The sequences and systems tracts that build transgressive/regressive facies cycles in carbonate settings

The present study documents the existence of four types of depositional sequences in carbonate settings on the basis of their stratal pattern an the facies within the systems tracts. Infilling sequences at the beginning of regression fill in the flooded platform and tend to form a large hemipelagic ramp with equal thicknesses of shelf and basin lithofacies. Forestepping sequences near the end of the regression are bounded by tectonically enhanced unconformities and form large lowstand systems tracts with progradational offlap at the platform margin. Transgressive and highstand systems tracts are thin on the platform, because little to no space is created by tectonic subsidence. Aggrading sequences in the early stage of the transgressive and highstand systems tracts are thick and consist mainly of lagoonal facies. It is the best period for platform growth because the rate of the relative rise of sea level is not too fast and carbonates are able to keep up. Backstepping sequences in the latest stage of the transgression are characterized by depositional environments that backstep due to the increasing rate of creation of accommodation space. Shelf-members of these sequences show mainly high-energy facies. Backstepping sequences thin upward to a surface of drowning (drowning unconformity), whichmore » corresponds to the maximum flooding surface of the last well-developed sequence. Sequences and systems tracts provide a way to subdivide sedimentary strata into genetic correlatable chronostratigraphic intervals. Transgressive/regressive facies cycles are useful as a framework for analysis and prediction of depositional environments and lithofacies within systems tracts.« less