Rhythms and events in carbonate stratigraphy
The standard facies model depicts depositional systems where sedimentation is in approximate equilibrium with intrinsic feedbacks and extrinsic controls. The assumption of equilibrium conditions is often unjustified and this chapter examines important causes of change in carbonate sedimentation with time.
Chapter 2 presented evidence that sedimentation is inherently episodic or pulsating and that the record is riddled with hiatuses in a wide range of scales. The scaling of sedimentation rates and the intense lamination of unbioturbated sediments are two major arguments in favor of a non-steady model of sedimentation. This “Cantor model” of sedimentation (Plotnick, 1986) does not invalidate facies models but limits their use. Facies models should be viewed as idealized equilibrium states that depositional systems strive to but do not always reach before being disturbed by extrinsic factors. The episodic nature of sedimentation implies that stratigraphic documentations solely in terms of standard facies belts, is inadequate and cannot do justice to the complexity of vertical successions. For carbonates, in particular, we need to consider changes through time imposed by system-internal feed back (“autocycles”), orbital cycles of the ocean-atmosphere system, organic evolution and long oscillations induced by plate tectonics and cosmic processes.
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.