carbonate platforms. recap: carbonate ramps consistent shallow gradient from shoreline to basin...

Carbonate Platforms

RECAP: Carbonate Ramps

Consistent shallow gradient from shoreline to basin (some may be distally steepened), somewhat analogous to siliciclastic shelf

Highest

Energy

Lowest

Energy

Carbonate Platforms

Low-gradient to flat, shallow, broad top with very steep slope

May have platform-edge barrier (reef or shoals)

Where is the energy maximum?

Frictional energy loss

Platform-edge energy maxBeach

energy max

Quiet-water lagoonFore-reef energy max

Platform interior

Slope and Basin

Platform Margin Shoals

Platform margin facies: Great Bahama Bank

Skeletal grainstone margin

Localized ooid grainstone shoals

Grainstone marginPackstone/wackestone interior

Platform-margin shoals of ooid or skeletal grainstone

Spillover sediment transport in windward direction

Planar cross-bedded shoal facies over lagoonal facies

Platform Interior Facies

Open platform peloidal grainstone

Protected peloidal packstone/wackestone

Characteristic meter-scale cyclicity in platform interior

Shallowing-upwards from subtidal to supratidal

Upper Triassic, Italy

Cycles may be allocyclic, driven by rising base level and creation of accommodation space for sediment accumulation

Typical carbonate accumulate rates are very high (> base level rise), so rapidly fill accommodation space and shallow upward

Exposure (karst) surface (Carboniferous, Nevada)

Primary relief on surface mantled and infilled by subsequent deposition

Cycles may also be autocyclic (properties inherent to the carbonate factory)

High shallow subtidal sedimentation rates = carbonate factory tends to aggrade rapidly to sea level

Carbonate production slows as sediments near sea level

May require “lag time” where carbonate sedimentation is slow or absent during early part of base level rise

Cycles are strongly asymmetrical – thin to absent retrogradational facies, thick progradational facies