Fundamentals of sequence stratigraphy
Careful definitions and consistent terminology have been a hallmark of sequence stratigraphy from the outset. By now, however, the field suffers from an inflation of terms. I will introduce and use only the most essential ones. Others are explained elsewhere and can be recovered via the index.
The discussion in this chapter assumes that the following principles hold.
Sedimentary bodies and thus all physical stratigraphic units are of finite extent - they are lenses in the most general sense of the term.
Surfaces of erosion or non-deposition are also of limited extent. On the flat basin floors, the ultimate limits are set by the principle that siliciclastic sediment mass is conserved. Basins continually receive sediment and the water column cannot hold this material for any geologically significant time. As dissolution of siliciclastic sediment is negligible, the bulk must come rest on the basin floor. Thus, erosion in part of the basin must be compensated by increased sedimentation in others. For carbonate material, the conservation of mass does not hold for the abyssal sea floors but it remains a reason-able assumption for neritic and bathyal settings.
The sediment record is dissected by hiatuses at all geologically relevant scales. The assumption of a “complete section” is an (often useful) idealization.
The sediment record at any given locality reflects the history of relative sea-level movements. The history of eustasy requires either evidence of a world-wide phenomenon (such as orbital signals), or a global stack of records plus constraints from geodynamics. Geodynamic input is necessary because crust and mantle deform under the changing water load and therefore the same eustatic signal is recorded differently in different regions (Watts, 2001).
Figures & Tables
Sedimentology and stratigraphy are neighbors yet distinctly separate entities within the earth sciences. Sedimentology searches for the common traits of sedimentary rocks regardless of age as it reconstructs environments and processes of deposition and erosion from the sediment record. Stratigraphy, by contrast, concentrates on changes with time, on measuring time and correlating coeval events. Sequence stratigraphy straddles the boundary between the two fields. This book, dedicated to carbonate rocks, approaches sequence stratigraphy from its sedimentologic background. This book attempts to communicate by combining different specialities and different lines of reasoning, and by searching for principles underlying the bewildering diversity of carbonate rocks. It provides enough general background, in introductory chapters and appendices, to be easily digestible for sedimentologists and stratigraphers as well as earth scientists at large.