Greenhouse, Transitional and Icehouse Eustasy Yield Distinctive Parasequence and Sequence Stacking on Carbonate Platforms

Read, J. Fred

Department of Geosciences, Virginia Tech, Blacksburg VA 24061 USA

Greenhouse platforms consist of stacked, meter scale parasequences capped updip by tidal flat facies and subtle disconformities; downdip they have poorly cyclic subtidal units.  Parasequences are precessional or even sub-Milankovitch.  Third-order sequences are bounded by stacked disconformities.  Icehouse platforms have disconformable fourth-order sequences (grossly 100 to 400 k.y) with local incision, and contain juxtaposed deep water and shallow water facies and rarely have tidal flat facies.  The large sea level changes (50 m to 100 m plus) are driven by obliquity and eccentricity.  Component parasequences may be shingled and contain local, high relief buildups.  Platform margins may show considerable vertical shifts in depth sensitive facies.  Third-order sequences are relatively subtle and tend to be masked by the dominant fourth order successions.  Transitional states when the earth contained moderate amounts of polar ice have sea level changes of a few tens of meters or less.  Platform successions contain well developed third- or fourth-order sequences dominated by shallow water facies and widespread carbonate sands, bounded by subtidal carbonate mudstones and variably developed regional disconformities.  Parasequences (obliquity/short term eccentricity?) are a few meters thick and are bounded updip by disconformities and thin eolianites, but tidal flat deposits are relatively rare.  Transitional states have been overlooked and occupy a substantial part of the record.   Reservoirs on greenhouse platforms are early peritidal dolomites, or shelf edge grainstones.  On transitional and icehouse platforms, reservoirs include widespread grainstones that extend into the interior, and on-platform buildups on icehouse platforms.