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Geol 464: Carbonate GeologySemester 0622007

Lecture 15

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Sequence stratigraphy of

carbonate depositional systems Sediment supply and carbonate platform flooding

Carbonate production is proportional to the area of flooded platform

top. * The greatest supply of sediment in carbonate platform systems occur

during periods of elevated relative sea-level because shallow-watercarbonate-secreting communities (the carbonate factories) will be

able to extend over the entire upper surface of carbonate platforms.

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Relationships between carbonate platform morphology, sea-level and carbonatesediment supply.

The graphs show vertical changes in carbonate production (in meters of vertical

thickness per 1000 years) with depth (in meters below sealevel). Production reaches a maximum between 10 and 30 m water depth and then

decreases with increasing water depth. (a) During periods of relatively low sea-level,the limited horizontal extent of the carbonate platform results in small total amountsof carbonate being generated because of the small area of high productivity.

(b) Flooding over the platform top significantly increases the horizontal extent of theproductive area, and sediment production will rapidly fill all the newly createdaccommodation space.

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Sequence stratigraphy of carbonatedepositional systems

Shallow-marine warm water carbonate factories produce sediment at greaterrates (1-10m per 1000 years) than rates of increase of accommodationspace due to long-term basin subsidence and low-order sea-level changes(0.01-0.1m per 1000 years).

This means that on many platforms shallow-water carbonate sediment may

infill or overfill accommodation space even when relative sea-level is rising. Such systems will show aggradation or progradation during both rising

relative sea-level and high sea-level.

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Dissolution and cementation

During relative sea-level fall, carbonate platform top sediments aredrained of the seawater contained within pore spaces and this is replaced

by meteoric water, derived from rainfall, which is CO2-rich and thereforeacidic.

Such fluid will dissolve unstable aragonitic grains (e.g. ooids, mulluscs,corals) and may precipitate this dissolved carbonate as a calcite cement.

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Sequence stratigraphy of rimmed

carbonate platformsTST

During times of relative sea-level rise, rimmed carbonate platforms mayrespond in two very different ways depending on the balance betweenthe rate of relative sea-level rise and rate of carbonate production:

1. Backstepping platforms: when rates of increase in accommodation

space are greater than rates of carbonate production, then the facies willretrograde, or carbonate platform margin may drown.

Offshore areas will accumulate pelagic facies and sediment starvationmay result in submarine cementation and hardground formation.

2. Aggrading and prograding platforms: if production is equal to or fasterthan the rate of increase in accommodation space, the platform willeither aggrade or prograde into the basin.

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Sequence stratigraphy of rimmed

carbonate platforms HST

When the platform top is flooded by shallow-marine waters, there is a very

large area for the carbonate factory to occupy and the platforms are likelyto produce the greatest amounts of reefal material, and bioclastic or ooidsand.

HST will be characterized by prograding clinoforms and regressivesuccessions.

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Sequence stratigraphy of rimmed carbonateplatforms

FSST and LST Falling sea-level will usually lead to the subaerial exposure of large areas

of platform top carbonate facies.

Basin margin facies may include rock fall deposits, and beaches andfringing reefs may form on the steep slope.

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Sequence stratigraphy of

carbonate ramp The different morphology of ramps means that they respond to relative sea-level changes in a different fashion form rimmed shelves.

Ramps dont have the highly productive carbonate communities such s

fringing and barrier reefs that characterize rimmed carbonate platforms.

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Diagrams illustrating variations in the sequence stratigraphy of carbonateramps: (a) distally steepened ramp; (b) arid climate (evaporitic) ramp.

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Sequence stratigraphy of

carbonate ramp TST

The shoreline and associated facies belts will migrate largedistances landward.

in most cases, the rate of relative sealevel rise exceed therate of carbonate production so that retrogradationalparasequence set is deposited.

This result in deepening-upward profiles with muddier facies ininner ramp settings, and condensed beds of shellyconcentrations, hardgrounds or pelagic facies may be found inmid to outer ramp locations.

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Sequence stratigraphy of

carbonate ramp HST

The decreasing rates of relative sea-level rise during the period of HST

times result in a decrease in accommodation space that is normallyexceeded by sediment production and development of progradationalparasequences.

The low angle of slope results in progradation taking place across largedistances.

Depending on the efficiency of wave and tidal current to redistributesediment on the ramp, a margin or slope break may develop.

The extensive progradational inner ramp packstones and grainstones willbecome leached and develop a secondary porosity and/or sediments willbecome cemented and lithified.

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Sequence stratigraphy of

carbonate ramp FSST and LST

Relative sealevel fall on a ramp will expose updip section that will developan unconformity and the locus of sedimentation will shift considerabledistances downslope.

FST and LST comprising breccias or debris flows are not generally found

on ramps unless the submarine slope is distally steepened by faulting.