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Carbonate Sequence Stratigraphy – First Principles Accommodate the Unruly Carbonate System

Abstract

Carbonates have been considered faithful recorders of sea level because the light-dependent sediment production maintains the base level close to sea level. Carbonates are, however, also known to be 180 degrees out of phase with siliciclastics in regards to export of sediment into the deep basin. In addition, carbonate-specific processes like platform drowning or changes of production mode over time seem to be in conflict with the rules established in siliciclastic sequence stratigraphy. Yet, several assumptions on sequence stratigraphy, like the chronostratigraphic significance of seismic reflections and the tie of sequence boundaries to sea level changes have been established in carbonates by coring of carbonate sequences along transects from the platform to the basin. These studies clearly document that the principles of sequence stratigraphy also apply to carbonates. If the original principles in sequence stratigraphy are applied, the unruly behavior of carbonates is largely eliminated. For example, the original definition of a depositional sequence as an unconformity-bounded unit has proved to be a robust definition for carbonate sequence stratigraphy. This original definition does not require the unconformity to be related to an exposure surface. As a consequence it also applies to drowning unconformities and unconformities caused by changes of the ecologic system. Furthermore, unconformities in the carbonate deep-water drift deposits that do not contain any evidence of subaerial exposure qualify as sequence boundaries. Yet, studies have shown that these sequence boundaries are genetically related to unconformities on the shelf as the shift of the current pattern that creates the unconformity is related to the same sea level change that generates the subaerial exposure on the shelf. Because carbonates are faithful recorders of sea level, depositional cycles in carbonates that are separated by an exposure surface have been called high-frequency sequences. These high-frequency sequences are thought to be caused by orbital forcing, with precession being the highest frequency for sea level changes. Recent studies, however, have documented that shallow-water carbonates record meter-scale oscillations that occur in the sea level highstand of the precession cycles, indicating that carbonates are sensitive recorders of meter-scale sea level changes that occur over a few thousand years.