Carbonate Platforms RECAP Carbonate Ramps Consistent shallow gradient

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? Quiet-water lagoon Beach energy max Fore-reef energy max Platform-edge energy max Frictional energy loss

Platform margin facies: Great Bahama Bank Platform interior Platform Margin Shoals Slope and Basin

Grainstone margin Packstone/wackestone interior Localized ooid grainstone shoals Skeletal grainstone margin

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 Protected peloidal packstone/wackestone Open platform peloidal grainstone

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

Cycles are strongly asymmetrical – thin to absent retrogradational facies, thick progradational facies May require “lag time” where carbonate sedimentation is slow or absent during early part of base level rise