JACQUIN, THIERRY, Universite de Bourgogne, Dijon, France, and P. R. VAIL, Rice University, Houston, TX
Tectonic and eustatic processes combine to cause relative changes of sea level. Tectonism, which causes changes in subsidence rates during the evolution of sedimentary basins, has the most profound effect on accommodation, but generally has a low rate of change. Transgressive/regressive facies cycles are their stratigraphic signature; they are commonly 3-50 my cycles. Eustatic changes in sea level produce systematic cycles at higher frequencies within the accommodation space created by tectonism. Depositional sequences and systems tracts are their stratigraphic signature; they are commonly .5-3.0 my cycles.
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 (major shelf and slope front erosion) 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. Without the infilling sequences to reduce the slope angle, forestepping sequences will not be able to form during this stage.
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), which corresponds to the maximum flooding surface of the last well-developed sequence. Several of the next sequences may be merged by sediment starvation.
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.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)