium carbonate saturation

Biotically controlled (light as main energy source)→ High temperature, shallow depth, water transparency → affects productivity of photozoa (corals, green algae, symbiotic foraminiferas

Alternatively controlled by hydrodynamics and water transparency → production of red algae

What controls carbonate deposition in low carbonate saturation

Organic matter as main energy source → supply of plankton and other pelagic organic matter to the food chain → affects productivity of hereozoa

M,C,T production modes

Name the types of carbonate platforms

Ramp

Epeiric shelves

Rimmed shelf

Isolated platform

Average angle of repose in loose sediment, and what happens if it is exceeded

30-35 degrees

If exceeded the friction between grains is not strong enough to hold them together, so becomes unstable

Why do carbonates have a high angle of repose

Interlocking chemical precipitates form a skeletal framework rock (as opposed to classic granular sediment)

Mineralisation/cementation as supersaturated water passes through the pore space (degassing of CO2) which lithophies carbonates as they form

Presence of microbes

Features of a carbonate ramp

Shallow dipping often <1 degree

Subdivided based on the influence of waves/storms on sedimentation (Sedimentation in the mid ramp is frequently storm influenced)

Sand shoals sometimes form protection from wave/storm energy

Low energy basin with negligible storm influence

e.g Arabian-Persian Gulf

Features of a carbonate rimmed shelf

Shelf margin break (High organic productivity), reef growth or carbonate sand bodies

Ocean facing shelf margin -> high energy + turbulent

e.g the Bahamas

Features of a carbonate isolated platform

Features of carbonate Epeiric shelves

Deposited in epi-continental seas

Platforms < a few thousand kms wide

No modern as sea level is too low

Shallow water, storm influenced, intertidal sedimentation

E,g permian-cretacrous middle east

Key factors favouring carbonate production

Warm water

High salinity

isolated from influx of eroded sediment (terrigenous)

Depositional processes in high energy platform margins (reefs)

In situ growth of carbonate build up

Reworking by storms and currents

Export to basins by gravity processes

Depositional processes in oolitic sand shoals

In situ precipitation of ooids

Reworking by tidal currents, storms and waves

Depositional processes of (low energy) lagoons

Accumulation of faecal pellets from worms and gastropods

Disintegration of algae into mud

Chemical precipitation (whitings)

Depositional processes of land attached tidal flats

Reworking by tidal currents and waves

Storms move subtidal sediment onto tidal flat and into lagoon

Why are carbonates important to study

Carbonate precipitation is driven by carbonate factories in the shallow ocean so can be used to track sea level change over time

The distribution and character of lithofacies are controlled by characteristic environmental parameters and can be used to interpret ancient carbonate platforms