GEOL 462: Principles of Sedimentology and Stratigraphy

Lecture 13: Thursday, February 26, 2009

Topics
1. Types of carbonate platforms
2. Carbonate Factories
3. Carbonate deposits: Controlling factors
4. Carbonate depositional models: overview
5. Carbonate platforms and sea level changes
6. Carbonate growth potential (accommodation space) and carbonate cycles

Types of carbonate platforms
a. Carbonate Platform: A large edifice formed by the accumulation of sediments in an area of subsidence
b. Carbonate Shelf: A platform attached to an adjacent continental land mass
c. Epeiric Platform: A craton many hundreds to thousands of kilometers acorss covered by shallow seawater
d. Carbonate Bank: An isolated platform surrounded by deep ocean water and cut off from terrigenous clastic sediments
e. Carbonate Atoll: A specific type of bank developed on a subsiding volcano
f. Reef Complexes or Carbonate Buildups: Atolls and Banks dominated by reefs

Different types of shelf morphologies:
a. rimmed shelf - pronounced shelf break with reef growth
b. unrimmed shelf - pronounced shelf break with little to no reef growth
c. ramp - no pronounced shelf break
d. isolated platform - steep pronounced shelf break with or without reef growth

Carbonate Factories
1. Carbonate Factory: Shallow, illuminated seafloor. Particles of all grain sized are born in the factory, either through secretion as skeletons or precipitating out of seawater;
2. Particle migration: Particles mostly remain in situ forming widespread ‘subtidal’ deposits or reefs and mounds; Fine-grained particles can move seaward and landward.
3. A healthy carbonate factory is the KEY to carbonate facies and stratigraphy
Br> Evaporites:
1. Evaporite Factory: Operates in both siliciclastic and carbonate setting due to supersaturation caused by restrictions or large alkalinity supply;
2. Particle migration: does occur but is rare because evaporite precipitates very quickly and sediment is lithified and cemented very fast.
3. Occurrences: Evaporites can be formed in both shallow- and deep-water basins

Carbonate deposits: Controlling factors
General Governing Principles:
a. carbonates are born, not made
diversity and abundance of organisms with time
abundance, composition, sediment composition
b. dependence on sunlight (photic zone)
depth versus growth rate; maximum growth in the upper 10 to 20 m
c. adequately warm water, but upper temperature threshold
latitudinal control +/- 32 degree; tropical and temperate carbonates (Australian and Ordovician)
d. adequate nutrient supply, but upper concentration threshold
clear tropical waters are actually submarine deserts
e. during sea level rise, platform depositional patterns shift abruptly; either:
backstepping, drowning
f. "bucket anatomy construction" (seafloor diagenesis)
wave-resistant margins, slump-resistant slopes

Carbonate depositional models: overview
Warm water and Cool water (Temperate) carbonates:
Modern examples:
Warm water--open shelves of West Florida (22-28 degree C), Campeche Bank (24-30 degree C), Persian (Arabian) Gulf
Cool water--Gulf of Gabes in the Miditerranean (15-26 degree C), Rottnest Shelf of Western Australia (16-20 degree C)
It is hard to identify an ancient temperate water carbonates. So temperate water models are not commonly used for anicent carbonate platforms

Carbonate ramps:
Warm Water: Beach/Barrier/Strand Plain Shoals and Patched Reefs
Cool Water: High energy near coastal areas but low energy in offshore settings: Carbonate Factory is closer to land
Rimmed shelves:
Warm Water: Shallow water carbonate in shelf lagoons
Cool Water: Starved deep water carbonate in shelf lagoons
Carbonate Banks:
Warm Water: Shallow reefs forming restricted areas and mud is easily produced
Cold Water: Different organisms and relatively deeper water, less carbonate mud

Epeiric Shelves: Simple model
No modern analogy, common in Precambrian such as Mesoproterozoic
Model is simplified according to Wave and Tidal activities
Much needs to do for better models
May have patched reefs, shoals, and restricted environments

Carbonate platforms and sea level changes
1. Progradation:
associated with sea level fall
associated with overgrowth
2. Formation of karstic surfaces
3. Siliciclastic filling of karstic depressions and incised valleys
4. Drowning of carbonate platforms: sea level rise or increase of siliciclastic supply, etc.

Carbonate growth potential (accommodation space) and carbonate cycles
Catch up, keep up, prograding, drowning Carbonates track sea level, or if not able to keep track of it, it would be drowned!
The major mechanism for the formation of carbonate cycles: different from siliciclastics