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